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

[David Sterba <dsterba@suse.cz>]

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
>