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

[Joshua Hahn <joshua.hahnjy@gmail.com>]

On Tue, 25 Mar 2025 14:16:34 +0800 Wupeng Ma <mawupeng1@huawei.com> 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>

Hi Wupeng,
Thank you for the fix! This is a problem that we've also seen happen in
our fleet at Meta. I was able to recreate the issue that you mentioned -- to
explicitly lay down the steps I used:

1. echo 1 > /proc/sys/vm/nr_hugepages
2. mkdir /mnt/hugetlb-pool
3.mount -t hugetlbfs -o min_size=2M none /mnt/hugetlb-pool
4. (./get_hugepage &) && (./get_hugepage &)
    # get_hugepage just opens a file in /mnt/hugetlb-pool and mmaps 2M into it.
5. sleep 3
6. (./get_hugepage &) && (./get_hugepage &)
7. cat /proc/meminfo | grep HugePages_Rsvd

... and (6) shows that HugePages_Rsvd has indeed underflowed to U64_MAX!

I've also verified that applying your fix and then re-running the reproducer
shows no underflow.

Reviewed-by: Joshua Hahn <joshua.hahnjy@gmail.com>
Tested-by: Joshua Hahn <joshua.hahnjy@gmail.com>

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