[PATCH v10 15/25] mm: Use memalloc_nofs_save in readahead path

Matthew Wilcox
2020-03-23T20:24:41Z

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.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reported-by: Cong Wang <xiyou.wangcong@gmail.com>
Suggested-by: Michal Hocko <mhocko@suse.com>
Reviewed-by: William Kucharski <william.kucharski@oracle.com>
---
mm/readahead.c | 14 ++++++++++++++
1 file changed, 14 insertions(+)

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"

@@ -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);

--
2.25.1