Re: [PATCH v2] f2fs: remove mappedtodisk flag usage in mmap write path

[Chao Yu <chao@kernel.org>]

On 10/11/26 12:02, Nanzhe Zhao wrote:
> PG_mappedtodisk was inherited from the nobh/buffer-head era.  However,
> f2fs has no buffer heads and the flag is no longer meaningful.  The only
> user of the flag is the "no holes" shortcut in f2fs_vm_page_mkwrite(),
> which makes mmap write faults skip marking the folio dirty.  Chao Yu
> pointed out that this is not only a cleanup: it causes the following
> race condition with filesystem freezing:
> 
>   CPU0 (write fault)     CPU1 (freeze_super)
>   ----                   ----
>   f2fs_vm_page_mkwrite()
>   sb_start_pagefault()
>   /* shortcut: no dirty */
>   sb_end_pagefault()
> 
>                          sb_wait_write(SB_FREEZE_PAGEFAULT)
>                          /* no waiter, passes */
>                          sync_filesystem()
>                          /* folio not dirty, skipped */
> 
>   finish_fault()
>   /* PTE writable */
>   fault_dirty_shared_page()
>   folio_mark_dirty()
>   /* dirty folio on a frozen fs */
> 
> It will also skip APP_MAPPED_IO iostat/REQ_TIME updates and post-EOF
> folio zeroing.
> 
> Remove this shortcut in f2fs_vm_page_mkwrite() and stop setting it in
> f2fs_read_single_page().
> 
> Reported-by: Chao Yu <chao@kernel.org>
> Link: https://lore.kernel.org/linux-f2fs-devel/496b913c-eb81-4fa8-be72-c6a06a10aba2@kernel.org/
> Fixes: fbfa2cc58d53 ("f2fs: add file operations")
> Cc: stable@vger.kernel.org
> Signed-off-by: Nanzhe Zhao <zhaonanzhe@xiaomi.com>

Reviewed-by: Chao Yu <chao@kernel.org>

Thanks,