linux-kernel - [PATCH 21/21] NFS: swap-out must always use STABLE writes.

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Message-ID: <164420916125.29374.15563574398247024921.stgit@noble.brown>
Date:   Mon, 07 Feb 2022 15:46:01 +1100
From:   NeilBrown <neilb@...e.de>
To:     Trond Myklebust <trond.myklebust@...merspace.com>,
        Anna Schumaker <anna.schumaker@...app.com>,
        Chuck Lever <chuck.lever@...cle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mark Hemment <markhemm@...glemail.com>,
        Christoph Hellwig <hch@...radead.org>,
        David Howells <dhowells@...hat.com>
Cc:     linux-nfs@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH 21/21] NFS: swap-out must always use STABLE writes.

The commit handling code is not safe against memory-pressure deadlocks
when writing to swap.  In particular, nfs_commitdata_alloc() blocks
indefinitely waiting for memory, and this can consume all available
workqueue threads.

swap-out most likely uses STABLE writes anyway as COND_STABLE indicates
that a stable write should be used if the write fits in a single
request, and it normally does.  However if we ever swap with a small
wsize, or gather unusually large numbers of pages for a single write,
this might change.

For safety, make it explicit in the code that direct writes used for swap
must always use FLUSH_STABLE.

Signed-off-by: NeilBrown <neilb@...e.de>
---
 fs/nfs/direct.c |   10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/fs/nfs/direct.c b/fs/nfs/direct.c
index c5c53219beeb..4eb2a8380a28 100644
--- a/fs/nfs/direct.c
+++ b/fs/nfs/direct.c
@@ -791,7 +791,7 @@ static const struct nfs_pgio_completion_ops nfs_direct_write_completion_ops = {
  */
 static ssize_t nfs_direct_write_schedule_iovec(struct nfs_direct_req *dreq,
 					       struct iov_iter *iter,
-					       loff_t pos)
+					       loff_t pos, int ioflags)
 {
 	struct nfs_pageio_descriptor desc;
 	struct inode *inode = dreq->inode;
@@ -799,7 +799,7 @@ static ssize_t nfs_direct_write_schedule_iovec(struct nfs_direct_req *dreq,
 	size_t requested_bytes = 0;
 	size_t wsize = max_t(size_t, NFS_SERVER(inode)->wsize, PAGE_SIZE);
 
-	nfs_pageio_init_write(&desc, inode, FLUSH_COND_STABLE, false,
+	nfs_pageio_init_write(&desc, inode, ioflags, false,
 			      &nfs_direct_write_completion_ops);
 	desc.pg_dreq = dreq;
 	get_dreq(dreq);
@@ -945,11 +945,13 @@ ssize_t nfs_file_direct_write(struct kiocb *iocb, struct iov_iter *iter,
 	pnfs_init_ds_commit_info_ops(&dreq->ds_cinfo, inode);
 
 	if (swap) {
-		requested = nfs_direct_write_schedule_iovec(dreq, iter, pos);
+		requested = nfs_direct_write_schedule_iovec(dreq, iter, pos,
+							    FLUSH_STABLE);
 	} else {
 		nfs_start_io_direct(inode);
 
-		requested = nfs_direct_write_schedule_iovec(dreq, iter, pos);
+		requested = nfs_direct_write_schedule_iovec(dreq, iter, pos,
+							    FLUSH_COND_STABLE);
 
 		if (mapping->nrpages) {
 			invalidate_inode_pages2_range(mapping,