| Message ID | patch.git-943f1e5dcff2.your-ad-here.call-01599856292-ext-8676@work.hours |
|---|---|
| State | Not Applicable |
| Delegated to: | David Miller |
| Headers | show |
| Series | [v2] mm/gup: fix gup_fast with dynamic page table folding | expand |
On Fri, Sep 11, 2020 at 10:36:43PM +0200, Vasily Gorbik wrote: > Currently to make sure that every page table entry is read just once > gup_fast walks perform READ_ONCE and pass pXd value down to the next > gup_pXd_range function by value e.g.: ...snip... > --- > v2: added brackets &pgd -> &(pgd) > > arch/s390/include/asm/pgtable.h | 42 +++++++++++++++++++++++---------- > include/linux/pgtable.h | 10 ++++++++ > mm/gup.c | 18 +++++++------- > 3 files changed, 49 insertions(+), 21 deletions(-) Andrew, any chance you would pick this up? There is an Ack from Linus. And I haven't seen any objections from Jason or John. This seems to be as safe for other architectures as possible. @Jason and John Any acks/nacks? Thank you, Vasily
On Fri, Sep 11, 2020 at 10:36:43PM +0200, Vasily Gorbik wrote: > Currently to make sure that every page table entry is read just once > gup_fast walks perform READ_ONCE and pass pXd value down to the next > gup_pXd_range function by value e.g.: > > static int gup_pud_range(p4d_t p4d, unsigned long addr, unsigned long end, > unsigned int flags, struct page **pages, int *nr) > ... > pudp = pud_offset(&p4d, addr); > > This function passes a reference on that local value copy to pXd_offset, > and might get the very same pointer in return. This happens when the > level is folded (on most arches), and that pointer should not be iterated. > > On s390 due to the fact that each task might have different 5,4 or > 3-level address translation and hence different levels folded the logic > is more complex and non-iteratable pointer to a local copy leads to > severe problems. > > Here is an example of what happens with gup_fast on s390, for a task > with 3-levels paging, crossing a 2 GB pud boundary: > > // addr = 0x1007ffff000, end = 0x10080001000 > static int gup_pud_range(p4d_t p4d, unsigned long addr, unsigned long end, > unsigned int flags, struct page **pages, int *nr) > { > unsigned long next; > pud_t *pudp; > > // pud_offset returns &p4d itself (a pointer to a value on stack) > pudp = pud_offset(&p4d, addr); > do { > // on second iteratation reading "random" stack value > pud_t pud = READ_ONCE(*pudp); > > // next = 0x10080000000, due to PUD_SIZE/MASK != PGDIR_SIZE/MASK on s390 > next = pud_addr_end(addr, end); > ... > } while (pudp++, addr = next, addr != end); // pudp++ iterating over stack > > return 1; > } > > This happens since s390 moved to common gup code with > commit d1874a0c2805 ("s390/mm: make the pxd_offset functions more robust") > and commit 1a42010cdc26 ("s390/mm: convert to the generic > get_user_pages_fast code"). s390 tried to mimic static level folding by > changing pXd_offset primitives to always calculate top level page table > offset in pgd_offset and just return the value passed when pXd_offset > has to act as folded. > > What is crucial for gup_fast and what has been overlooked is > that PxD_SIZE/MASK and thus pXd_addr_end should also change > correspondingly. And the latter is not possible with dynamic folding. > > To fix the issue in addition to pXd values pass original > pXdp pointers down to gup_pXd_range functions. And introduce > pXd_offset_lockless helpers, which take an additional pXd > entry value parameter. This has already been discussed in > https://lkml.kernel.org/r/20190418100218.0a4afd51@mschwideX1 > > Cc: <stable@vger.kernel.org> # 5.2+ > Fixes: 1a42010cdc26 ("s390/mm: convert to the generic get_user_pages_fast code") > Reviewed-by: Gerald Schaefer <gerald.schaefer@linux.ibm.com> > Reviewed-by: Alexander Gordeev <agordeev@linux.ibm.com> > Signed-off-by: Vasily Gorbik <gor@linux.ibm.com> > --- > v2: added brackets &pgd -> &(pgd) Reviewed-by: Jason Gunthorpe <jgg@nvidia.com> Regards, Jason
