| Message ID | 1515777968-867-4-git-send-email-ldufour@linux.vnet.ibm.com (mailing list archive) |
|---|---|
| State | Not Applicable |
| Headers | show |
| Series | Speculative page faults | expand |
Laurent Dufour <ldufour@linux.vnet.ibm.com> writes: > From: Peter Zijlstra <peterz@infradead.org> > > One of the side effects of speculating on faults (without holding > mmap_sem) is that we can race with free_pgtables() and therefore we > cannot assume the page-tables will stick around. > > Remove the reliance on the pte pointer. This needs a lot more explanation. So why is this code not needed with SPF only? -Andi
On 17/01/2018 04:04, Andi Kleen wrote: > Laurent Dufour <ldufour@linux.vnet.ibm.com> writes: > >> From: Peter Zijlstra <peterz@infradead.org> >> >> One of the side effects of speculating on faults (without holding >> mmap_sem) is that we can race with free_pgtables() and therefore we >> cannot assume the page-tables will stick around. >> >> Remove the reliance on the pte pointer. > > This needs a lot more explanation. So why is this code not needed with > SPF only? Hi Andi, This is a good question, and I should detail that more in the commit's log. Here is my response to Balbir when he asked for: On 10/07/2017 19:48, Laurent Dufour wrote: > On 07/07/2017 09:07, Balbir Singh wrote: >> On Fri, 2017-06-16 at 19:52 +0200, Laurent Dufour wrote: >>> From: Peter Zijlstra <peterz@infradead.org> >>> >>> One of the side effects of speculating on faults (without holding >>> mmap_sem) is that we can race with free_pgtables() and therefore we >>> cannot assume the page-tables will stick around. >>> >>> Remove the relyance on the pte pointer. >> ^^ reliance >> >> Looking at the changelog and the code the impact is not clear. >> It looks like after this patch we always assume the pte is not >> the same. What is the impact of this patch? > > Hi Balbir, > > In most of the case pte_unmap_same() was returning 1, which meaning that > do_swap_page() should do its processing. > > So in most of the case there will be no impact. > > Now regarding the case where pte_unmap_safe() was returning 0, and thus > do_swap_page return 0 too, this happens when the page has already been > swapped back. This may happen before do_swap_page() get called or while in > the call to do_swap_page(). In that later case, the check done when > swapin_readahead() returns will detect that case. > > The worst case would be that a page fault is occuring on 2 threads at the > same time on the same swapped out page. In that case one thread will take > much time looping in __read_swap_cache_async(). But in the regular page > fault path, this is even worse since the thread would wait for semaphore to > be released before starting anything. > > Cheers, > Laurent. > I'll add that to the commit's log. Thanks, Laurent.
diff --git a/mm/memory.c b/mm/memory.c index 8a80986fff48..259f621345b2 100644 --- a/mm/memory.c +++ b/mm/memory.c @@ -2274,6 +2274,7 @@ int apply_to_page_range(struct mm_struct *mm, unsigned long addr, } EXPORT_SYMBOL_GPL(apply_to_page_range); +#ifndef CONFIG_SPF /* * handle_pte_fault chooses page fault handler according to an entry which was * read non-atomically. Before making any commitment, on those architectures @@ -2297,6 +2298,7 @@ static inline int pte_unmap_same(struct mm_struct *mm, pmd_t *pmd, pte_unmap(page_table); return same; } +#endif /* CONFIG_SPF */ static inline void cow_user_page(struct page *dst, struct page *src, unsigned long va, struct vm_area_struct *vma) { @@ -2884,11 +2886,13 @@ int do_swap_page(struct vm_fault *vmf) swapcache = page; } +#ifndef CONFIG_SPF if (!pte_unmap_same(vma->vm_mm, vmf->pmd, vmf->pte, vmf->orig_pte)) { if (page) put_page(page); goto out; } +#endif entry = pte_to_swp_entry(vmf->orig_pte); if (unlikely(non_swap_entry(entry))) {