Re: [PATCH v2 2/3] mm/migrate: make isolate_movable_page() skip slab pages

[Hyeonggon Yoo]

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