Re: [PATCH] mm: hugetlb: Fix incorrect fallback for subpool

[Oscar Salvador <osalvador@suse.de>]

On Thu, Apr 10, 2025 at 02:26:33PM +0800, Wupeng Ma wrote:
> During our testing with hugetlb subpool enabled, we observe that
> hstate->resv_huge_pages may underflow into negative values. Root cause
> analysis reveals a race condition in subpool reservation fallback handling
> as follow:
> 
> hugetlb_reserve_pages()
>     /* Attempt subpool reservation */
>     gbl_reserve = hugepage_subpool_get_pages(spool, chg);
> 
>     /* Global reservation may fail after subpool allocation */
>     if (hugetlb_acct_memory(h, gbl_reserve) < 0)
>         goto out_put_pages;
> 
> out_put_pages:
>     /* This incorrectly restores reservation to subpool */
>     hugepage_subpool_put_pages(spool, chg);
> 
> When hugetlb_acct_memory() fails after subpool allocation, the current
> implementation over-commits subpool reservations by returning the full
> 'chg' value instead of the actual allocated 'gbl_reserve' amount. This
> discrepancy propagates to global reservations during subsequent releases,
> eventually causing resv_huge_pages underflow.
> 
> This problem can be trigger easily with the following steps:
> 1. reverse hugepage for hugeltb allocation
> 2. mount hugetlbfs with min_size to enable hugetlb subpool
> 3. alloc hugepages with two task(make sure the second will fail due to
>    insufficient amount of hugepages)
> 4. with for a few seconds and repeat step 3 which will make
>    hstate->resv_huge_pages to go below zero.
> 
> To fix this problem, return corrent amount of pages to subpool during the
> fallback after hugepage_subpool_get_pages is called.
> 
> Fixes: 1c5ecae3a93f ("hugetlbfs: add minimum size accounting to subpools")
> Signed-off-by: Wupeng Ma <mawupeng1@huawei.com>
> Tested-by: Joshua Hahn <joshua.hahnjy@gmail.com>

Reviewed-by: Oscar Salvador <osalvador@suse.de>


-- 
Oscar Salvador
SUSE Labs