mm/migrate: make isolate_movable_page() skip slab pages

In the next commit we want to rearrange struct slab fields to allow a larger
rcu_head. Afterwards, the page->mapping field will overlap with SLUB's "struct
list_head slab_list", where the value of prev pointer can become LIST_POISON2,
which is 0x122 + POISON_POINTER_DELTA.  Unfortunately the bit 1 being set can
confuse PageMovable() to be a false positive and cause a GPF as reported by lkp
[1].

To fix this, make isolate_movable_page() skip pages with the PageSlab flag set.
This is a bit tricky as we need to add memory barriers to SLAB and SLUB's page
allocation and freeing, and their counterparts to isolate_movable_page().

Based on my RFC from [2]. Added a comment update from Matthew's variant in [3]
and, as done there, moved the PageSlab checks to happen before trying to take
the page lock.

[1] https://lore.kernel.org/all/208c1757-5edd-fd42-67d4-1940cc43b50f@intel.com/
[2] https://lore.kernel.org/all/aec59f53-0e53-1736-5932-25407125d4d4@suse.cz/
[3] https://lore.kernel.org/all/YzsVM8eToHUeTP75@casper.infradead.org/

Reported-by: kernel test robot <yujie.liu@intel.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>

diff --git a/mm/migrate.c b/mm/migrate.c
index 1379e1912772e0409b78e776b2777a0c3cf0a33b..959c99cff814e541252688d19ef5a51017c9d095 100644 (file)
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -74,13 +74,22 @@ int isolate_movable_page(struct page *page, isolate_mode_t mode)
        if (unlikely(!get_page_unless_zero(page)))
                goto out;
 
+       if (unlikely(PageSlab(page)))
+               goto out_putpage;
+       /* Pairs with smp_wmb() in slab freeing, e.g. SLUB's __free_slab() */
+       smp_rmb();
        /*
-        * Check PageMovable before holding a PG_lock because page's owner
-        * assumes anybody doesn't touch PG_lock of newly allocated page
-        * so unconditionally grabbing the lock ruins page's owner side.
+        * Check movable flag before taking the page lock because
+        * we use non-atomic bitops on newly allocated page flags so
+        * unconditionally grabbing the lock ruins page's owner side.
         */
        if (unlikely(!__PageMovable(page)))
                goto out_putpage;
+       /* Pairs with smp_wmb() in slab allocation, e.g. SLUB's alloc_slab_page() */
+       smp_rmb();
+       if (unlikely(PageSlab(page)))
+               goto out_putpage;
+
        /*
         * As movable pages are not isolated from LRU lists, concurrent
         * compaction threads can race against page migration functions

diff --git a/mm/slab.c b/mm/slab.c
index 59c8e28f7b6ab7498ba13a244168f3e021f21155..219beb48588ee64101bb039c7f333626b5cf6628 100644 (file)
--- a/mm/slab.c
+++ b/mm/slab.c
@@ -1370,6 +1370,8 @@ static struct slab *kmem_getpages(struct kmem_cache *cachep, gfp_t flags,
 
        account_slab(slab, cachep->gfporder, cachep, flags);
        __folio_set_slab(folio);
+       /* Make the flag visible before any changes to folio->mapping */
+       smp_wmb();
        /* Record if ALLOC_NO_WATERMARKS was set when allocating the slab */
        if (sk_memalloc_socks() && page_is_pfmemalloc(folio_page(folio, 0)))
                slab_set_pfmemalloc(slab);
@@ -1387,9 +1389,11 @@ static void kmem_freepages(struct kmem_cache *cachep, struct slab *slab)
 
        BUG_ON(!folio_test_slab(folio));
        __slab_clear_pfmemalloc(slab);
-       __folio_clear_slab(folio);
        page_mapcount_reset(folio_page(folio, 0));
        folio->mapping = NULL;
+       /* Make the mapping reset visible before clearing the flag */
+       smp_wmb();
+       __folio_clear_slab(folio);
 
        if (current->reclaim_state)
                current->reclaim_state->reclaimed_slab += 1 << order;

diff --git a/mm/slub.c b/mm/slub.c
index 99ba865afc4a3a80dfa53abf10baba9a9db9fe64..5e6519d5169c5b352bcdb69b6ebaf4b4866e1abf 100644 (file)
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -1800,6 +1800,8 @@ static inline struct slab *alloc_slab_page(gfp_t flags, int node,
 
        slab = folio_slab(folio);
        __folio_set_slab(folio);
+       /* Make the flag visible before any changes to folio->mapping */
+       smp_wmb();
        if (page_is_pfmemalloc(folio_page(folio, 0)))
                slab_set_pfmemalloc(slab);
 
@@ -2000,8 +2002,10 @@ static void __free_slab(struct kmem_cache *s, struct slab *slab)
        int pages = 1 << order;
 
        __slab_clear_pfmemalloc(slab);
-       __folio_clear_slab(folio);
        folio->mapping = NULL;
+       /* Make the mapping reset visible before clearing the flag */
+       smp_wmb();
+       __folio_clear_slab(folio);
        if (current->reclaim_state)
                current->reclaim_state->reclaimed_slab += pages;
        unaccount_slab(slab, order, s);