mm: use memalloc_nofs_save in readahead path

Ensure that memory allocations in the readahead path do not attempt to
reclaim file-backed pages, which could lead to a deadlock.  It is
possible, though unlikely this is the root cause of a problem observed
by Cong Wang.

Reported-by: Cong Wang <[email protected]>
Suggested-by: Michal Hocko <[email protected]>
Signed-off-by: Matthew Wilcox (Oracle) <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Reviewed-by: William Kucharski <[email protected]>
Cc: Chao Yu <[email protected]>
Cc: Christoph Hellwig <[email protected]>
Cc: Darrick J. Wong <[email protected]>
Cc: Dave Chinner <[email protected]>
Cc: Eric Biggers <[email protected]>
Cc: Gao Xiang <[email protected]>
Cc: Jaegeuk Kim <[email protected]>
Cc: John Hubbard <[email protected]>
Cc: Joseph Qi <[email protected]>
Cc: Junxiao Bi <[email protected]>
Cc: Zi Yan <[email protected]>
Cc: Johannes Thumshirn <[email protected]>
Cc: Miklos Szeredi <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Linus Torvalds <[email protected]>

1 files changed, 14 insertions, 0 deletions

diff --git a/mm/readahead.c b/mm/readahead.c
index 73cb59ed5cff..3c9a8dd7c56c 100644
--- a/mm/readahead.c
+++ b/mm/readahead.c
@@ -22,6 +22,7 @@
 #include <linux/mm_inline.h>
 #include <linux/blk-cgroup.h>
 #include <linux/fadvise.h>
+#include <linux/sched/mm.h>
 
 #include "internal.h"
 
@@ -186,6 +187,18 @@ void page_cache_readahead_unbounded(struct address_space *mapping,
 	unsigned long i;
 
 	/*
+	 * Partway through the readahead operation, we will have added
+	 * locked pages to the page cache, but will not yet have submitted
+	 * them for I/O.  Adding another page may need to allocate memory,
+	 * which can trigger memory reclaim.  Telling the VM we're in
+	 * the middle of a filesystem operation will cause it to not
+	 * touch file-backed pages, preventing a deadlock.  Most (all?)
+	 * filesystems already specify __GFP_NOFS in their mapping's
+	 * gfp_mask, but let's be explicit here.
+	 */
+	unsigned int nofs = memalloc_nofs_save();
+
+	/*
 	 * Preallocate as many pages as we will need.
 	 */
 	for (i = 0; i < nr_to_read; i++) {
@@ -229,6 +242,7 @@ void page_cache_readahead_unbounded(struct address_space *mapping,
 	 * will then handle the error.
 	 */
 	read_pages(&rac, &page_pool, false);
+	memalloc_nofs_restore(nofs);
 }
 EXPORT_SYMBOL_GPL(page_cache_readahead_unbounded);