On Fri, Sep 11, 2020 at 10:36:43PM +0200, Vasily Gorbik wrote: > Currently to make sure that every page table entry is read just once > gup_fast walks perform READ_ONCE and pass pXd value down to the next > gup_pXd_range function by value e.g.: > > static int gup_pud_range(p4d_t p4d, unsigned long addr, unsigned long end, > unsigned int flags, struct page **pages, int *nr) > ... > pudp = pud_offset(&p4d, addr); > > This function passes a reference on that local value copy to pXd_offset, > and might get the very same pointer in return. This happens when the > level is folded (on most arches), and that pointer should not be iterated. > > On s390 due to the fact that each task might have different 5,4 or > 3-level address translation and hence different levels folded the logic > is more complex and non-iteratable pointer to a local copy leads to > severe problems. > > Here is an example of what happens with gup_fast on s390, for a task > with 3-levels paging, crossing a 2 GB pud boundary: > > // addr = 0x1007ffff000, end = 0x10080001000 > static int gup_pud_range(p4d_t p4d, unsigned long addr, unsigned long end, > unsigned int flags, struct page **pages, int *nr) > { > unsigned long next; > pud_t *pudp; > > // pud_offset returns &p4d itself (a pointer to a value on stack) > pudp = pud_offset(&p4d, addr); > do { > // on second iteratation reading "random" stack value > pud_t pud = READ_ONCE(*pudp); > > // next = 0x10080000000, due to PUD_SIZE/MASK != PGDIR_SIZE/MASK on s390 > next = pud_addr_end(addr, end); > ... > } while (pudp++, addr = next, addr != end); // pudp++ iterating over stack > > return 1; > } > > This happens since s390 moved to common gup code with > commit d1874a0c2805 ("s390/mm: make the pxd_offset functions more robust") > and commit 1a42010cdc26 ("s390/mm: convert to the generic > get_user_pages_fast code"). s390 tried to mimic static level folding by > changing pXd_offset primitives to always calculate top level page table > offset in pgd_offset and just return the value passed when pXd_offset > has to act as folded. > > What is crucial for gup_fast and what has been overlooked is > that PxD_SIZE/MASK and thus pXd_addr_end should also change > correspondingly. And the latter is not possible with dynamic folding. > > To fix the issue in addition to pXd values pass original > pXdp pointers down to gup_pXd_range functions. And introduce > pXd_offset_lockless helpers, which take an additional pXd > entry value parameter. This has already been discussed in > https://lkml.kernel.org/r/20190418100218.0a4afd51@mschwideX1 > > Cc: <stable@vger.kernel.org> # 5.2+ > Fixes: 1a42010cdc26 ("s390/mm: convert to the generic get_user_pages_fast code") > Reviewed-by: Gerald Schaefer <gerald.schaefer@linux.ibm.com> > Reviewed-by: Alexander Gordeev <agordeev@linux.ibm.com> > Signed-off-by: Vasily Gorbik <gor@linux.ibm.com> Reviewed-by: Mike Rapoport <rppt@linux.ibm.com> > --- > v2: added brackets &pgd -> &(pgd) > > arch/s390/include/asm/pgtable.h | 42 +++++++++++++++++++++++---------- > include/linux/pgtable.h | 10 ++++++++ > mm/gup.c | 18 +++++++------- > 3 files changed, 