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SubjectRe: [PATCH v7 04/24] mm: Dont assume page-table invariance during faults
On Tue, Feb 06, 2018 at 05:49:50PM +0100, 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 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>

I have a great deal of trouble connecting all of the words above to the
contents of the patch.

>
> +#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
>

This feels to me like we want:

#ifdef CONFIG_SPECULATIVE_PAGE_FAULT
[current code]
#else
/*
* Some words here which explains why we always want to return this
* value if we support speculative page faults.
*/
static inline int pte_unmap_same(struct mm_struct *mm, pmd_t *pmd,
pte_t *page_table, pte_t orig_pte)
{
return 1;
}
#endif

instead of cluttering do_swap_page with an ifdef.

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