Re: [PATCH v2 29/29] ext4: Use a folio in ext4_read_merkle_tree_page

[Eric Biggers <ebiggers@kernel.org>]

Hi Matthew,

On Fri, Mar 24, 2023 at 06:01:29PM +0000, Matthew Wilcox (Oracle) wrote:
> This is an implementation of fsverity_operations read_merkle_tree_page,
> so it must still return the precise page asked for, but we can use the
> folio API to reduce the number of conversions between folios & pages.
> 
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> ---
>  fs/ext4/verity.c | 14 +++++++-------
>  1 file changed, 7 insertions(+), 7 deletions(-)
> 
> diff --git a/fs/ext4/verity.c b/fs/ext4/verity.c
> index afe847c967a4..3b01247066dd 100644
> --- a/fs/ext4/verity.c
> +++ b/fs/ext4/verity.c
> @@ -361,21 +361,21 @@ static struct page *ext4_read_merkle_tree_page(struct inode *inode,
>  					       pgoff_t index,
>  					       unsigned long num_ra_pages)
>  {
> -	struct page *page;
> +	struct folio *folio;
>  
>  	index += ext4_verity_metadata_pos(inode) >> PAGE_SHIFT;
>  
> -	page = find_get_page_flags(inode->i_mapping, index, FGP_ACCESSED);
> -	if (!page || !PageUptodate(page)) {
> +	folio = __filemap_get_folio(inode->i_mapping, index, FGP_ACCESSED, 0);
> +	if (!folio || !folio_test_uptodate(folio)) {
>  		DEFINE_READAHEAD(ractl, NULL, NULL, inode->i_mapping, index);
>  
> -		if (page)
> -			put_page(page);
> +		if (folio)
> +			folio_put(folio);
>  		else if (num_ra_pages > 1)
>  			page_cache_ra_unbounded(&ractl, num_ra_pages, 0);
> -		page = read_mapping_page(inode->i_mapping, index, NULL);
> +		folio = read_mapping_folio(inode->i_mapping, index, NULL);
>  	}
> -	return page;
> +	return folio_file_page(folio, index);

This is not working at all, since it dereferences ERR_PTR(-ENOENT).  I think it
needs:

diff --git a/fs/ext4/verity.c b/fs/ext4/verity.c
index 3b01247066dd..dbc655a6c443 100644
--- a/fs/ext4/verity.c
+++ b/fs/ext4/verity.c
@@ -366,15 +366,17 @@ static struct page *ext4_read_merkle_tree_page(struct inode *inode,
 	index += ext4_verity_metadata_pos(inode) >> PAGE_SHIFT;
 
 	folio = __filemap_get_folio(inode->i_mapping, index, FGP_ACCESSED, 0);
-	if (!folio || !folio_test_uptodate(folio)) {
+	if (folio == ERR_PTR(-ENOENT) || !folio_test_uptodate(folio)) {
 		DEFINE_READAHEAD(ractl, NULL, NULL, inode->i_mapping, index);
 
-		if (folio)
+		if (!IS_ERR(folio))
 			folio_put(folio);
 		else if (num_ra_pages > 1)
 			page_cache_ra_unbounded(&ractl, num_ra_pages, 0);
 		folio = read_mapping_folio(inode->i_mapping, index, NULL);
 	}
+	if (IS_ERR(folio))
+		return ERR_CAST(folio);
 	return folio_file_page(folio, index);
 }