diff mbox series

[v2,2/2] vmalloc: Remove work as from vfree path

Message ID 20190520233841.17194-3-rick.p.edgecombe@intel.com
State Not Applicable
Delegated to: David Miller
Headers show
Series Fix issues with vmalloc flush flag | expand

Commit Message

Edgecombe, Rick P May 20, 2019, 11:38 p.m. UTC 
From: Rick Edgecombe <redgecombe.lkml@gmail.com>

Calling vm_unmap_alias() in vm_remove_mappings() could potentially be a
lot of work to do on a free operation. Simply flushing the TLB instead of
the whole vm_unmap_alias() operation makes the frees faster and pushes
the heavy work to happen on allocation where it would be more expected.
In addition to the extra work, vm_unmap_alias() takes some locks including
a long hold of vmap_purge_lock, which will make all other
VM_FLUSH_RESET_PERMS vfrees wait while the purge operation happens.

Lastly, page_address() can involve locking and lookups on some
configurations, so skip calling this by exiting out early when
!CONFIG_ARCH_HAS_SET_DIRECT_MAP.

Cc: Meelis Roos <mroos@linux.ee>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Nadav Amit <namit@vmware.com>
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
---
 mm/vmalloc.c | 18 ++++++++++--------
 1 file changed, 10 insertions(+), 8 deletions(-)

Comments

Andy Lutomirski May 21, 2019, 4:17 p.m. UTC | #1 
On Mon, May 20, 2019 at 4:39 PM Rick Edgecombe
<rick.p.edgecombe@intel.com> wrote:
>
> From: Rick Edgecombe <redgecombe.lkml@gmail.com>
>
> Calling vm_unmap_alias() in vm_remove_mappings() could potentially be a
> lot of work to do on a free operation. Simply flushing the TLB instead of
> the whole vm_unmap_alias() operation makes the frees faster and pushes
> the heavy work to happen on allocation where it would be more expected.
> In addition to the extra work, vm_unmap_alias() takes some locks including
> a long hold of vmap_purge_lock, which will make all other
> VM_FLUSH_RESET_PERMS vfrees wait while the purge operation happens.
>
> Lastly, page_address() can involve locking and lookups on some
> configurations, so skip calling this by exiting out early when
> !CONFIG_ARCH_HAS_SET_DIRECT_MAP.

Hmm.  I would have expected that the major cost of vm_unmap_aliases()
would be the flush, and at least informing the code that the flush
happened seems valuable.  So would guess that this patch is actually a
loss in throughput.

--Andy
Edgecombe, Rick P May 21, 2019, 4:51 p.m. UTC | #2 
On Tue, 2019-05-21 at 09:17 -0700, Andy Lutomirski wrote:
> On Mon, May 20, 2019 at 4:39 PM Rick Edgecombe
> <rick.p.edgecombe@intel.com> wrote:
> > From: Rick Edgecombe <redgecombe.lkml@gmail.com>
> > 
> > Calling vm_unmap_alias() in vm_remove_mappings() could potentially
> > be a
> > lot of work to do on a free operation. Simply flushing the TLB
> > instead of
> > the whole vm_unmap_alias() operation makes the frees faster and
> > pushes
> > the heavy work to happen on allocation where it would be more
> > expected.
> > In addition to the extra work, vm_unmap_alias() takes some locks
> > including
> > a long hold of vmap_purge_lock, which will make all other
> > VM_FLUSH_RESET_PERMS vfrees wait while the purge operation happens.
> > 
> > Lastly, page_address() can involve locking and lookups on some
> > configurations, so skip calling this by exiting out early when
> > !CONFIG_ARCH_HAS_SET_DIRECT_MAP.
> 
> Hmm.  I would have expected that the major cost of vm_unmap_aliases()
> would be the flush, and at least informing the code that the flush
> happened seems valuable.  So would guess that this patch is actually
> a
> loss in throughput.
> 
You are probably right about the flush taking the longest. The original
idea of using it was exactly to improve throughput by saving a flush.
However with vm_unmap_aliases() the flush will be over a larger range
than before for most arch's since it will likley span from the module
space to vmalloc. From poking around the sparc tlb flush history, I
guess the lazy purges used to be (still are?) a problem for them
because it would try to flush each page individually for some CPUs. Not
sure about all of the other architectures, but for any implementation
like that, using vm_unmap_alias() would turn an occasional long
operation into a more frequent one.

