Re: [BUG] vfio device assignment regression with THP ref counting redesign

Alex Williamson
2016-04-29T22:34:33Z

On Fri, 29 Apr 2016 18:34:44 +0200
Andrea Arcangeli <aarcange@redhat.com> wrote:

> On Fri, Apr 29, 2016 at 10:06:11AM +0300, Kirill A. Shutemov wrote:
> > Hm. I just woke up and haven't got any coffee yet, but I don't why my
> > approach would be worse for performance. Both have the same algorithmic
> > complexity.
>
> Even before looking at the overall performance, I'm not sure your
> patch is really fixing it all: you didn't touch reuse_swap_page which
> is used by do_wp_page to know if it can call do_wp_page_reuse. Your
> patch would still trigger a COW instead of calling do_wp_page_reuse,
> but it would only happen if the page was pinned after the pmd split,
> which is probably not what the testcase is triggering. My patch
> instead fixed that too.
>
> total_mapcount returns the wrong value for reuse_swap_page, which is
> probably why you didn't try to use it there.
>
> The main issue of my patch is that it has a performance downside that
> is page_mapcount becomes expensive for all other usages, which is
> better than breaking vfio but I couldn't use total_mapcount again
> because it counts things wrong in reuse_swap_page.
>
> Like I said there's room for optimizations so today I tried to
> optimize more stuff...

I've had this under test for several hours without error. Thanks!

Alex

> From 74f1fd7fab71a2cce0d1796fb38241acde2c1224 Mon Sep 17 00:00:00 2001
> From: Andrea Arcangeli <aarcange@redhat.com>
> Date: Fri, 29 Apr 2016 01:05:06 +0200
> Subject: [PATCH 1/1] mm: thp: calculate the mapcount correctly for THP pages
> during WP faults
>
> This will provide fully accuracy to the mapcount calculation in the
> write protect faults, so page pinning will not get broken by false
> positive copy-on-writes.
>
> total_mapcount() isn't the right calculation needed in
> reuse_swap_page, so this introduces a page_trans_huge_mapcount() that
> is effectively the full accurate return value for page_mapcount() if
> dealing with Transparent Hugepages, however we only use the
> page_trans_huge_mapcount() during COW faults where it strictly needed,
> due to its higher runtime cost.
>
> Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
> ---
> include/linux/mm.h | 5 +++++
> include/linux/swap.h | 3 +--
> mm/huge_memory.c | 44 ++++++++++++++++++++++++++++++++++++--------
> mm/swapfile.c | 5 +----
> 4 files changed, 43 insertions(+), 14 deletions(-)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 8fb3604..c2026a1 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -501,11 +501,16 @@ static inline int page_mapcount(struct page *page)
>
> #ifdef CONFIG_TRANSPARENT_HUGEPAGE
> int total_mapcount(struct page *page);
> +int page_trans_huge_mapcount(struct page *page);
> #else
> static inline int total_mapcount(struct page *page)
> {
> return page_mapcount(page);
> }
> +static inline int page_trans_huge_mapcount(struct page *page)
> +{
> + return page_mapcount(page);
> +}
> #endif
>
> static inline struct page *virt_to_head_page(const void *x)
> diff --git a/include/linux/swap.h b/include/linux/swap.h
> index 2f6478f..905bf8e 100644
> --- a/include/linux/swap.h
> +++ b/include/linux/swap.h
> @@ -517,8 +517,7 @@ static inline int swp_swapcount(swp_entry_t entry)
> return 0;
> }
>
> -#define reuse_swap_page(page) \
> - (!PageTransCompound(page) && page_mapcount(page) == 1)
> +#define reuse_swap_page(page) (page_trans_huge_mapcount(page) == 1)
>
> static inline int try_to_free_swap(struct page *page)
> {
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 06bce0f..6a6d9c0 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -1298,15 +1298,9 @@ int do_huge_pmd_wp_page(struct mm_struct *mm, struct vm_area_struct *vma,
> VM_BUG_ON_PAGE(!PageCompound(page) || !PageHead(page), page);
> /*
> * We can only reuse the page if nobody else maps the huge page or it's
> - * part. We can do it by checking page_mapcount() on each sub-page, but
> - * it's expensive.
> - * The cheaper way is to check page_count() to be equal 1: every
> - * mapcount takes page reference reference, so this way we can
> - * guarantee, that the PMD is the only mapping.
> - * This can give false negative if somebody pinned the page, but that's
> - * fine.
> + * part.
> */
> - if (page_mapcount(page) == 1 && page_count(page) == 1) {
> + if (page_trans_huge_mapcount(page) == 1) {
> pmd_t entry;
> entry = pmd_mkyoung(orig_pmd);
> entry = maybe_pmd_mkwrite(pmd_mkdirty(entry), vma);
> @@ -3226,6 +3220,40 @@ int total_mapcount(struct page *page)
> }
>
> /*
> + * This calculates accurately how many mappings a transparent hugepage
> + * has (unlike page_mapcount() which isn't fully accurate). This full
> + * accuracy is primarily needed to know if copy-on-write faults can
> + * takeover the page and change the mapping to read-write instead of
> + * copying them. This is different from total_mapcount() too: we must
> + * not count all mappings on the subpages individually, but instead we
> + * must check the highest mapcount any one of the subpages has.
> + *
> + * It would be entirely safe and even more correct to replace
> + * page_mapcount() with page_trans_huge_mapcount(), however we only
> + * use page_trans_huge_mapcount() in the copy-on-write faults where we
> + * need full accuracy to avoid breaking page pinning.
> + */
> +int page_trans_huge_mapcount(struct page *page)
> +{
> + int i, ret;
> +
> + VM_BUG_ON_PAGE(PageTail(page), page);
> +
> + if (likely(!PageCompound(page)))
> + return atomic_read(&page->_mapcount) + 1;
> +
> + ret = 0;
> + if (likely(!PageHuge(page))) {
> + for (i = 0; i < HPAGE_PMD_NR; i++)
> + ret = max(ret, atomic_read(&page[i]._mapcount) + 1);
> + if (PageDoubleMap(page))
> + ret -= 1;
> + }
> + ret += compound_mapcount(page);
> + return ret;
> +}
> +
> +/*
> * This function splits huge page into normal pages. @page can point to any
> * subpage of huge page to split. Split doesn't change the position of @page.
> *
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 83874ec..984470a 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -930,10 +930,7 @@ int reuse_swap_page(struct page *page)
> VM_BUG_ON_PAGE(!PageLocked(page), page);
> if (unlikely(PageKsm(page)))
> return 0;
> - /* The page is part of THP and cannot be reused */
> - if (PageTransCompound(page))
> - return 0;
> - count = page_mapcount(page);
> + count = page_trans_huge_mapcount(page);
> if (count <= 1 && PageSwapCache(page)) {
> count += page_swapcount(page);
> if (count == 1 && !PageWriteback(page)) {