Messages in this thread Patch in this message | | | From | Laurent Dufour <> | Subject | [PATCH v7 04/24] mm: Dont assume page-table invariance during faults | Date | Tue, 6 Feb 2018 17:49:50 +0100 |
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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.
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
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.
[Remove only if !CONFIG_SPECULATIVE_PAGE_FAULT] Signed-off-by: Laurent Dufour <ldufour@linux.vnet.ibm.com> --- mm/memory.c | 4 ++++ 1 file changed, 4 insertions(+)
diff --git a/mm/memory.c b/mm/memory.c index 5ec6433d6a5c..32b9eb77d95c 100644 --- a/mm/memory.c +++ b/mm/memory.c @@ -2288,6 +2288,7 @@ int apply_to_page_range(struct mm_struct *mm, unsigned long addr, } EXPORT_SYMBOL_GPL(apply_to_page_range); +#ifndef CONFIG_SPECULATIVE_PAGE_FAULT /* * handle_pte_fault chooses page fault handler according to an entry which was * read non-atomically. Before making any commitment, on those architectures @@ -2311,6 +2312,7 @@ static inline int pte_unmap_same(struct mm_struct *mm, pmd_t *pmd, pte_unmap(page_table); return same; } +#endif /* CONFIG_SPECULATIVE_PAGE_FAULT */ static inline void cow_user_page(struct page *dst, struct page *src, unsigned long va, struct vm_area_struct *vma) { @@ -2898,11 +2900,13 @@ int do_swap_page(struct vm_fault *vmf) swapcache = page; } +#ifndef CONFIG_SPECULATIVE_PAGE_FAULT 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))) { -- 2.7.4
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