linux-kernel - [PATCH 18/18] NFS: swap-out must always use STABLE writes.

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Message-ID: <163969850347.20885.5566025433915169963.stgit@noble.brown>
Date:   Fri, 17 Dec 2021 10:48:23 +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>,
        Mel Gorman <mgorman@...e.de>,
        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 18/18] 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_COND_STABLE.

Signed-off-by: NeilBrown <neilb@...e.de>
---
 fs/nfs/direct.c |    7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/fs/nfs/direct.c b/fs/nfs/direct.c
index eeff1b4e1a7c..1317465150a6 100644
--- a/fs/nfs/direct.c
+++ b/fs/nfs/direct.c
@@ -790,7 +790,7 @@ 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;
@@ -798,7 +798,7 @@ 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);
@@ -904,6 +904,7 @@ ssize_t nfs_file_direct_write(struct kiocb *iocb, struct iov_iter *iter,
 	struct nfs_direct_req *dreq;
 	struct nfs_lock_context *l_ctx;
 	loff_t pos, end;
+	int ioflags = swap ? FLUSH_COND_STABLE : FLUSH_STABLE;
 
 	dfprintk(FILE, "NFS: direct write(%pD2, %zd@%Ld)\n",
 		file, iov_iter_count(iter), (long long) iocb->ki_pos);
@@ -946,7 +947,7 @@ ssize_t nfs_file_direct_write(struct kiocb *iocb, struct iov_iter *iter,
 	if (!swap)
 		nfs_start_io_direct(inode);
 
-	requested = nfs_direct_write_schedule_iovec(dreq, iter, pos);
+	requested = nfs_direct_write_schedule_iovec(dreq, iter, pos, ioflags);
 
 	if (mapping->nrpages) {
 		invalidate_inode_pages2_range(mapping,