linux-kernel - Re: [PATCH 3/6] mm/hugetlb: fix missing call to restore_reserve_on

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Date:   Tue, 16 Aug 2022 16:31:08 -0700
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Miaohe Lin <linmiaohe@...wei.com>
Cc:     akpm@...ux-foundation.org, songmuchun@...edance.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/6] mm/hugetlb: fix missing call to
 restore_reserve_on_error()

On 08/16/22 21:05, Miaohe Lin wrote:
> When huge_add_to_page_cache() fails, the page is freed directly without
> calling restore_reserve_on_error() to restore reserve for newly allocated
> pages not in page cache. Fix this by calling restore_reserve_on_error()
> when huge_add_to_page_cache fails.
> 
> Signed-off-by: Miaohe Lin <linmiaohe@...wei.com>
> ---
>  mm/hugetlb.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index ff991e5bdf1f..b69d7808f457 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -5603,6 +5603,7 @@ static vm_fault_t hugetlb_no_page(struct mm_struct *mm,
>  		if (vma->vm_flags & VM_MAYSHARE) {
>  			int err = huge_add_to_page_cache(page, mapping, idx);
>  			if (err) {
> +				restore_reserve_on_error(h, vma, haddr, page);

Hmmmm.  I was going to comment that restore_reserve_on_error would not handle
the situation where 'err == -EEXIST' below.  This is because it implies we
raced with someone else that added the page to the cache.  And, that other
allocation, not this one, consumed the reservation.  However, I am not sure
how that could be possible?  The hugetlb fault mutex (which we hold)
must be held to add a page to the page cache.

Searching git history I see that code was added (or at least existed) before
the hugetlb fault mutex was introduced.  So, I believe that check for -EEXIST
and retry can go.

With that said, restore_reserve_on_error can be called here.  But, let's
look into removing that err == -EEXIST check to avoid confusion.
-- 
Mike Kravetz

>  				put_page(page);
>  				if (err == -EEXIST)
>  					goto retry;
> -- 
> 2.23.0
>