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

From: David Hildenbrand <david@redhat.com>
To: Baolin Wang <baolin.wang@linux.alibaba.com>,
	Mike Kravetz <mike.kravetz@oracle.com>
Cc: akpm@linux-foundation.org, songmuchun@bytedance.com,
	linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 1/5] mm/hugetlb: fix races when looking up a CONT-PTE size hugetlb page
Date: Thu, 25 Aug 2022 09:10:41 +0200	[thread overview]
Message-ID: <50703dda-bbee-7484-97ff-87d6ea75bf80@redhat.com> (raw)
In-Reply-To: <f3ee3581-5d0b-f564-7016-783a0d91fea2@linux.alibaba.com>

>> +		/*
>> +		 * 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).

-- 
Thanks,

David / dhildenb