linux-kernel - Re: [PATCH 3/9] f2fs: change retry waiting for f2fs_write_single_data

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Date:   Thu, 27 Jan 2022 17:34:31 -0800
From:   Jaegeuk Kim <jaegeuk@...nel.org>
To:     NeilBrown <neilb@...e.de>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Chao Yu <chao@...nel.org>, Jeff Layton <jlayton@...nel.org>,
        Ilya Dryomov <idryomov@...il.com>,
        Miklos Szeredi <miklos@...redi.hu>,
        Trond Myklebust <trond.myklebust@...merspace.com>,
        Anna Schumaker <anna.schumaker@...app.com>,
        Ryusuke Konishi <konishi.ryusuke@...il.com>,
        "Darrick J. Wong" <djwong@...nel.org>,
        Philipp Reisner <philipp.reisner@...bit.com>,
        Lars Ellenberg <lars.ellenberg@...bit.com>,
        Paolo Valente <paolo.valente@...aro.org>,
        Jens Axboe <axboe@...nel.dk>, linux-mm@...ck.org,
        linux-nilfs@...r.kernel.org, linux-nfs@...r.kernel.org,
        linux-fsdevel@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net, linux-ext4@...r.kernel.org,
        ceph-devel@...r.kernel.org, drbd-dev@...ts.linbit.com,
        linux-kernel@...r.kernel.org, linux-block@...r.kernel.org
Subject: Re: [PATCH 3/9] f2fs: change retry waiting for
 f2fs_write_single_data_page()

On 01/27, NeilBrown wrote:
> f2fs_write_single_data_page() can return -EAGAIN if it cannot get
> the cp_rwsem lock - it holds a page lock and so cannot wait for it.
> 
> Some code which calls f2fs_write_single_data_page() use
> congestion_wait() and then tries again.  congestion_wait() doesn't do
> anything useful as congestion is no longer tracked.  So this is just a
> simple sleep.
> 
> A better approach is it wait until the cp_rwsem lock can be taken - then
> try again.  There is certainly no point trying again *before* the lock
> can be taken.
> 
> Signed-off-by: NeilBrown <neilb@...e.de>
> ---
>  fs/f2fs/compress.c |    6 +++---
>  fs/f2fs/data.c     |    9 ++++++---
>  2 files changed, 9 insertions(+), 6 deletions(-)
> 
> diff --git a/fs/f2fs/compress.c b/fs/f2fs/compress.c
> index d0c3aeba5945..58ff7f4b296c 100644
> --- a/fs/f2fs/compress.c
> +++ b/fs/f2fs/compress.c
> @@ -1505,9 +1505,9 @@ static int f2fs_write_raw_pages(struct compress_ctx *cc,
>  				if (IS_NOQUOTA(cc->inode))
>  					return 0;
>  				ret = 0;
> -				cond_resched();
> -				congestion_wait(BLK_RW_ASYNC,
> -						DEFAULT_IO_TIMEOUT);
> +				/* Wait until we can get the lock, then try again. */
> +				f2fs_lock_op(F2FS_I_SB(cc->inode));
> +				f2fs_unlock_op(F2FS_I_SB(cc->inode));

Since checkpoint uses down_write(cp_rwsem), I'm not sure the write path is safe
and needs to wait for checkpoint. Can we just do io_schedule_timeout()?

>  				goto retry_write;
>  			}
>  			return ret;
> diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
> index 8c417864c66a..1d2341163e2c 100644
> --- a/fs/f2fs/data.c
> +++ b/fs/f2fs/data.c
> @@ -3047,9 +3047,12 @@ static int f2fs_write_cache_pages(struct address_space *mapping,
>  				} else if (ret == -EAGAIN) {
>  					ret = 0;
>  					if (wbc->sync_mode == WB_SYNC_ALL) {
> -						cond_resched();
> -						congestion_wait(BLK_RW_ASYNC,
> -							DEFAULT_IO_TIMEOUT);
> +						/* Wait until we can get the
> +						 * lock, then try again.
> +						 */
> +						f2fs_lock_op(F2FS_I_SB(mapping->host));
> +						f2fs_unlock_op(F2FS_I_SB(mapping->host));
> +
>  						goto retry_write;
>  					}
>  					goto next;
>