49 insertions(+), 21 deletions(-) > > diff --git a/arch/s390/include/asm/pgtable.h b/arch/s390/include/asm/pgtable.h > index 7eb01a5459cd..b55561cc8786 100644 > --- a/arch/s390/include/asm/pgtable.h > +++ b/arch/s390/include/asm/pgtable.h > @@ -1260,26 +1260,44 @@ static inline pgd_t *pgd_offset_raw(pgd_t *pgd, unsigned long address) > > #define pgd_offset(mm, address) pgd_offset_raw(READ_ONCE((mm)->pgd), address) > > -static inline p4d_t *p4d_offset(pgd_t *pgd, unsigned long address) > +static inline p4d_t *p4d_offset_lockless(pgd_t *pgdp, pgd_t pgd, unsigned long address) > { > - if ((pgd_val(*pgd) & _REGION_ENTRY_TYPE_MASK) >= _REGION_ENTRY_TYPE_R1) > - return (p4d_t *) pgd_deref(*pgd) + p4d_index(address); > - return (p4d_t *) pgd; > + if ((pgd_val(pgd) & _REGION_ENTRY_TYPE_MASK) >= _REGION_ENTRY_TYPE_R1) > + return (p4d_t *) pgd_deref(pgd) + p4d_index(address); > + return (p4d_t *) pgdp; > } > +#define p4d_offset_lockless p4d_offset_lockless > > -static inline pud_t *pud_offset(p4d_t *p4d, unsigned long address) > +static inline p4d_t *p4d_offset(pgd_t *pgdp, unsigned long address) > { > - if ((p4d_val(*p4d) & _REGION_ENTRY_TYPE_MASK) >= _REGION_ENTRY_TYPE_R2) > - return (pud_t *) p4d_deref(*p4d) + pud_index(address); > - return (pud_t *) p4d; > + return p4d_offset_lockless(pgdp, *pgdp, address); > +} > + > +static inline pud_t *pud_offset_lockless(p4d_t *p4dp, p4d_t p4d, unsigned long address) > +{ > + if ((p4d_val(p4d) & _REGION_ENTRY_TYPE_MASK) >= _REGION_ENTRY_TYPE_R2) > + return (pud_t *) p4d_deref(p4d) + pud_index(address); > + return (pud_t *) p4dp; > +} > +#define pud_offset_lockless pud_offset_lockless > + > +static inline pud_t *pud_offset(p4d_t *p4dp, unsigned long address) > +{ > + return pud_offset_lockless(p4dp, *p4dp, address); > } > #define pud_offset pud_offset > > -static inline pmd_t *pmd_offset(pud_t *pud, unsigned long address) > +static inline pmd_t *pmd_offset_lockless(pud_t *pudp, pud_t pud, unsigned long address) > +{ > + if ((pud_val(pud) & _REGION_ENTRY_TYPE_MASK) >= _REGION_ENTRY_TYPE_R3) > + return (pmd_t *) pud_deref(pud) + pmd_index(address); > + return (pmd_t *) pudp; > +} > +#define pmd_offset_lockless pmd_offset_lockless > + > +static inline pmd_t *pmd_offset(pud_t *pudp, unsigned long address) > { > - if ((pud_val(*pud) & _REGION_ENTRY_TYPE_MASK) >= _REGION_ENTRY_TYPE_R3) > - return (pmd_t *) pud_deref(*pud) + pmd_index(address); > - return (pmd_t *) pud; > + return pmd_offset_lockless(pudp, *pudp, address); > } > #define pmd_offset pmd_offset > > diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h > index e8cbc2e795d5..90654cb63e9e 100644 > --- a/include/linux/pgtable.h > +++ b/include/linux/pgtable.h > @@ -1427,6 +1427,16 @@ typedef unsigned int pgtbl_mod_mask; > #define mm_pmd_folded(mm) __is_defined(__PAGETABLE_PMD_FOLDED) > #endif > > +#ifndef p4d_offset_lockless > +#define p4d_offset_lockless(pgdp, pgd, address) p4d_offset(&(pgd), address) > +#endif > +#ifndef pud_offset_lockless > +#define pud_offset_lockless(p4dp, p4d, address) pud_offset(&(p4d), address) > +#endif > +#ifndef pmd_offset_lockless > +#define pmd_offset_lockless(pudp, pud, address) pmd_offset(&(pud), address) > +#endif > + > /* > * p?d_leaf() - true if this entry is a final mapping to a physical address. > * This differs from p?d_huge() by the fact that they are always available (if > diff --git a/mm/gup.c b/mm/gup.c > index e5739a1974d5..578bf5bd8bf8 100644 > --- a/mm/gup.c > +++ b/mm/gup.c > @@ -2485,13 +2485,13 @@ static int gup_huge_pgd(pgd_t orig, pgd_t *pgdp, unsigned long addr, > return 1; > } > > -static int gup_pmd_range(pud_t pud, unsigned long addr, unsigned long end, > +static int gup_pmd_range(pud_t *pudp, pud_t pud, unsigned long addr, unsigned long end, > unsigned int flags, struct page **pages, int *nr) > { > unsigned long next; > pmd_t *pmdp; > > - pmdp = pmd_offset(&pud, addr); > + pmdp = pmd_offset_lockless(pudp, pud, addr); > do { > pmd_t pmd = READ_ONCE(*pmdp); > > @@ -2528,13 +2528,13 @@ static int gup_pmd_range(pud_t pud, unsigned long addr, unsigned long end, > return 1; > } > > -static int gup_pud_range(p4d_t p4d, unsigned long addr, unsigned long end, > +static int gup_pud_range(p4d_t *p4dp, p4d_t p4d, unsigned long addr, unsigned long end, > unsigned int flags, struct page **pages, int *nr) > { > unsigned long next; > pud_t *pudp; > > - pudp = pud_offset(&p4d, addr); > + pudp = pud_offset_lockless(p4dp, p4d, addr); > do { > pud_t pud = READ_ONCE(*pudp); > > @@ -2549,20 +2549,20 @@ static int gup_pud_range(p4d_t p4d, unsigned long addr, unsigned long end, > if (!gup_huge_pd(__hugepd(pud_val(pud)), addr, > PUD_SHIFT, next, flags, pages, nr)) > return 0; > - } else if (!gup_pmd_range(pud, addr, next, flags, pages, nr)) > + } else if (!gup_pmd_range(pudp, pud, addr, next, flags, pages, nr)) > return 0; > } while (pudp++, addr = next, addr != end); > > return 1; > } > > -static int gup_p4d_range(pgd_t pgd, unsigned long addr, unsigned long end, > +static int gup_p4d_range(pgd_t *pgdp, pgd_t pgd, unsigned long addr, unsigned long end, > unsigned int flags, struct page **pages, int *nr) > { > unsigned long next; > p4d_t *p4dp; > > - p4dp = p4d_offset(&pgd, addr); > + p4dp = p4d_offset_lockless(pgdp, pgd, addr); > do { > p4d_t p4d = READ_ONCE(*p4dp); > > @@ -2574,7 +2574,7 @@ static int gup_p4d_range(pgd_t pgd, unsigned long addr, unsigned long end, > if (!gup_huge_pd(__hugepd(p4d_val(p4d)), addr, > P4D_SHIFT, next, flags, pages, nr)) > return 0; > - } else if (!gup_pud_range(p4d, addr, next, flags, pages, nr)) > + } else if (!gup_pud_range(p4dp, p4d, addr, next, flags, pages, nr)) > return 0; > } while (p4dp++, addr = next, addr != end); > > @@ -2602,7 +2602,7 @@ static void gup_pgd_range(unsigned long addr, unsigned long end, > if (!gup_huge_pd(__hugepd(pgd_val(pgd)), addr, > PGDIR_SHIFT, next, flags, pages, nr)) > return; > - } else if (!gup_p4d_range(pgd, addr, next, flags, pages, nr)) > + } else if (!gup_p4d_range(pgdp, pgd, addr, next, flags, pages, nr)) > return; > } while (pgdp++, addr = next, addr != end); > } > -- > ⣿⣿⣿⣿⢋⡀⣀⠹⣿⣿⣿⣿ > ⣿⣿⣿⣿⠠⣶⡦⠀⣿⣿⣿⣿ > ⣿⣿⣿⠏⣴⣮⣴⣧⠈⢿⣿⣿ > ⣿⣿⡏⢰⣿⠖⣠⣿⡆⠈⣿⣿ > ⣿⢛⣵⣄⠙⣶⣶⡟⣅⣠⠹⣿ > ⣿⣜⣛⠻⢎⣉⣉⣀⠿⣫⣵⣿
On 9/11/20 1:36 PM, Vasily Gorbik wrote: > Currently to make sure that every page table entry is read just once > gup_fast walks perform READ_ONCE and pass pXd value down to the next > gup_pXd_range function by value e.g.: > > static int gup_pud_range(p4d_t p4d, unsigned long addr, unsigned long end, > unsigned int flags, struct page **pages, int *nr) > ... > pudp = pud_offset(&p4d, addr); > > This function passes a reference on that local value copy to pXd_offset, > and might get the very same pointer in return. This happens when the > level is folded (on most arches), and that pointer should not be iterated. > > On s390 due to the fact that each task might have different 5,4 or > 3-level address translation and hence different levels folded the logic > is more complex and non-iteratable pointer to a local copy leads to > severe problems. > > Here is an example of what happens with gup_fast on s390, for a task > with 