The patch titled
Subject: mm: swap: enforce updating inuse_pages at the end of swap_range_free()
has been added to the -mm mm-unstable branch. Its filename is
mm-swap-enforce-updating-inuse_pages-at-the-end-of-swap_range_free.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-swap-enforce-updating-inuse_pages-at-the-end-of-swap_range_free.patch
This patch will later appear in the mm-unstable branch at
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
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------------------------------------------------------
From: Yosry Ahmed <yosryahmed@xxxxxxxxxx>
Subject: mm: swap: enforce updating inuse_pages at the end of swap_range_free()
Date: Wed, 24 Jan 2024 04:51:11 +0000
Patch series "mm: zswap: simplify zswap_swapoff()", v2.
These patches aim to simplify zswap_swapoff() by removing the unnecessary
trees cleanup code. Patch 1 makes sure that the order of operations
during swapoff is enforced correctly, making sure the simplification in
patch 2 is correct in a future-proof manner.
This patch (of 2):
In swap_range_free(), we update inuse_pages then do some cleanups (arch
invalidation, zswap invalidation, swap cache cleanups, etc). During
swapoff, try_to_unuse() checks that inuse_pages is 0 to make sure all swap
entries are freed. Make sure we only update inuse_pages after we are done
with the cleanups in swap_range_free(), and use the proper memory barriers
to enforce it. This makes sure that code following try_to_unuse() can
safely assume that swap_range_free() ran for all entries in thr swapfile
(e.g. swap cache cleanup, zswap_swapoff()).
In practice, this currently isn't a problem because swap_range_free() is
called with the swap info lock held, and the swapoff code happens to spin
for that after try_to_unuse(). However, this seems fragile and
unintentional, so make it more relable and future-proof. This also
facilitates a following simplification of zswap_swapoff().
Link: https://lkml.kernel.org/r/20240124045113.415378-1-yosryahmed@xxxxxxxxxx
Link: https://lkml.kernel.org/r/20240124045113.415378-2-yosryahmed@xxxxxxxxxx
Signed-off-by: Yosry Ahmed <yosryahmed@xxxxxxxxxx>
Reviewed-by: "Huang, Ying" <ying.huang@xxxxxxxxx>
Cc: Chengming Zhou <zhouchengming@xxxxxxxxxxxxx>
Cc: Chris Li <chrisl@xxxxxxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: Nhat Pham <nphamcs@xxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/swapfile.c | 18 +++++++++++++++---
1 file changed, 15 insertions(+), 3 deletions(-)
--- a/mm/swapfile.c~mm-swap-enforce-updating-inuse_pages-at-the-end-of-swap_range_free
+++ a/mm/swapfile.c
@@ -737,8 +737,6 @@ static void swap_range_free(struct swap_
if (was_full && (si->flags & SWP_WRITEOK))
add_to_avail_list(si);
}
- atomic_long_add(nr_entries, &nr_swap_pages);
- WRITE_ONCE(si->inuse_pages, si->inuse_pages - nr_entries);
if (si->flags & SWP_BLKDEV)
swap_slot_free_notify =
si->bdev->bd_disk->fops->swap_slot_free_notify;
@@ -752,6 +750,14 @@ static void swap_range_free(struct swap_
offset++;
}
clear_shadow_from_swap_cache(si->type, begin, end);
+
+ /*
+ * Make sure that try_to_unuse() observes si->inuse_pages reaching 0
+ * only after the above cleanups are done.
+ */
+ smp_wmb();
+ atomic_long_add(nr_entries, &nr_swap_pages);
+ WRITE_ONCE(si->inuse_pages, si->inuse_pages - nr_entries);
}
static void set_cluster_next(struct swap_info_struct *si, unsigned long next)
@@ -2049,7 +2055,7 @@ static int try_to_unuse(unsigned int typ
unsigned int i;
if (!READ_ONCE(si->inuse_pages))
- return 0;
+ goto success;
retry:
retval = shmem_unuse(type);
@@ -2130,6 +2136,12 @@ retry:
return -EINTR;
}
+success:
+ /*
+ * Make sure that further cleanups after try_to_unuse() returns happen
+ * after swap_range_free() reduces si->inuse_pages to 0.
+ */
+ smp_mb();
return 0;
}
_
Patches currently in -mm which might be from yosryahmed@xxxxxxxxxx are
maintainers-supplement-of-zswap-maintainers-update.patch
mm-swap-enforce-updating-inuse_pages-at-the-end-of-swap_range_free.patch
mm-zswap-remove-unnecessary-trees-cleanups-in-zswap_swapoff.patch
