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/208c1757-5edd-fd42-67d4-1940cc43b50f@xxxxxxxxx/ > [2] https://lore.kernel.org/all/aec59f53-0e53-1736-5932-25407125d4d4@xxxxxxx/ > [3] https://lore.kernel.org/all/YzsVM8eToHUeTP75@xxxxxxxxxxxxxxxxxxxx/ > > Reported-by: kernel test robot <yujie.liu@xxxxxxxxx> > Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx> > --- > 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 <42.hyeyoo@xxxxxxxxx> 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
