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Date: Sun, 8 Aug 2021 11:49:33 -0600
From: Yu Zhao <yuzhao@...gle.com>
To: Yang Shi <shy828301@...il.com>
Cc: Linux MM <linux-mm@...ck.org>,
Andrew Morton <akpm@...ux-foundation.org>,
Hugh Dickins <hughd@...gle.com>,
"Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
Matthew Wilcox <willy@...radead.org>,
Vlastimil Babka <vbabka@...e.cz>, Zi Yan <ziy@...dia.com>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
Shuang Zhai <zhais@...gle.com>
Subject: Re: [PATCH 2/3] mm: free zapped tail pages when splitting isolated thp
On Wed, Aug 4, 2021 at 6:13 PM Yang Shi <shy828301@...il.com> wrote:
>
> On Fri, Jul 30, 2021 at 11:39 PM Yu Zhao <yuzhao@...gle.com> wrote:
> >
> > If a tail page has only two references left, one inherited from the
> > isolation of its head and the other from lru_add_page_tail() which we
> > are about to drop, it means this tail page was concurrently zapped.
> > Then we can safely free it and save page reclaim or migration the
> > trouble of trying it.
> >
> > Signed-off-by: Yu Zhao <yuzhao@...gle.com>
> > Tested-by: Shuang Zhai <zhais@...gle.com>
> > ---
> > include/linux/vm_event_item.h | 1 +
> > mm/huge_memory.c | 28 ++++++++++++++++++++++++++++
> > mm/vmstat.c | 1 +
> > 3 files changed, 30 insertions(+)
> >
> > diff --git a/include/linux/vm_event_item.h b/include/linux/vm_event_item.h
> > index ae0dd1948c2b..829eeac84094 100644
> > --- a/include/linux/vm_event_item.h
> > +++ b/include/linux/vm_event_item.h
> > @@ -99,6 +99,7 @@ enum vm_event_item { PGPGIN, PGPGOUT, PSWPIN, PSWPOUT,
> > #ifdef CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD
> > THP_SPLIT_PUD,
> > #endif
> > + THP_SPLIT_FREE,
> > THP_ZERO_PAGE_ALLOC,
> > THP_ZERO_PAGE_ALLOC_FAILED,
> > THP_SWPOUT,
> > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > index d8b655856e79..5120478bca41 100644
> > --- a/mm/huge_memory.c
> > +++ b/mm/huge_memory.c
> > @@ -2432,6 +2432,8 @@ static void __split_huge_page(struct page *page, struct list_head *list,
> > struct address_space *swap_cache = NULL;
> > unsigned long offset = 0;
> > unsigned int nr = thp_nr_pages(head);
> > + LIST_HEAD(pages_to_free);
> > + int nr_pages_to_free = 0;
> > int i;
> >
> > VM_BUG_ON_PAGE(list && PageLRU(head), head);
> > @@ -2506,6 +2508,25 @@ static void __split_huge_page(struct page *page, struct list_head *list,
> > continue;
> > unlock_page(subpage);
> >
> > + /*
> > + * If a tail page has only two references left, one inherited
> > + * from the isolation of its head and the other from
> > + * lru_add_page_tail() which we are about to drop, it means this
> > + * tail page was concurrently zapped. Then we can safely free it
> > + * and save page reclaim or migration the trouble of trying it.
> > + */
> > + if (list && page_ref_freeze(subpage, 2)) {
> > + VM_BUG_ON_PAGE(PageLRU(subpage), subpage);
> > + VM_BUG_ON_PAGE(PageCompound(subpage), subpage);
> > + VM_BUG_ON_PAGE(page_mapped(subpage), subpage);
> > +
> > + ClearPageActive(subpage);
> > + ClearPageUnevictable(subpage);
> > + list_move(&subpage->lru, &pages_to_free);
> > + nr_pages_to_free++;
> > + continue;
> > + }
>
> Yes, such page could be freed instead of swapping out. But I'm
> wondering if we could have some simpler implementation. Since such
> pages will be re-added to page list, so we should be able to check
> their refcount in shrink_page_list(). If the refcount is 1, the
> refcount inc'ed by lru_add_page_tail() has been put by later
> put_page(), we know it is freed under us since the only refcount comes
> from isolation, we could just jump to "keep" (the label in
> shrink_page_list()), then such page will be freed later by
> shrink_inactive_list().
>
> For MADV_PAGEOUT, I think we could add some logic to handle such page
> after shrink_page_list(), just like what shrink_inactive_list() does.
>
> Migration already handles refcount == 1 page, so should not need any change.
>
> Is this idea feasible?
Yes, but then we would have to loop over the tail pages twice, here
and in shrink_page_list(), right?
In addition, if we try to freeze the refcount of a page in
shrink_page_list(), we couldn't be certain whether this page used to
be a tail page. So we would have to test every page. If a page wasn't
a tail page, it's unlikely for its refcount to drop unless there is a
race. But this patch isn't really intended to optimize such a race.
It's mainly for the next, i.e., we know there is a good chance to drop
tail pages (~10% on our systems). Sounds reasonable? Thanks.
> > +
> > /*
> > * Subpages may be freed if there wasn't any mapping
> > * like if add_to_swap() is running on a lru page that
> > @@ -2515,6 +2536,13 @@ static void __split_huge_page(struct page *page, struct list_head *list,
> > */
> > put_page(subpage);
> > }
> > +
> > + if (!nr_pages_to_free)
> > + return;
> > +
> > + mem_cgroup_uncharge_list(&pages_to_free);
> > + free_unref_page_list(&pages_to_free);
> > + count_vm_events(THP_SPLIT_FREE, nr_pages_to_free);
> > }
> >
> > int total_mapcount(struct page *page)
> > diff --git a/mm/vmstat.c b/mm/vmstat.c
> > index b0534e068166..f486e5d98d96 100644
> > --- a/mm/vmstat.c
> > +++ b/mm/vmstat.c
> > @@ -1300,6 +1300,7 @@ const char * const vmstat_text[] = {
> > #ifdef CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD
> > "thp_split_pud",
> > #endif
> > + "thp_split_free",
> > "thp_zero_page_alloc",
> > "thp_zero_page_alloc_failed",
> > "thp_swpout",
> > --
> > 2.32.0.554.ge1b32706d8-goog
> >
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