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 <xiyou.wangcong@gmail.com>
Suggested-by: Michal Hocko <mhocko@suse.com>
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: William Kucharski <william.kucharski@oracle.com>
Cc: Chao Yu <yuchao0@huawei.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Darrick J. Wong <darrick.wong@oracle.com>
Cc: Dave Chinner <dchinner@redhat.com>
Cc: Eric Biggers <ebiggers@google.com>
Cc: Gao Xiang <gaoxiang25@huawei.com>
Cc: Jaegeuk Kim <jaegeuk@kernel.org>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Joseph Qi <joseph.qi@linux.alibaba.com>
Cc: Junxiao Bi <junxiao.bi@oracle.com>
Cc: Zi Yan <ziy@nvidia.com>
Cc: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Cc: Miklos Szeredi <mszeredi@redhat.com>
Link: http://lkml.kernel.org/r/20200414150233.24495-16-willy@infradead.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/readahead.c b/mm/readahead.c
index 73cb59e..3c9a8dd 100644 (file)
--- 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"
 
@@ -185,6 +186,18 @@ 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.
         */
@@ -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);