On x86, it shouldn't be a problem to use it. We already used to call
this function several times around a exec permission vfree. 

I guess its a tradeoff that depends on how fast large range TLB flushes
usually are compared to small ones. I am ok dropping it, if it doesn't
seem worth it.
Andy Lutomirski May 21, 2019, 5 p.m. UTC | #3 
On Tue, May 21, 2019 at 9:51 AM Edgecombe, Rick P
<rick.p.edgecombe@intel.com> wrote:
>
> On Tue, 2019-05-21 at 09:17 -0700, Andy Lutomirski wrote:
> > On Mon, May 20, 2019 at 4:39 PM Rick Edgecombe
> > <rick.p.edgecombe@intel.com> wrote:
> > > From: Rick Edgecombe <redgecombe.lkml@gmail.com>
> > >
> > > Calling vm_unmap_alias() in vm_remove_mappings() could potentially
> > > be a
> > > lot of work to do on a free operation. Simply flushing the TLB
> > > instead of
> > > the whole vm_unmap_alias() operation makes the frees faster and
> > > pushes
> > > the heavy work to happen on allocation where it would be more
> > > expected.
> > > In addition to the extra work, vm_unmap_alias() takes some locks
> > > including
> > > a long hold of vmap_purge_lock, which will make all other
> > > VM_FLUSH_RESET_PERMS vfrees wait while the purge operation happens.
> > >
> > > Lastly, page_address() can involve locking and lookups on some
> > > configurations, so skip calling this by exiting out early when
> > > !CONFIG_ARCH_HAS_SET_DIRECT_MAP.
> >
> > Hmm.  I would have expected that the major cost of vm_unmap_aliases()
> > would be the flush, and at least informing the code that the flush
> > happened seems valuable.  So would guess that this patch is actually
> > a
> > loss in throughput.
> >
> You are probably right about the flush taking the longest. The original
> idea of using it was exactly to improve throughput by saving a flush.
> However with vm_unmap_aliases() the flush will be over a larger range
> than before for most arch's since it will likley span from the module
> space to vmalloc. From poking around the sparc tlb flush history, I
> guess the lazy purges used to be (still are?) a problem for them
> because it would try to flush each page individually for some CPUs. Not
> sure about all of the other architectures, but for any implementation
> like that, using vm_unmap_alias() would turn an occasional long
> operation into a more frequent one.
>
> On x86, it shouldn't be a problem to use it. We already used to call
> this function several times around a exec permission vfree.
>
> I guess its a tradeoff that depends on how fast large range TLB flushes
> usually are compared to small ones. I am ok dropping it, if it doesn't
> seem worth it.

On x86, a full flush is probably not much slower than just flushing a
page or two -- the main cost is in the TLB refill.  I don't know about
other architectures.  I would drop this patch unless you have numbers
suggesting that it's a win.
Edgecombe, Rick P May 21, 2019, 7:47 p.m. UTC | #4 
On Tue, 2019-05-21 at 10:00 -0700, Andy Lutomirski wrote:
> On Tue, May 21, 2019 at 9:51 AM Edgecombe, Rick P
> <rick.p.edgecombe@intel.com> wrote:
> > On Tue, 2019-05-21 at 09:17 -0700, Andy Lutomirski wrote:
> > > On Mon, May 20, 2019 at 4:39 PM Rick Edgecombe
> > > <rick.p.edgecombe@intel.com> wrote:
> > > > From: Rick Edgecombe <redgecombe.lkml@gmail.com>
> > > > 
> > > > Calling vm_unmap_alias() in vm_remove_mappings() could
> > > > potentially
> > > > be a
> > > > lot of work to do on a free operation. Simply flushing the TLB
> > > > instead of
> > > > the whole vm_unmap_alias() operation makes the frees faster and
> > > > pushes
> > > > the heavy work to happen on allocation where it would be more
> > > > expected.
> > > > In addition to the extra work, vm_unmap_alias() takes some
> > > > locks
> > > > including
> > > > a long hold of vmap_purge_lock, which will make all other
> > > > VM_FLUSH_RESET_PERMS vfrees wait while the purge operation
> > > > happens.
> > > > 
> > > > Lastly, page_address() can involve locking and lookups on some
> > > > configurations, so skip calling this by exiting out early when
> > > > !CONFIG_ARCH_HAS_SET_DIRECT_MAP.
> > > 
> > > Hmm.  I would have expected that the major cost of
> > > vm_unmap_aliases()
> > > would be the flush, and at least informing the code that the
> > > flush
> > > happened seems valuable.  So would guess that this patch is
> > > actually
> > > a
> > > loss in throughput.
> > > 
> > You are probably right about the flush taking the longest. The
> > original
> > idea of using it was exactly to improve throughput by saving a
> > flush.
> > However with vm_unmap_aliases() the flush will be over a larger
> > range
> > than before for most arch's since it will likley span from the
> > module
> > space to vmalloc. From poking around the sparc tlb flush history, I
> > guess the lazy purges used to be (still are?) a problem for them
> > because it would try to flush each page individually for some CPUs.
> > Not
> > sure about all of the other architectures, but for any
> > implementation
> > like that, using vm_unmap_alias() would turn an occasional long
> > operation into a more frequent one.
> > 
> > On x86, it shouldn't be a problem to use it. We already used to
> > call
> > this function several times around a exec permission vfree.
> > 
> > I guess its a tradeoff that depends on how fast large range TLB
> > flushes
> > usually are compared to small ones. I am ok dropping it, if it
> > doesn't
> > seem worth it.
> 
> On x86, a full flush is probably not much slower than just flushing a
> page or two -- the main cost is in the TLB refill.  I don't know
> about
> other architectures.  I would drop this patch unless you have numbers
> suggesting that it's a win.

