linux-kernel - Re: [PATCH mm-unstable] mm: folio_add_new_anon_rmap() careful __folio_set

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Date: Tue, 25 Jun 2024 09:40:22 +0200
From: David Hildenbrand <david@...hat.com>
To: Hugh Dickins <hughd@...gle.com>, Andrew Morton <akpm@...ux-foundation.org>
Cc: Barry Song <21cnbao@...il.com>, baolin.wang@...ux.alibaba.com,
 chrisl@...nel.org, linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 mhocko@...e.com, ryan.roberts@....com, shy828301@...il.com,
 surenb@...gle.com, v-songbaohua@...o.com, willy@...radead.org,
 ying.huang@...el.com, yosryahmed@...gle.com, yuanshuai@...o.com,
 yuzhao@...gle.com
Subject: Re: [PATCH mm-unstable] mm: folio_add_new_anon_rmap() careful
 __folio_set_swapbacked()

On 25.06.24 07:00, Hugh Dickins wrote:
> Commit "mm: use folio_add_new_anon_rmap() if folio_test_anon(folio)==
> false" has extended folio_add_new_anon_rmap() to use on non-exclusive
> folios, already visible to others in swap cache and on LRU.
> 
> That renders its non-atomic __folio_set_swapbacked() unsafe: it risks
> overwriting concurrent atomic operations on folio->flags, losing bits
> added or restoring bits cleared.  Since it's only used in this risky
> way when folio_test_locked and !folio_test_anon, many such races are
> excluded; but, for example, isolations by folio_test_clear_lru() are
> vulnerable, and setting or clearing active.
> 
> It could just use the atomic folio_set_swapbacked(); but this function
> does try to avoid atomics where it can, so use a branch instead: just
> avoid setting swapbacked when it is already set, that is good enough.
> (Swapbacked is normally stable once set: lazyfree can undo it, but
> only later, when found anon in a page table.)
> 
> This fixes a lot of instability under compaction and swapping loads:
> assorted "Bad page"s, VM_BUG_ON_FOLIO()s, apparently even page double
> frees - though I've not worked out what races could lead to the latter.
> 
> Signed-off-by: Hugh Dickins <hughd@...gle.com>
> ---
>   mm/rmap.c | 4 +++-
>   1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/rmap.c b/mm/rmap.c
> index df1a43295c85..5394c1178bf1 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1408,7 +1408,9 @@ void folio_add_new_anon_rmap(struct folio *folio, struct vm_area_struct *vma,
>   	VM_WARN_ON_FOLIO(folio_test_hugetlb(folio), folio);
>   	VM_BUG_ON_VMA(address < vma->vm_start ||
>   			address + (nr << PAGE_SHIFT) > vma->vm_end, vma);
> -	__folio_set_swapbacked(folio);
> +
> +	if (!folio_test_swapbacked(folio))
> +		__folio_set_swapbacked(folio);
>   	__folio_set_anon(folio, vma, address, exclusive);
>   
>   	if (likely(!folio_test_large(folio))) {

LGTM.

I'll point out that it's sufficient for a PFN walker to do a tryget + 
trylock to cause trouble.

Fortunately isolate_movable_page() will check __folio_test_movable() 
before doing the trylock.

Reviewed-by: David Hildenbrand <david@...hat.com>

-- 
Cheers,

David / dhildenb