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

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Date:   Tue, 16 Nov 2021 13:44:04 +1100
From:   NeilBrown <neilb@...e.de>
To:     Trond Myklebust <trond.myklebust@...merspace.com>,
        Anna Schumaker <anna.schumaker@...app.com>,
        Chuck Lever <chuck.lever@...cle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mel Gorman <mgorman@...e.de>
Cc:     linux-nfs@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH 03/13] 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@...e.de>
---
 mm/vmscan.c |   12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index fb9584641ac7..049ff4494081 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1513,8 +1513,14 @@ static unsigned int shrink_page_list(struct list_head *page_list,
 		if (!sc->may_unmap && page_mapped(page))
 			goto keep_locked;
 
+		/* ->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) && (sc->gfp_mask & __GFP_IO));
+			(PageSwapCache(page) &&
+			 !data_race(page_swap_info(page)->flags & SWP_FS_OPS) &&
+			 (sc->gfp_mask & __GFP_IO));
 
 		/*
 		 * The number of dirty pages determines if a node is marked
@@ -1682,7 +1688,9 @@ static unsigned int shrink_page_list(struct list_head *page_list,
 						goto activate_locked_split;
 				}
 
-				may_enter_fs = true;
+				if ((sc->gfp_mask & __GFP_FS) ||
+				    !data_race(page_swap_info(page)->flags & SWP_FS_OPS))
+					may_enter_fs = true;
 
 				/* Adding to swap updated mapping */
 				mapping = page_mapping(page);