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Message-ID: <CACT4Y+Z6kTgH7vV7+nX9G5h4hSteZxszPqvi3ThE8Gb5V-NkCQ@mail.gmail.com>
Date: Sun, 13 Nov 2022 17:44:45 +0100
From: Dmitry Vyukov <dvyukov@...gle.com>
To: Hugh Dickins <hughd@...gle.com>
Cc: David Hildenbrand <david@...hat.com>,
syzbot <syzbot+273b547b15eb58ea35e8@...kaller.appspotmail.com>,
akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
linux-mm@...ck.org, syzkaller-bugs@...glegroups.com,
Mel Gorman <mgorman@...hsingularity.net>
Subject: Re: [syzbot] WARNING in __split_huge_page_tail
On Wed, 26 Oct 2022 at 18:18, Hugh Dickins <hughd@...gle.com> wrote:
> > >> On 26.10.22 08:59, syzbot wrote:
> > >>> Hello,
> > >>>
> > >>> syzbot found the following issue on:
> > >>>
> > >>> HEAD commit: 4da34b7d175d Merge tag 'thermal-6.1-rc2' of
> > >>> git://git.kern..
> > >>> git tree: upstream
> > >>> console+strace: https://syzkaller.appspot.com/x/log.txt?x=113bd8bc880000
> > >>> kernel config: https://syzkaller.appspot.com/x/.config?x=4789759e8a6d5f57
> > >>> dashboard link:
> > >>> https://syzkaller.appspot.com/bug?extid=273b547b15eb58ea35e8
> > >>> compiler: Debian clang version
> > >>> 13.0.1-++20220126092033+75e33f71c2da-1~exp1~20220126212112.63, GNU ld (GNU
> > >>> Binutils for Debian) 2.35.2
> > >>> syz repro: https://syzkaller.appspot.com/x/repro.syz?x=161e1f62880000
> > >>> C reproducer: https://syzkaller.appspot.com/x/repro.c?x=16dd4fe6880000
> > >>>
> > >>> Downloadable assets:
> > >>> disk image:
> > >>> https://storage.googleapis.com/syzbot-assets/a61ddb36c296/disk-4da34b7d.raw.xz
> > >>> vmlinux:
> > >>> https://storage.googleapis.com/syzbot-assets/ceee41246252/vmlinux-4da34b7d.xz
> > >>>
> > >>> IMPORTANT: if you fix the issue, please add the following tag to the
> > >>> commit:
> > >>> Reported-by: syzbot+273b547b15eb58ea35e8@...kaller.appspotmail.com
> > >>>
> > >>> tlb_finish_mmu+0xcb/0x200 mm/mmu_gather.c:363
> > >>> exit_mmap+0x2b1/0x670 mm/mmap.c:3098
> > >>> __mmput+0x114/0x3b0 kernel/fork.c:1185
> > >>> exit_mm+0x217/0x2f0 kernel/exit.c:516
> > >>> do_exit+0x5e7/0x2070 kernel/exit.c:807
> > >>> do_group_exit+0x1fd/0x2b0 kernel/exit.c:950
> > >>> __do_sys_exit_group kernel/exit.c:961 [inline]
> > >>> __se_sys_exit_group kernel/exit.c:959 [inline]
> > >>> __x64_sys_exit_group+0x3b/0x40 kernel/exit.c:959
> > >>> do_syscall_x64 arch/x86/entry/common.c:50 [inline]
> > >>> do_syscall_64+0x2b/0x70 arch/x86/entry/common.c:80
> > >>> entry_SYSCALL_64_after_hwframe+0x63/0xcd
> > >>> ------------[ cut here ]------------
> > >>> WARNING: CPU: 0 PID: 3908 at mm/huge_memory.c:2465
> > >>> __split_huge_page_tail+0x81c/0x1080 mm/huge_memory.c:2465
> > >>
> > >> Is this the
> > >>
> > >> VM_BUG_ON_PAGE(atomic_read(&page_tail->_mapcount) != -1, page_tail);
> > >>
> > >> assertion?
> > >
> > > Hi David,
> > >
> > > You can check the sources for that revision, but on the dashboard
> > > there are clickable links for all source references:
> > > https://syzkaller.appspot.com/bug?extid=273b547b15eb58ea35e8
> > >
> > > In this case it points to:
> > > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/mm/huge_memory.c?id=4da34b7d175dc99b8befebd69e96546c960d526c#n2465
> > >
> >
> > Ah, thanks!
> >
> > ... so
> >
> > if (!folio_test_swapcache(page_folio(head))) {
> > VM_WARN_ON_ONCE_PAGE(page_tail->private != 0, head);
> > page_tail->private = 0;
> > }
> >
> > I recall that there was a patch either from Hugh or Mel floating around that
> > might be related.
>
> Yes, it's in akpm's mm-hotfixes-unstable branch, currently at
> 826367c8c422 ("mm: prep_compound_tail() clear page->private")
Let's tell syzbot about the fix so that it reports similar bugs in future:
#syz fix: mm: prep_compound_tail() clear page->private
> [PATCH] mm: prep_compound_tail() clear page->private
>
> Although page allocation always clears page->private in the first page
> or head page of an allocation, it has never made a point of clearing
> page->private in the tails (though 0 is often what is already there).
>
> But now commit 71e2d666ef85 ("mm/huge_memory: do not clobber swp_entry_t
> during THP split") issues a warning when page_tail->private is found to
> be non-0 (unless it's swapcache).
>
> Change that warning to dump page_tail (which also dumps head), instead
> of just the head: so far we have seen dead000000000122, dead000000000003,
> dead000000000001 or 0000000000000002 in the raw output for tail private.
>
> We could just delete the warning, but today's consensus appears to want
> page->private to be 0, unless there's a good reason for it to be set:
> so now clear it in prep_compound_tail() (more general than just for THP;
> but not for high order allocation, which makes no pass down the tails).
>
> Fixes: 71e2d666ef85 ("mm/huge_memory: do not clobber swp_entry_t during THP split")
> Signed-off-by: Hugh Dickins <hughd@...gle.com>
> Cc: Mel Gorman <mgorman@...hsingularity.net>
> Cc: Matthew Wilcox (Oracle) <willy@...radead.org>
> Cc: <stable@...r.kernel.org>
> ---
> mm/huge_memory.c | 2 +-
> mm/page_alloc.c | 1 +
> 2 files changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 03fc7e5edf07..561a42567477 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -2462,7 +2462,7 @@ static void __split_huge_page_tail(struct page *head, int tail,
> * Fix up and warn once if private is unexpectedly set.
> */
> if (!folio_test_swapcache(page_folio(head))) {
> - VM_WARN_ON_ONCE_PAGE(page_tail->private != 0, head);
> + VM_WARN_ON_ONCE_PAGE(page_tail->private != 0, page_tail);
> page_tail->private = 0;
> }
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index b5a6c815ae28..218b28ee49ed 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -807,6 +807,7 @@ static void prep_compound_tail(struct page *head, int tail_idx)
>
> p->mapping = TAIL_MAPPING;
> set_compound_head(p, head);
> + set_page_private(p, 0);
> }
>
> void prep_compound_page(struct page *page, unsigned int order)
> --
> 2.35.3
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