Re: [PATCH] iomap: Handle memory allocation failure in readahead

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Tue, Mar 31, 2020 at 08:04:21PM -0700, Matthew Wilcox wrote:
> From: "Matthew Wilcox (Oracle)" <willy@infradead.org>
> 
> bio_alloc() can fail when we use GFP_NORETRY.  If it does, allocate
> a bio large enough for a single page like mpage_readpages() does.

Why does mpage_readpages() do that?

Is this a means to guarantee some kind of forward (readahead?) progress?
Forgive my ignorance, but if memory is so tight we can't allocate a bio
for readahead then why not exit having accomplished nothing?

--D

> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> ---
>  fs/iomap/buffered-io.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> index 417115bfaf6b..c258801f18d4 100644
> --- a/fs/iomap/buffered-io.c
> +++ b/fs/iomap/buffered-io.c
> @@ -302,6 +302,7 @@ iomap_readpage_actor(struct inode *inode, loff_t pos, loff_t length, void *data,
>  
>  	if (!ctx->bio || !is_contig || bio_full(ctx->bio, plen)) {
>  		gfp_t gfp = mapping_gfp_constraint(page->mapping, GFP_KERNEL);
> +		gfp_t orig_gfp = gfp;
>  		int nr_vecs = (length + PAGE_SIZE - 1) >> PAGE_SHIFT;
>  
>  		if (ctx->bio)
> @@ -310,6 +311,8 @@ iomap_readpage_actor(struct inode *inode, loff_t pos, loff_t length, void *data,
>  		if (ctx->is_readahead) /* same as readahead_gfp_mask */
>  			gfp |= __GFP_NORETRY | __GFP_NOWARN;
>  		ctx->bio = bio_alloc(gfp, min(BIO_MAX_PAGES, nr_vecs));
> +		if (!ctx->bio)
> +			ctx->bio = bio_alloc(orig_gfp, 1);
>  		ctx->bio->bi_opf = REQ_OP_READ;
>  		if (ctx->is_readahead)
>  			ctx->bio->bi_opf |= REQ_RAHEAD;
> -- 
> 2.25.1
>