On Mon 06-02-17 10:32:37, Darrick J. Wong wrote:

On Mon, Feb 06, 2017 at 06:44:15PM +0100, Michal Hocko wrote:

quoted

On Mon 06-02-17 07:39:23, Matthew Wilcox wrote:

quoted

On Mon, Feb 06, 2017 at 03:07:16PM +0100, Michal Hocko wrote:

quoted

+++ b/fs/xfs/xfs_buf.c

@@ -442,17 +442,17 @@ _xfs_buf_map_pages(
 		bp->b_addr = NULL;
 	} else {
 		int retried = 0;
-		unsigned noio_flag;
+		unsigned nofs_flag;
 
 		/*
 		 * vm_map_ram() will allocate auxillary structures (e.g.
 		 * pagetables) with GFP_KERNEL, yet we are likely to be under
 		 * GFP_NOFS context here. Hence we need to tell memory reclaim
-		 * that we are in such a context via PF_MEMALLOC_NOIO to prevent
+		 * that we are in such a context via PF_MEMALLOC_NOFS to prevent
 		 * memory reclaim re-entering the filesystem here and
 		 * potentially deadlocking.
 		 */

This comment feels out of date ... how about:

which part is out of date?

quoted

		/*
		 * vm_map_ram will allocate auxiliary structures (eg page
		 * tables) with GFP_KERNEL.  If that tries to reclaim memory
		 * by calling back into this filesystem, we may deadlock.
		 * Prevent that by setting the NOFS flag.
		 */

dunno, the previous wording seems clear enough to me. Maybe little bit
more chatty than yours but I am not sure this is worth changing.

I prefer to keep the "...yet we are likely to be under GFP_NOFS..."
wording of the old comment because it captures the uncertainty of
whether or not we actually are already under NOFS.  If someone actually
has audited this code well enough to know for sure then yes let's change
the comment, but I haven't gone that far.

I believe we can drop the memalloc_nofs_save then as well because either
we are called from a potentially dangerous context and thus we are in
the nofs scope we we do not need the protection at all.
-- 
Michal Hocko
SUSE Labs

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