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/[email protected]/
[2] https://lore.kernel.org/all/[email protected]/
[3] https://lore.kernel.org/all/[email protected]/
Reported-by: kernel test robot <[email protected]>
Signed-off-by: Vlastimil Babka <[email protected]>
---
mm/migrate.c | 15 ++++++++++++---
mm/slab.c | 6 +++++-
mm/slub.c | 6 +++++-
3 files changed, 22 insertions(+), 5 deletions(-)
diff --git a/mm/migrate.c b/mm/migrate.c
index 1379e1912772..959c99cff814 100644
--- 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 59c8e28f7b6a..219beb48588e 100644
--- 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 99ba865afc4a..5e6519d5169c 100644
--- 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);
--
2.38.0
On Mon, Nov 07, 2022 at 06:05:53PM +0100, Vlastimil Babka wrote:
> 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/[email protected]/
> [2] https://lore.kernel.org/all/[email protected]/
> [3] https://lore.kernel.org/all/[email protected]/
>
> Reported-by: kernel test robot <[email protected]>
> Signed-off-by: Vlastimil Babka <[email protected]>
> ---
> mm/migrate.c | 15 ++++++++++++---
> mm/slab.c | 6 +++++-
> mm/slub.c | 6 +++++-
> 3 files changed, 22 insertions(+), 5 deletions(-)
>
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 1379e1912772..959c99cff814 100644
> --- 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 59c8e28f7b6a..219beb48588e 100644
> --- 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 99ba865afc4a..5e6519d5169c 100644
> --- 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);
> --
> 2.38.0
This looks correct to me.
Acked-by: Hyeonggon Yoo <[email protected]>
Just noting to myself to avoid confusion in the future:
- When one sees PageSlab() == false, __PageMovable() == true should not be false positive
from slab page because resetting ->mapping is visible first and then it clears PG_slab.
- When one sees __PageMoveable() == true for slab page, PageSlab() must be true because
setting PG_slab in slab allocation is visible first and then it writes to ->mapping field.
I hope it's nicely reshaped after Matthew's frozen refcount series.
--
Thanks,
Hyeonggon