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Subject: [PATCH 2/3] nfs: remove reliance on bdi congestion
From:   NeilBrown <neilb@suse.de>
To:     Andrew Morton <akpm@linux-foundation.org>,
        Jeff Layton <jlayton@kernel.org>,
        Ilya Dryomov <idryomov@gmail.com>,
        Miklos Szeredi <miklos@szeredi.hu>,
        Trond Myklebust <trond.myklebust@hammerspace.com>,
        Anna Schumaker <anna.schumaker@netapp.com>
Cc:     linux-mm@kvack.org, linux-nfs@vger.kernel.org,
        linux-fsdevel@vger.kernel.org, ceph-devel@vger.kernel.org,
        linux-kernel@vger.kernel.org
Date:   Mon, 31 Jan 2022 15:03:53 +1100
Message-ID: <164360183350.4233.691070075155620959.stgit@noble.brown>
In-Reply-To: <164360127045.4233.2606812444285122570.stgit@noble.brown>
References: <164360127045.4233.2606812444285122570.stgit@noble.brown>
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The bdi congestion tracking in not widely used and will be removed.

NFS is one of a small number of filesystems that uses it, setting just
the async (write) congestion flag at what it determines are appropriate
times.

The only remaining effect of the async flag is to cause (some)
WB_SYNC_NONE writes to be skipped.

So instead of setting the flag, set an internal flag and change:
 - .writepages to do nothing if WB_SYNC_NONE and the flag is set
 - .writepage to return AOP_WRITEPAGE_ACTIVATE if WB_SYNC_NONE
    and the flag is set.

The writepages change causes a behavioural change in that pageout() can
now return PAGE_ACTIVATE instead of PAGE_KEEP, so SetPageActive() will
be called on the page which (I think) wil further delay the next attempt
at writeout.  This might be a good thing.

Signed-off-by: NeilBrown <neilb@suse.de>
---
 fs/nfs/write.c            |   12 ++++++++++--
 include/linux/nfs_fs_sb.h |    1 +
 2 files changed, 11 insertions(+), 2 deletions(-)

diff --git a/fs/nfs/write.c b/fs/nfs/write.c
index 987a187bd39a..b7c6721dd36d 100644
--- a/fs/nfs/write.c
+++ b/fs/nfs/write.c
@@ -417,7 +417,7 @@ static void nfs_set_page_writeback(struct page *page)
 
 	if (atomic_long_inc_return(&nfss->writeback) >
 			NFS_CONGESTION_ON_THRESH)
-		set_bdi_congested(inode_to_bdi(inode), BLK_RW_ASYNC);
+		nfss->write_congested = 1;
 }
 
 static void nfs_end_page_writeback(struct nfs_page *req)
@@ -433,7 +433,7 @@ static void nfs_end_page_writeback(struct nfs_page *req)
 
 	end_page_writeback(req->wb_page);
 	if (atomic_long_dec_return(&nfss->writeback) < NFS_CONGESTION_OFF_THRESH)
-		clear_bdi_congested(inode_to_bdi(inode), BLK_RW_ASYNC);
+		nfss->write_congested = 0;
 }
 
 /*
@@ -672,6 +672,10 @@ static int nfs_writepage_locked(struct page *page,
 	struct inode *inode = page_file_mapping(page)->host;
 	int err;
 
+	if (wbc->sync_mode == WB_SYNC_NONE &&
+	    NFS_SERVER(inode)->write_congested)
+		return AOP_WRITEPAGE_ACTIVATE;
+
 	nfs_inc_stats(inode, NFSIOS_VFSWRITEPAGE);
 	nfs_pageio_init_write(&pgio, inode, 0,
 				false, &nfs_async_write_completion_ops);
@@ -719,6 +723,10 @@ int nfs_writepages(struct address_space *mapping, struct writeback_control *wbc)
 	int priority = 0;
 	int err;
 
+	if (wbc->sync_mode == WB_SYNC_NONE &&
+	    NFS_SERVER(inode)->write_congested)
+		return 0;
+
 	nfs_inc_stats(inode, NFSIOS_VFSWRITEPAGES);
 
 	if (!(mntflags & NFS_MOUNT_WRITE_EAGER) || wbc->for_kupdate ||
diff --git a/include/linux/nfs_fs_sb.h b/include/linux/nfs_fs_sb.h
index ca0959e51e81..6aa2a200676a 100644
--- a/include/linux/nfs_fs_sb.h
+++ b/include/linux/nfs_fs_sb.h
@@ -138,6 +138,7 @@ struct nfs_server {
 	struct nlm_host		*nlm_host;	/* NLM client handle */
 	struct nfs_iostats __percpu *io_stats;	/* I/O statistics */
 	atomic_long_t		writeback;	/* number of writeback pages */
+	unsigned int		write_congested;/* flag set when writeback gets too high */
 	unsigned int		flags;		/* various flags */
 
 /* The following are for internal use only. Also see uapi/linux/nfs_mount.h */