Re: [PATCH] erofs: fix folio reuse from a different address_space in erofs_bread()

[Gao Xiang]

On Wed, Sep 30, 2026 at 07:21:27PM +0800, Binglei Wang wrote:
> From: Binglei Wang <l3b2w1@xxxxxxxxx>
> 
> erofs_bread() reuses a cached folio on page index match without checking folio->mapping.

Each line in the commit message should not exceed 72 chars.

> 
> xattr.c calls erofs_init_metabuf() twice on the same buffer without erofs_put_metabuf();
> the two in_metabox sources differ, so buf->mapping can switch
> while buf->page still belongs to the old address_space.
> The next erofs_bread() then reads the wrong mapping, dropping shared xattrs with METABOX.
> 
> We require folio->mapping == buf->mapping in the reuse check;
> otherwise drop the cached folio and re-read via the slow path.
> 
> Fixes: 414091322c63 ("erofs: implement metadata compression")
> Cc: Bo Liu (OpenAnolis) <liubo03@xxxxxxxxxx>
> Signed-off-by: Binglei Wang <l3b2w1@xxxxxxxxx>
> ---
>  fs/erofs/data.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/erofs/data.c b/fs/erofs/data.c
> index be63b89f0862..d8b6523bc218 100644
> --- a/fs/erofs/data.c
> +++ b/fs/erofs/data.c
> @@ -33,8 +33,13 @@ void *erofs_bread(struct erofs_buf *buf, erofs_off_t offset, bool need_kmap)
>  	if (buf->page) {
>  		folio = page_folio(buf->page);
> -		if (folio_file_page(folio, index) != buf->page)
> +		if (folio->mapping != buf->mapping) {
> +			/* the cached folio belongs to another address_space */
> +			erofs_put_metabuf(buf);
> +			folio = NULL;

As I said, you could just drop `erofs_put_metabuf(buf);` here,
and folio = NULL will do the correct thing instead.

Thanks,
Gao Xiang

> +		} else if (folio_file_page(folio, index) != buf->page) {
>  			erofs_unmap_metabuf(buf);
> +		}
>  	}
>  	if (!folio || !folio_contains(folio, index)) {
>  		erofs_put_metabuf(buf);
> --
> 2.43.0
> 
>