Re: [PATCH v3] mm/hugetlb: update nr_huge_pages and surplus_huge_pages together

[Peter Xu]

On Wed, Mar 05, 2025 at 11:54:09AM +0800, Liu Shixin wrote:
> In alloc_surplus_hugetlb_folio(), we increase nr_huge_pages and
> surplus_huge_pages separately. In the middle window, if we set
> nr_hugepages to smaller and satisfy count < persistent_huge_pages(h),
> the surplus_huge_pages will be increased by adjust_pool_surplus().
> 
> After adding delay in the middle window, we can reproduce the problem
> easily by following step:
> 
>  1. echo 3 > /proc/sys/vm/nr_overcommit_hugepages
>  2. mmap two hugepages. When nr_huge_pages=2 and surplus_huge_pages=1,
>     goto step 3.
>  3. echo 0 > /proc/sys/vm/nr_huge_pages
> 
> Finally, nr_huge_pages is less than surplus_huge_pages.
> 
> To fix the problem, call only_alloc_fresh_hugetlb_folio() instead and
> move down __prep_account_new_huge_page() into the hugetlb_lock.
> 
> Fixes: 0c397daea1d4 ("mm, hugetlb: further simplify hugetlb allocation API")
> Signed-off-by: Liu Shixin <liushixin2@xxxxxxxxxx>

Acked-by: Peter Xu <peterx@xxxxxxxxxx>

-- 
Peter Xu