3-levels paging, crossing a 2 GB pud boundary: > > // addr = 0x1007ffff000, end = 0x10080001000 > static int gup_pud_range(p4d_t p4d, unsigned long addr, unsigned long end, > unsigned int flags, struct page **pages, int *nr) > { > unsigned long next; > pud_t *pudp; > > // pud_offset returns &p4d itself (a pointer to a value on stack) > pudp = pud_offset(&p4d, addr); > do { > // on second iteratation reading "random" stack value > pud_t pud = READ_ONCE(*pudp); > > // next = 0x10080000000, due to PUD_SIZE/MASK != PGDIR_SIZE/MASK on s390 > next = pud_addr_end(addr, end); > ... > } while (pudp++, addr = next, addr != end); // pudp++ iterating over stack > > return 1; > } > > This happens since s390 moved to common gup code with > commit d1874a0c2805 ("s390/mm: make the pxd_offset functions more robust") > and commit 1a42010cdc26 ("s390/mm: convert to the generic > get_user_pages_fast code"). s390 tried to mimic static level folding by > changing pXd_offset primitives to always calculate top level page table > offset in pgd_offset and just return the value passed when pXd_offset > has to act as folded. > > What is crucial for gup_fast and what has been overlooked is > that PxD_SIZE/MASK and thus pXd_addr_end should also change > correspondingly. And the latter is not possible with dynamic folding. > > To fix the issue in addition to pXd values pass original > pXdp pointers down to gup_pXd_range functions. And introduce > pXd_offset_lockless helpers, which take an additional pXd > entry value parameter. This has already been discussed in > https://lkml.kernel.org/r/20190418100218.0a4afd51@mschwideX1 > > Cc: <stable@vger.kernel.org> # 5.2+ > Fixes: 1a42010cdc26 ("s390/mm: convert to the generic get_user_pages_fast code") > Reviewed-by: Gerald Schaefer <gerald.schaefer@linux.ibm.com> > Reviewed-by: Alexander Gordeev <agordeev@linux.ibm.com> > Signed-off-by: Vasily Gorbik <gor@linux.ibm.com> > --- Looks cleaner than I'd dared hope for. :) Reviewed-by: John Hubbard <jhubbard@nvidia.com> thanks,
diff --git a/arch/s390/include/asm/pgtable.h b/arch/s390/include/asm/pgtable.h index 7eb01a5459cd..b55561cc8786 100644 --- a/arch/s390/include/asm/pgtable.h +++ b/arch/s390/include/asm/pgtable.h @@ -1260,26 +1260,44 @@ static inline pgd_t *pgd_offset_raw(pgd_t *pgd, unsigned long address) #define pgd_offset(mm, address) pgd_offset_raw(READ_ONCE((mm)->pgd), address) -static inline p4d_t *p4d_offset(pgd_t *pgd, unsigned long address) +static inline p4d_t *p4d_offset_lockless(pgd_t *pgdp, pgd_t pgd, unsigned long address) { - if ((pgd_val(*pgd) & _REGION_ENTRY_TYPE_MASK) >= _REGION_ENTRY_TYPE_R1) - return (p4d_t *) pgd_deref(*pgd) + p4d_index(address); - return (p4d_t *) pgd; + if ((pgd_val(pgd) & _REGION_ENTRY_TYPE_MASK) >= _REGION_ENTRY_TYPE_R1) + return (p4d_t *) pgd_deref(pgd) + p4d_index(address); + return (p4d_t *) pgdp; } +#define p4d_offset_lockless p4d_offset_lockless -static inline pud_t *pud_offset(p4d_t *p4d, unsigned long address) +static inline p4d_t *p4d_offset(pgd_t *pgdp, unsigned long address) { - if ((p4d_val(*p4d) & _REGION_ENTRY_TYPE_MASK) >= _REGION_ENTRY_TYPE_R2) - return (pud_t *) p4d_deref(*p4d) + pud_index(address); - return (pud_t *) p4d; + return p4d_offset_lockless(pgdp, *pgdp, address); +} + +static inline pud_t *pud_offset_lockless(p4d_t *p4dp, p4d_t p4d, unsigned long address) +{ + if ((p4d_val(p4d) & _REGION_ENTRY_TYPE_MASK) >= _REGION_ENTRY_TYPE_R2) + return (pud_t *) p4d_deref(p4d) + pud_index(address); + return (pud_t *) p4dp; +} +#define pud_offset_lockless