From: "David Hildenbrand (Arm)" <david@kernel.org>
To: Lance Yang <lance.yang@linux.dev>,
ljs@kernel.org, akpm@linux-foundation.org
Cc: harry@kernel.org, jannh@google.com, liam@infradead.org,
linux-ext4@vger.kernel.org, linux-kernel@vger.kernel.org,
linux-mm@kvack.org, riel@surriel.com,
syzkaller-bugs@googlegroups.com, vbabka@kernel.org,
syzbot+c181d3198e98f8aef8b9@syzkaller.appspotmail.com
Subject: Re: [syzbot] [mm?] [ext4?] WARNING in __folio_set_anon
Date: Thu, 24 Sep 2026 09:43:22 +0200 [thread overview]
Message-ID: <0a0a07f8-8ac9-4d75-b4c6-c403865f0be9@kernel.org> (raw)
In-Reply-To: <20260924065458.49698-1-lance.yang@linux.dev>
On 9/24/26 08:54, Lance Yang wrote:
>
> On Wed, Sep 23, 2026 at 10:00:37PM -0700, syzbot wrote:
>> Hello,
>>
>> syzbot found the following issue on:
>>
>> HEAD commit: 38872197cae2 Merge branch 'for-next/fixes' into for-kernelci
>> git tree: git://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux.git for-kernelci
>> console output: https://syzkaller.appspot.com/x/log.txt?x=17a44d25580000
>> kernel config: https://syzkaller.appspot.com/x/.config?x=56ed23170c168d4c
>> dashboard link: https://syzkaller.appspot.com/bug?extid=c181d3198e98f8aef8b9
>> compiler: Debian clang version 22.1.8 (++20260613092233+e80beda6e255-1~exp1~20260613092250.77), Debian LLD 22.1.8
>> userspace arch: arm64
>> syz repro: https://syzkaller.appspot.com/x/repro.syz?x=16a36515580000
>> C reproducer: https://syzkaller.appspot.com/x/repro.c?x=16dcf4c9580000
>
> Looking at the repro, emm ... the repro maps an O_RDONLY /dev/zero fd with
Does this trigger upstream or only after Lorenzo's rework (not upstream yet IIRC)
46827ac1ab221 mm/vma: make MAP_PRIVATE-mapped /dev/zero mappings truly anonymous
db7438ea23180 mm/vma: only permit MAP_PRIVATE /dev/zero to be mapped anonymous
54e8e096ea86b mm: implement file_is_dev_zero() to uniquely identify /dev/zero
fb24843cfd9eb mm: move drivers/char/mem.c to mm/char-mem.c
I assume it triggers upstream. Does it also trigger with lorenzo's changes?
> MAP_SHARED | PROT_READ. do_mmap() clears VM_SHARED and VM_MAYWRITE, so
Clearing VM_MAYWRITE for a private mapping is odd. Can you point me at the code
that clears both things?
I assume we still have the file pointer, and as the file is read-only we remove
VM_MAYWRITE. But why are we removing MAP_SHARED? (where?)
> mmap_zero_prepare() marks the VMA anonymous. MADV_COLLAPSE then allocates
> a new folio and adds an anon rmap, hitting the warning in
> linear_anon_page_index().
>
> Can an anonymous VMA without VM_MAYWRITE legitimately have an anon rmap?
It's not a COW mapping (by definition) so we don't expect anonymous folios in there.
--
Cheers,
David
next prev parent reply other threads:[~2026-09-24 7:43 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-09-24 5:00 syzbot
2026-09-24 6:54 ` Lance Yang
2026-09-24 7:43 ` David Hildenbrand (Arm) [this message]
2026-09-24 7:50 ` David Hildenbrand (Arm)
2026-09-24 8:52 ` Lorenzo Stoakes (ARM)
2026-09-24 9:27 ` Lance Yang
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