Re: [PATCH 3/5] btrfs: add manual PSI accounting for compressed reads

David Sterba
2022-09-12T15:33:56Z

On Sat, Sep 10, 2022 at 08:50:56AM +0200, Christoph Hellwig wrote:
> btrfs compressed reads try to always read the entire compressed chunk,
> even if only a subset is requested. Currently this is covered by the
> magic PSI accounting underneath submit_bio, but that is about to go
> away. Instead add manual psi_memstall_{enter,leave} annotations.
>
> Note that for readahead this really should be using readahead_expand,
> but the additionals reads are also done for plain ->read_folio where
> readahead_expand can't work, so this overall logic is left as-is for
> now.
>
> Signed-off-by: Christoph Hellwig <hch@lst.de>

With some small fixups,

Acked-by: David Sterba <dsterba@suse.com>

> fs/btrfs/compression.c | 14 ++++++++++++--
> 1 file changed, 12 insertions(+), 2 deletions(-)
>
> diff --git a/fs/btrfs/compression.c b/fs/btrfs/compression.c
> index e84d22c5c6a83..f7889a00e0055 100644
> --- a/fs/btrfs/compression.c
> +++ b/fs/btrfs/compression.c
> @@ -15,6 +15,7 @@
> #include <linux/string.h>
> #include <linux/backing-dev.h>
> #include <linux/writeback.h>
> +#include <linux/psi.h>
> #include <linux/slab.h>
> #include <linux/sched/mm.h>
> #include <linux/log2.h>
> @@ -519,7 +520,8 @@ static u64 bio_end_offset(struct bio *bio)
> */
> static noinline int add_ra_bio_pages(struct inode *inode,
> u64 compressed_end,
> - struct compressed_bio *cb)
> + struct compressed_bio *cb,
> + unsigned long *pflags)
> {
> struct btrfs_fs_info *fs_info = btrfs_sb(inode->i_sb);
> unsigned long end_index;
> @@ -588,6 +590,9 @@ static noinline int add_ra_bio_pages(struct inode *inode,
> continue;
> }
>
> + if (unlikely(PageWorkingset(page)))

Please drop the 'unlikely', in this case it does not seem to make much
sense.

> + psi_memstall_enter(pflags);
> +
> ret = set_page_extent_mapped(page);
> if (ret < 0) {
> unlock_page(page);
> @@ -674,6 +679,8 @@ void btrfs_submit_compressed_read(struct inode *inode, struct bio *bio,
> u64 em_len;
> u64 em_start;
> struct extent_map *em;
> + /* initialize to 1 to make skip psi_memstall_leave unless needed */

First letter in comment should be upper case unless it's an identifier.

> + unsigned long pflags = 1;
> blk_status_t ret;
> int ret2;
> int i;
> @@ -729,7 +736,7 @@ void btrfs_submit_compressed_read(struct inode *inode, struct bio *bio,
> goto fail;
> }
>
> - add_ra_bio_pages(inode, em_start + em_len, cb);
> + add_ra_bio_pages(inode, em_start + em_len, cb, &pflags);
>
> /* include any pages we added in add_ra-bio_pages */
> cb->len = bio->bi_iter.bi_size;
> @@ -810,6 +817,9 @@ void btrfs_submit_compressed_read(struct inode *inode, struct bio *bio,
> }
> }
>
> + if (!pflags)
> + psi_memstall_leave(&pflags);
> +
> if (refcount_dec_and_test(&cb->pending_ios))
> finish_compressed_bio_read(cb);
> return;
> --
> 2.30.2
>