Re: [PATCH v6 19/19] mm: Use memalloc_nofs_save in readahead path

On Wed, Feb 19, 2020 at 02:43:24PM +1100, Dave Chinner wrote:

On Mon, Feb 17, 2020 at 10:46:13AM -0800, Matthew Wilcox wrote:

quoted

From: "Matthew Wilcox (Oracle)" <willy@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@infradead.org>
Reported-by: Cong Wang <redacted>
Suggested-by: Michal Hocko <mhocko@suse.com>
---
 mm/readahead.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/mm/readahead.c b/mm/readahead.c
index 94d499cfb657..8f9c0dba24e7 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"
 

@@ -174,6 +175,18 @@ void page_cache_readahead_limit(struct address_space *mapping,
 		._nr_pages = 0,
 	};
 
+	/*
+	 * 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();
+

So doesn't this largely remove the need for all the gfp flag futzing
in the readahead path? i.e. almost all readahead allocations are now
going to be GFP_NOFS | GFP_NORETRY | GFP_NOWARN ?

I don't think it ensures the GFP_NORETRY | GFP_NOWARN, just the GFP_NOFS
part.  IOW, we'll still need a readahead_gfp() macro at some point ... I
don't want to add that to this already large series though.

Michal also wants to kill mapping->gfp_mask, btw.