Re: [f2fs-dev] [PATCH] f2fs: get out of a repeat loop when getting a locked data page

[Chao Yu]

On 2023/3/24 5:39, Jaegeuk Kim wrote:
> https://bugzilla.kernel.org/show_bug.cgi?id=216050
>
> Somehow we're getting a page which has a different mapping.
> Let's avoid the infinite loop.
>
> Cc: <stable@xxxxxxxxxxxxxxx>
> Signed-off-by: Jaegeuk Kim <jaegeuk@xxxxxxxxxx>
> ---
>   fs/f2fs/data.c | 8 ++------
>   1 file changed, 2 insertions(+), 6 deletions(-)
>
> diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
> index bf51e6e4eb64..80702c93e885 100644
> --- a/fs/f2fs/data.c
> +++ b/fs/f2fs/data.c
> @@ -1329,18 +1329,14 @@ struct page *f2fs_get_lock_data_page(struct inode *inode, pgoff_t index,
>   {
>   	struct address_space *mapping = inode->i_mapping;
>   	struct page *page;
> -repeat:
> +
>   	page = f2fs_get_read_data_page(inode, index, 0, for_write, NULL);
>   	if (IS_ERR(page))
>   		return page;
>   
>   	/* wait for read completion */
>   	lock_page(page);
> -	if (unlikely(page->mapping != mapping)) {

How about using such logic only for move_data_page() to limit affect for
other paths?

Jaegeuk, any thoughts about why mapping is mismatch in between page's one and
inode->i_mapping?

After several times code review, I didn't get any clue about why f2fs always
get the different mapping in a loop.

Maybe we can loop MM guys to check whether below folio_file_page() may return
page which has different mapping?

struct page *pagecache_get_page(struct address_space *mapping, pgoff_t index,
		int fgp_flags, gfp_t gfp)
{
	struct folio *folio;

	folio = __filemap_get_folio(mapping, index, fgp_flags, gfp);
	if (IS_ERR(folio))
		return NULL;
	return folio_file_page(folio, index);
}

Thanks,

> -		f2fs_put_page(page, 1);
> -		goto repeat;
> -	}
> -	if (unlikely(!PageUptodate(page))) {
> +	if (unlikely(page->mapping != mapping || !PageUptodate(page))) {
>   		f2fs_put_page(page, 1);
>   		return ERR_PTR(-EIO);
>   	}