linux-kernel - Re: [PATCH v2 1/5] mm/hugetlb: fix races when looking up a CONT-PTE size hugetlb page

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Message-ID: <3c381602-26ea-883c-859a-4007cb7cb289@linux.alibaba.com>
Date:   Thu, 25 Aug 2022 15:58:45 +0800
From:   Baolin Wang <baolin.wang@...ux.alibaba.com>
To:     David Hildenbrand <david@...hat.com>,
        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 v2 1/5] mm/hugetlb: fix races when looking up a CONT-PTE
 size hugetlb page



On 8/25/2022 3:10 PM, David Hildenbrand wrote:
> 
>>> +		/*
>>> +		 * try_grab_page() should always succeed here, because we hold
>>> +		 * the ptl lock and have verified pte_present().
>>> +		 */
>>> +		if (WARN_ON_ONCE(!try_grab_page(page, flags))) {
>>> +			page = NULL;
>>> +			goto out;
>>> +		}
>>> +	} else {
>>
>> Should add FOLL_MIGRATION validation before waiting a migration entry.
> 
> We really only need FOLL_MIGRATION for KSM. As hugetlb pages cannot be
> KSM pages, we don't need this.
> 
> Actually, I do have patches in the works that rip out FOLL_MIGRATION
> completely by adjusting KSM code.
> 
> So let's try to not add dead code (although it would make sense for
> feature completeness as is -- but then, FOLL_MIGRATION really needs to go).

Make sense. Thanks for your explanation.