Re: [PATCH 03/13] MM: reclaim mustn't enter FS for swap-over-NFS

["NeilBrown" <neilb@suse.de>]

On Tue, 16 Nov 2021, Christoph Hellwig wrote:
> On Tue, Nov 16, 2021 at 01:44:04PM +1100, NeilBrown wrote:
> > +		/* ->flags can be updated non-atomicially (scan_swap_map_slots),
> > +		 * but that will never affect SWP_FS_OPS, so the data_race
> > +		 * is safe.
> > +		 */
> >  		may_enter_fs = (sc->gfp_mask & __GFP_FS) ||
> > +			(PageSwapCache(page) &&
> > +			 !data_race(page_swap_info(page)->flags & SWP_FS_OPS) &&
> > +			 (sc->gfp_mask & __GFP_IO));
> 
> You might want to move the comment and SWP_FS_OPS into a little
> inline helper.  That makes it a lot more readable and also avoids the
> overly long line in the second hunk.

Yes, that's a good idea.  Something like this....

Thanks,
NeilBrown

From a85d09cc3d671c45e32d782454afeeaaaece96c7 Mon Sep 17 00:00:00 2001
From: NeilBrown <neilb@suse.de>
Date: Fri, 29 Oct 2021 13:35:56 +1100
Subject: [PATCH] MM: reclaim mustn't enter FS for swap-over-NFS

If swap-out is using filesystem operations (SWP_FS_OPS), then it is not
safe to enter the FS for reclaim.
So only down-grade the requirement for swap pages to __GFP_IO after
checking that SWP_FS_OPS are not being used.

Signed-off-by: NeilBrown <neilb@suse.de>
---
 mm/vmscan.c | 21 ++++++++++++++++++---
 1 file changed, 18 insertions(+), 3 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index fb9584641ac7..e672fcc14bac 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1464,6 +1464,21 @@ static unsigned int demote_page_list(struct list_head *demote_pages,
 	return nr_succeeded;
 }
 
+static bool test_may_enter_fs(struct page *page, gfp_t gfp_mask)
+{
+	if (gfp_mask & __GFP_FS)
+		return true;
+	if (!PageSwapCache(page) || !(gfp_mask & __GFP_IO))
+		return false;
+	/* We can "enter_fs" for swap-cache with only __GFP_IO
+	 * providing this isn't SWP_FS_OPS.
+	 * ->flags can be updated non-atomicially (scan_swap_map_slots),
+	 * but that will never affect SWP_FS_OPS, so the data_race
+	 * is safe.
+	 */
+	return !data_race(page_swap_info(page)->flags & SWP_FS_OPS);
+}
+
 /*
  * shrink_page_list() returns the number of reclaimed pages
  */
@@ -1513,8 +1528,7 @@ static unsigned int shrink_page_list(struct list_head *page_list,
 		if (!sc->may_unmap && page_mapped(page))
 			goto keep_locked;
 
-		may_enter_fs = (sc->gfp_mask & __GFP_FS) ||
-			(PageSwapCache(page) && (sc->gfp_mask & __GFP_IO));
+		may_enter_fs = test_may_enter_fs(page, sc->gfp_mask);
 
 		/*
 		 * The number of dirty pages determines if a node is marked
@@ -1682,7 +1696,8 @@ static unsigned int shrink_page_list(struct list_head *page_list,
 						goto activate_locked_split;
 				}
 
-				may_enter_fs = true;
+				may_enter_fs = test_may_enter_fs(page,
+								 sc->gfp_mask);
 
 				/* Adding to swap updated mapping */
 				mapping = page_mapping(page);
-- 
2.33.1