linux-kernel - Re: [PATCH v2] ext4: check folio uptodate state in ext4_page

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Message-ID: <CADhLXY5k9nmFGRLLxguWB9sQ4_B6-Cxu=xHs71c5kCEyj49Vuw@mail.gmail.com>
Date: Sun, 30 Nov 2025 07:36:31 +0530
From: Deepanshu Kartikey <kartikey406@...il.com>
To: tytso@....edu, adilger.kernel@...ger.ca, djwong@...nel.org
Cc: linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org, 
	syzbot+b0a0670332b6b3230a0a@...kaller.appspotmail.com
Subject: Re: [PATCH v2] ext4: check folio uptodate state in ext4_page_mkwrite()

On Sat, Nov 22, 2025 at 7:27 AM Deepanshu Kartikey
<kartikey406@...il.com> wrote:
>
> When delayed block allocation fails due to filesystem corruption,
> ext4's writeback error handling invalidates affected folios by calling
> mpage_release_unused_pages() with invalidate=true, which explicitly
> clears the uptodate flag:
>
>     static void mpage_release_unused_pages(..., bool invalidate)
>     {
>         ...
>         if (invalidate) {
>             block_invalidate_folio(folio, 0, folio_size(folio));
>             folio_clear_uptodate(folio);
>         }
>     }
>
> If ext4_page_mkwrite() is subsequently called on such a non-uptodate
> folio, it can proceed to mark the folio dirty without checking its
> state. This triggers a warning in __folio_mark_dirty():
>
>     WARNING: CPU: 0 PID: 5 at mm/page-writeback.c:2960
>     __folio_mark_dirty+0x578/0x880
>
>     Call Trace:
>      fault_dirty_shared_page+0x16e/0x2d0
>      do_wp_page+0x38b/0xd20
>      handle_pte_fault+0x1da/0x450
>      __handle_mm_fault+0x652/0x13b0
>      handle_mm_fault+0x22a/0x6f0
>      do_user_addr_fault+0x200/0x8a0
>      exc_page_fault+0x81/0x1b0
>
> This scenario occurs when:
> 1. A write with delayed allocation marks a folio dirty (uptodate=1)
> 2. Writeback attempts block allocation but detects filesystem corruption
> 3. Error handling calls mpage_release_unused_pages(invalidate=true),
>    which clears the uptodate flag via folio_clear_uptodate()
> 4. A subsequent ftruncate() triggers ext4_truncate()
> 5. ext4_block_truncate_page() attempts to zero the page tail
> 6. This triggers a write fault on the mmap'd page
> 7. ext4_page_mkwrite() is called with the non-uptodate folio
> 8. Without checking uptodate, it proceeds to mark the folio dirty
> 9. __folio_mark_dirty() triggers: WARN_ON_ONCE(!folio_test_uptodate())
>
> Fix this by checking folio_test_uptodate() early in ext4_page_mkwrite()
> and returning VM_FAULT_SIGBUS if the folio is not uptodate. This prevents
> attempting to write to invalidated folios and properly signals the error
> to userspace.
>
> The check is placed early, before the delalloc/journal/normal code paths,
> as none of these paths should proceed with a non-uptodate folio.
>
> Reported-by: syzbot+b0a0670332b6b3230a0a@...kaller.appspotmail.com
> Tested-by: syzbot+b0a0670332b6b3230a0a@...kaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=b0a0670332b6b3230a0a
> Signed-off-by: Deepanshu Kartikey <kartikey406@...il.com>
> ---
>  fs/ext4/inode.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
>
> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> index e99306a8f47c..18a029362c1f 100644
> --- a/fs/ext4/inode.c
> +++ b/fs/ext4/inode.c
> @@ -6688,6 +6688,14 @@ vm_fault_t ext4_page_mkwrite(struct vm_fault *vmf)
>         if (err)
>                 goto out_ret;
>
> +       folio_lock(folio);
> +       if (!folio_test_uptodate(folio)) {
> +               folio_unlock(folio);
> +               ret = VM_FAULT_SIGBUS;
> +               goto out;
> +       }
> +       folio_unlock(folio);
> +
>         /*
>          * On data journalling we skip straight to the transaction handle:
>          * there's no delalloc; page truncated will be checked later; the
> --
> 2.43.0
>

Hi Ted and ext4 maintainers,

I wanted to follow up on this patch submitted a week ago. This fixes
a syzbot-reported WARNING in __folio_mark_dirty() that occurs when
ext4_page_mkwrite() is called with a non-uptodate folio after delayed
allocation writeback failure.

Please let me know if there's any feedback or if I should make any
changes.

Thanks,
Deepanshu