Ok. This patch also inadvertently improved some correctness in calls to
flush_tlb_kernel_range() for a rare situation. I'll work that into a
different patch.

Thanks,

Rick
diff mbox series

Patch

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index 836888ae01f6..8d03427626dc 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -2122,9 +2122,10 @@  static inline void set_area_direct_map(const struct vm_struct *area,
 /* Handle removing and resetting vm mappings related to the vm_struct. */
 static void vm_remove_mappings(struct vm_struct *area, int deallocate_pages)
 {
+	const bool has_set_direct = IS_ENABLED(CONFIG_ARCH_HAS_SET_DIRECT_MAP);
+	const bool flush_reset = area->flags & VM_FLUSH_RESET_PERMS;
 	unsigned long addr = (unsigned long)area->addr;
-	unsigned long start = ULONG_MAX, end = 0;
-	int flush_reset = area->flags & VM_FLUSH_RESET_PERMS;
+	unsigned long start = addr, end = addr + area->size;
 	int i;
 
 	/*
@@ -2133,7 +2134,7 @@  static void vm_remove_mappings(struct vm_struct *area, int deallocate_pages)
 	 * This is concerned with resetting the direct map any an vm alias with
 	 * execute permissions, without leaving a RW+X window.
 	 */
-	if (flush_reset && !IS_ENABLED(CONFIG_ARCH_HAS_SET_DIRECT_MAP)) {
+	if (flush_reset && !has_set_direct) {
 		set_memory_nx(addr, area->nr_pages);
 		set_memory_rw(addr, area->nr_pages);
 	}
@@ -2146,17 +2147,18 @@  static void vm_remove_mappings(struct vm_struct *area, int deallocate_pages)
 
 	/*
 	 * If not deallocating pages, just do the flush of the VM area and
-	 * return.
+	 * return. If the arch doesn't have set_direct_map_(), also skip the
+	 * below work.
 	 */
-	if (!deallocate_pages) {
-		vm_unmap_aliases();
+	if (!deallocate_pages || !has_set_direct) {
+		flush_tlb_kernel_range(start, end);
 		return;
 	}
 
 	/*
 	 * If execution gets here, flush the vm mapping and reset the direct
 	 * map. Find the start and end range of the direct mappings to make sure
-	 * the vm_unmap_aliases() flush includes the direct map.
+	 * the flush_tlb_kernel_range() includes the direct map.
 	 */
 	for (i = 0; i < area->nr_pages; i++) {
 		addr = (unsigned long)page_address(area->pages[i]);
@@ -2172,7 +2174,7 @@  static void vm_remove_mappings(struct vm_struct *area, int deallocate_pages)
 	 * reset the direct map permissions to the default.
 	 */
 	set_area_direct_map(area, set_direct_map_invalid_noflush);
-	_vm_unmap_aliases(start, end, 1);
+	flush_tlb_kernel_range(start, end);
 	set_area_direct_map(area, set_direct_map_default_noflush);
 }