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

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This comment feels out of date ...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which part is out of date?

		/*
		 * 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.

quoted

-		noio_flag = memalloc_noio_save();
+		nofs_flag = memalloc_nofs_save();
 		do {
 			bp->b_addr = vm_map_ram(bp->b_pages, bp->b_page_count,
 						-1, PAGE_KERNEL);

Also, I think it shows that this is the wrong place in XFS to be calling
memalloc_nofs_save().  I'm not arguing against including this patch;
it's a step towards where we want to be.  I also don't know XFS well
enough to know where to set that flag ;-)  Presumably when we start a
transaction ... ?

Yes that is what I would like to achieve longterm. And the reason why I
didn't want to mimic this pattern in kvmalloc as some have suggested.
It just takes much more time to get there from the past experience and
we should really start somewhere.
-- 
Michal Hocko
SUSE Labs

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