[PATCH v10 15/25] mm: Use memalloc_nofs_save in readahead path
willy at infradead.org
Tue Mar 24 07:22:49 AEDT 2020
From: "Matthew Wilcox (Oracle)" <willy at infradead.org>
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 at infradead.org>
Reported-by: Cong Wang <xiyou.wangcong at gmail.com>
Suggested-by: Michal Hocko <mhocko at suse.com>
Reviewed-by: William Kucharski <william.kucharski at 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
@@ -22,6 +22,7 @@
@@ -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);
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