Re: [PATCH 2/2] btrfs: prevent direct reclaim during compressed readahead

[Mark Harmstone <mark@harmstone.com>]

Reviewed-by: Mark Harmstone <mark@harmstone.com>

On 20/03/2026 7.34 am, JP Kobryn (Meta) wrote:
> Prevent direct reclaim during compressed readahead. This is achieved by
> passing specific GFP flags whenever the bio is marked for readahead. The
> flags are similar to GFP_NOFS but stripped of __GFP_DIRECT_RECLAIM. Also,
> __GFP_NOWARN is added since these allocations are allowed to fail. Demand
> reads still use full GFP_NOFS and will enter reclaim if needed.
> 
> btrfs_submit_compressed_read() now makes use of the new gfp_t API for
> allocations within. Since non-readahead code may call this function, the
> bio flags are inspected to determine whether direct reclaim should be
> restricted or not.
> 
> add_ra_bio_pages() gains a bool parameter which allows callers to specify
> if they want to allow direct reclaim or not. In either case, the NOWARN
> flag was added unconditionally since the allocations are speculative.
> 
> Signed-off-by: JP Kobryn (Meta) <jp.kobryn@linux.dev>
> ---
>   fs/btrfs/compression.c | 33 ++++++++++++++++++++++++++++-----
>   1 file changed, 28 insertions(+), 5 deletions(-)
> 
> diff --git a/fs/btrfs/compression.c b/fs/btrfs/compression.c
> index ae9cb5b7676c..f32cfc933bee 100644
> --- a/fs/btrfs/compression.c
> +++ b/fs/btrfs/compression.c
> @@ -372,7 +372,8 @@ struct compressed_bio *btrfs_alloc_compressed_write(struct btrfs_inode *inode,
>   static noinline int add_ra_bio_pages(struct inode *inode,
>   				     u64 compressed_end,
>   				     struct compressed_bio *cb,
> -				     int *memstall, unsigned long *pflags)
> +				     int *memstall, unsigned long *pflags,
> +				     bool direct_reclaim)
>   {
>   	struct btrfs_fs_info *fs_info = inode_to_fs_info(inode);
>   	pgoff_t end_index;
> @@ -380,6 +381,7 @@ static noinline int add_ra_bio_pages(struct inode *inode,
>   	u64 cur = cb->orig_bbio->file_offset + orig_bio->bi_iter.bi_size;
>   	u64 isize = i_size_read(inode);
>   	int ret;
> +	gfp_t constraint_gfp, cache_gfp;
>   	struct folio *folio;
>   	struct extent_map *em;
>   	struct address_space *mapping = inode->i_mapping;
> @@ -409,6 +411,14 @@ static noinline int add_ra_bio_pages(struct inode *inode,
>   
>   	end_index = (i_size_read(inode) - 1) >> PAGE_SHIFT;
>   
> +	if (!direct_reclaim) {
> +		constraint_gfp = ~(__GFP_FS | __GFP_DIRECT_RECLAIM);
> +		cache_gfp = (GFP_NOFS & ~__GFP_DIRECT_RECLAIM) | __GFP_NOWARN;
> +	} else {
> +		constraint_gfp = ~__GFP_FS;
> +		cache_gfp = GFP_NOFS | __GFP_NOWARN;
> +	}
> +
>   	while (cur < compressed_end) {
>   		pgoff_t page_end;
>   		pgoff_t pg_index = cur >> PAGE_SHIFT;
> @@ -438,12 +448,13 @@ static noinline int add_ra_bio_pages(struct inode *inode,
>   			continue;
>   		}
>   
> -		folio = filemap_alloc_folio(mapping_gfp_constraint(mapping, ~__GFP_FS),
> +		folio = filemap_alloc_folio(mapping_gfp_constraint(mapping,
> +					    constraint_gfp) | __GFP_NOWARN,
>   					    0, NULL);
>   		if (!folio)
>   			break;
>   
> -		if (filemap_add_folio(mapping, folio, pg_index, GFP_NOFS)) {
> +		if (filemap_add_folio(mapping, folio, pg_index, cache_gfp)) {
>   			/* There is already a page, skip to page end */
>   			cur += folio_size(folio);
>   			folio_put(folio);
> @@ -536,6 +547,7 @@ void btrfs_submit_compressed_read(struct btrfs_bio *bbio)
>   	unsigned int compressed_len;
>   	const u32 min_folio_size = btrfs_min_folio_size(fs_info);
>   	u64 file_offset = bbio->file_offset;
> +	gfp_t gfp;
>   	u64 em_len;
>   	u64 em_start;
>   	struct extent_map *em;
> @@ -543,6 +555,17 @@ void btrfs_submit_compressed_read(struct btrfs_bio *bbio)
>   	int memstall = 0;
>   	int ret;
>   
> +	/*
> +	 * If this is a readahead bio, prevent direct reclaim. This is done to
> +	 * avoid stalling on speculative allocations when memory pressure is
> +	 * high. The demand fault will retry with GFP_NOFS and enter direct
> +	 * reclaim if needed.
> +	 */
> +	if (bbio->bio.bi_opf & REQ_RAHEAD)
> +		gfp = (GFP_NOFS & ~__GFP_DIRECT_RECLAIM) | __GFP_NOWARN;
> +	else
> +		gfp = GFP_NOFS;
> +
>   	/* we need the actual starting offset of this extent in the file */
>   	read_lock(&em_tree->lock);
>   	em = btrfs_lookup_extent_mapping(em_tree, file_offset, fs_info->sectorsize);
> @@ -573,7 +596,7 @@ void btrfs_submit_compressed_read(struct btrfs_bio *bbio)
>   		struct folio *folio;
>   		u32 cur_len = min(compressed_len - i * min_folio_size, min_folio_size);
>   
> -		folio = btrfs_alloc_compr_folio(fs_info);
> +		folio = btrfs_alloc_compr_folio_gfp(fs_info, gfp);
>   		if (!folio) {
>   			ret = -ENOMEM;
>   			goto out_free_bio;
> @@ -589,7 +612,7 @@ void btrfs_submit_compressed_read(struct btrfs_bio *bbio)
>   	ASSERT(cb->bbio.bio.bi_iter.bi_size == compressed_len);
>   
>   	add_ra_bio_pages(&inode->vfs_inode, em_start + em_len, cb, &memstall,
> -			 &pflags);
> +			 &pflags, !(bbio->bio.bi_opf & REQ_RAHEAD));
>   
>   	cb->len = bbio->bio.bi_iter.bi_size;
>   	cb->bbio.bio.bi_iter.bi_sector = bbio->bio.bi_iter.bi_sector;