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Subject[PATCH 05/18] MM: reclaim mustn't enter FS for SWP_FS_OPS swap-space
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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 969bcdb4ca80..5f460d174b1b 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1465,6 +1465,21 @@ 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
*/
@@ -1514,8 +1529,7 @@ 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
@@ -1683,7 +1697,8 @@ 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);

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