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

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Message-ID: <14198f4d-f9ff-8e36-d985-3719e365f913@huawei.com>
Date:   Wed, 17 Aug 2022 09:59:57 +0800
From:   Miaohe Lin <linmiaohe@...wei.com>
To:     Mike Kravetz <mike.kravetz@...cle.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 2022/8/17 7:31, Mike Kravetz wrote:
> 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

Thanks for pointing this out.

> 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.

Agree with you. All call sites of huge_add_to_page_cache is protected by hugetlb fault mutex.

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

Will do it in next version. Many thanks for your review and comment.

Thanks,
Miaohe Lin