pud_offset_lockless + +static inline pud_t *pud_offset(p4d_t *p4dp, unsigned long address) +{ + return pud_offset_lockless(p4dp, *p4dp, address); } #define pud_offset pud_offset -static inline pmd_t *pmd_offset(pud_t *pud, unsigned long address) +static inline pmd_t *pmd_offset_lockless(pud_t *pudp, pud_t pud, unsigned long address) +{ + if ((pud_val(pud) & _REGION_ENTRY_TYPE_MASK) >= _REGION_ENTRY_TYPE_R3) + return (pmd_t *) pud_deref(pud) + pmd_index(address); + return (pmd_t *) pudp; +} +#define pmd_offset_lockless pmd_offset_lockless + +static inline pmd_t *pmd_offset(pud_t *pudp, unsigned long address) { - if ((pud_val(*pud) & _REGION_ENTRY_TYPE_MASK) >= _REGION_ENTRY_TYPE_R3) - return (pmd_t *) pud_deref(*pud) + pmd_index(address); - return (pmd_t *) pud; + return pmd_offset_lockless(pudp, *pudp, address); } #define pmd_offset pmd_offset diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h index e8cbc2e795d5..90654cb63e9e 100644 --- a/include/linux/pgtable.h +++ b/include/linux/pgtable.h @@ -1427,6 +1427,16 @@ typedef unsigned int pgtbl_mod_mask; #define mm_pmd_folded(mm) __is_defined(__PAGETABLE_PMD_FOLDED) #endif +#ifndef p4d_offset_lockless +#define p4d_offset_lockless(pgdp, pgd, address) p4d_offset(&(pgd), address) +#endif +#ifndef pud_offset_lockless +#define pud_offset_lockless(p4dp, p4d, address) pud_offset(&(p4d), address) +#endif +#ifndef pmd_offset_lockless +#define pmd_offset_lockless(pudp, pud, address) pmd_offset(&(pud), address) +#endif + /* * p?d_leaf() - true if this entry is a final mapping to a physical address. * This differs from p?d_huge() by the fact that they are always available (if diff --git a/mm/gup.c b/mm/gup.c index e5739a1974d5..578bf5bd8bf8 100644 --- a/mm/gup.c +++ b/mm/gup.c @@ -2485,13 +2485,13 @@ static int gup_huge_pgd(pgd_t orig, pgd_t *pgdp, unsigned long addr, return 1; } -static int gup_pmd_range(pud_t pud, unsigned long addr, unsigned long end, +static int gup_pmd_range(pud_t *pudp, pud_t pud, unsigned long addr, unsigned long end, unsigned int flags, struct page **pages, int *nr) { unsigned long next; pmd_t *pmdp; - pmdp = pmd_offset(&pud, addr); + pmdp = pmd_offset_lockless(pudp, pud, addr); do { pmd_t pmd = READ_ONCE(*pmdp); @@ -2528,13 +2528,13 @@ static int gup_pmd_range(pud_t pud, unsigned long addr, unsigned long end, return 1; } -static int gup_pud_range(p4d_t p4d, unsigned long addr, unsigned long end, +static int gup_pud_range(p4d_t *p4dp, p4d_t p4d, unsigned long addr, unsigned long end, unsigned int flags, struct page **pages, int *nr) { unsigned long next; pud_t *pudp; - pudp = pud_offset(&p4d, addr); + pudp = pud_offset_lockless(p4dp, p4d, addr); do { pud_t pud = READ_ONCE(*pudp); @@ -2549,20 +2549,20 @@ static int gup_pud_range(p4d_t p4d, unsigned long addr, unsigned long end, if (!gup_huge_pd(__hugepd(pud_val(pud)), addr, PUD_SHIFT, next, flags, pages, nr)) return 0; - } else if (!gup_pmd_range(pud, addr, next, flags, pages, nr)) + } else if (!gup_pmd_range(pudp, pud, addr, next, flags, pages, nr)) return 0; } while (pudp++, addr = next, addr != end); return 1; } -static int gup_p4d_range(pgd_t pgd, unsigned long addr, unsigned long end, +static int gup_p4d_range(pgd_t *pgdp, pgd_t pgd, unsigned long addr, unsigned long end, unsigned int flags, struct page **pages, int *nr) { unsigned long next; p4d_t *p4dp; - p4dp = p4d_offset(&pgd, addr); + p4dp = p4d_offset_lockless(pgdp, pgd, addr); do { p4d_t p4d = READ_ONCE(*p4dp); @@ -2574,7 +2574,7 @@ static int gup_p4d_range(pgd_t pgd, unsigned long addr, unsigned long end, if (!gup_huge_pd(__hugepd(p4d_val(p4d)), addr, P4D_SHIFT, next, flags, pages, nr)) return 0; - } else if (!gup_pud_range(p4d, addr, next, flags, pages, nr)) + } else if (!gup_pud_range(p4dp, p4d, addr, next, flags, pages, nr)) return 0; } while (p4dp++, addr = next, addr != end); @@ -2602,7 +2602,7 @@ static void gup_pgd_range(unsigned long addr, unsigned long end, if (!gup_huge_pd(__hugepd(pgd_val(pgd)), addr, PGDIR_SHIFT, next, flags, pages, nr)) return; - } else if (!gup_p4d_range(pgd, addr, next, flags, pages, nr)) + } else if (!gup_p4d_range(pgdp, pgd, addr, next, flags, pages, nr)) return; } while (pgdp++, addr = next, addr != end); }
Currently to make sure that every page table entry is read just once gup_fast walks perform READ_ONCE and pass pXd value down to the next gup_pXd_range function by value e.g.: static int gup_pud_range(p4d_t p4d, unsigned long addr, unsigned long end, unsigned int flags, struct page **pages, int *nr) ... pudp = pud_offset(&p4d, addr); This function passes a reference on that local value copy to pXd_offset, and might get the very same pointer in return. This happens when the level is folded (on most arches), and that pointer should not be iterated. On s390 due to the fact that each task might have different 5,4 or 3-level address translation and hence different levels folded the logic is more complex and non-iteratable pointer to a local copy leads to severe problems. Here is an example of what happens with gup_fast on s390, for a task with 3-levels paging, crossing a 2 GB pud boundary: // addr = 0x1007ffff000, end = 0x10080001000 static int gup_pud_range(p4d_t p4d, unsigned long addr, unsigned long end, unsigned int flags, struct page **pages, int *nr) { unsigned long next; pud_t *pudp; // pud_offset returns &p4d itself (a pointer to a value on stack) pudp = pud_offset(&p4d, addr); do { // on second iteratation reading "random" stack value pud_t pud = READ_ONCE(*pudp); // next = 0x10080000000, due to PUD_SIZE/MASK != PGDIR_SIZE/MASK on s390 next = pud_addr_end(addr, end); ... } while (pudp++, addr = next, addr != end); // pudp++ iterating over stack return 1; } This happens since s390 moved to common gup code with commit d1874a0c2805 ("s390/mm: make the pxd_offset functions more robust") and commit 1a42010cdc26 ("s390/mm: convert to the generic get_user_pages_fast code"). s390 tried to mimic static level folding by changing pXd_offset primitives to always calculate top level page table offset in pgd_offset and just return the value passed when pXd_offset has to act as folded. What is crucial for gup_fast and what has been overlooked is that PxD_SIZE/MASK and thus pXd_addr_end should also change correspondingly. And the latter is not possible with dynamic folding. To fix the issue in addition to pXd values pass original pXdp pointers down to gup_pXd_range functions. And introduce pXd_offset_lockless helpers, which take an additional pXd entry value parameter. This has already been discussed in https://lkml.kernel.org/r/20190418100218.0a4afd51@mschwideX1 Cc: <stable@vger.kernel.org> # 5.2+ Fixes: 1a42010cdc26 ("s390/mm: convert to the generic get_user_pages_fast code") Reviewed-by: Gerald Schaefer <gerald.schaefer@linux.ibm.com> Reviewed-by: Alexander Gordeev <agordeev@linux.ibm.com> Signed-off-by: Vasily Gorbik <gor@linux.ibm.com> --- v2: added brackets &pgd -> &(pgd) arch/s390/include/asm/pgtable.h | 42 +++++++++++++++++++++++---------- include/linux/pgtable.h | 10 ++++++++ mm/gup.c | 18 +++++++------- 3 files changed, 49 insertions(+), 21 deletions(-)