linux-kernel - Re: [PATCH 1/2] mm: swap: update inuse

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Date: Tue, 23 Jan 2024 01:54:03 -0800
From: Yosry Ahmed <yosryahmed@...gle.com>
To: "Huang, Ying" <ying.huang@...el.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, Johannes Weiner <hannes@...xchg.org>, 
	Nhat Pham <nphamcs@...il.com>, Chris Li <chrisl@...nel.org>, 
	Chengming Zhou <zhouchengming@...edance.com>, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] mm: swap: update inuse_pages after all cleanups are done

> Alternatively, we may just hold the spinlock in try_to_unuse() when we
> check si->inuse_pages at the end. This will also ensure that any calls
> to swap_range_free() have completed. Let me know what you prefer.

To elaborate, I mean replacing this patch and the memory barriers with
the diff below.

diff --git a/mm/swapfile.c b/mm/swapfile.c
index 2fedb148b9404..9b932ecbd80a8 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -2046,6 +2046,7 @@ static int try_to_unuse(unsigned int type)
        struct swap_info_struct *si = swap_info[type];
        struct folio *folio;
        swp_entry_t entry;
+       unsigned int inuse;
        unsigned int i;

        if (!READ_ONCE(si->inuse_pages))
@@ -2123,8 +2124,14 @@ static int try_to_unuse(unsigned int type)
         * and even shmem_writepage() could have been preempted after
         * folio_alloc_swap(), temporarily hiding that swap.  It's easy
         * and robust (though cpu-intensive) just to keep retrying.
+        *
+        * Read si->inuse_pages with the lock held to make sure that cleanups in
+        * swap_range_free() are completed when we read si->inuse_pages == 0.
         */
-       if (READ_ONCE(si->inuse_pages)) {
+       spin_lock(&si->lock);
+       inuse = si->inuse_pages;
+       spin_unlock(&si->lock);
+       if (inuse) {
                if (!signal_pending(current))
                        goto retry;
                return -EINTR;