linux-kernel - Re: [v3 PATCH 2/2] mm: thp: fix false negative of shmem vma's THP eligibility

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Date:   Thu, 18 Jul 2019 23:52:04 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        Yang Shi <yang.shi@...ux.alibaba.com>
Cc:     hughd@...gle.com, kirill.shutemov@...ux.intel.com, mhocko@...e.com,
        rientjes@...gle.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [v3 PATCH 2/2] mm: thp: fix false negative of shmem vma's THP
 eligibility

On 7/18/19 11:44 PM, Andrew Morton wrote:
> On Wed, 19 Jun 2019 09:28:42 -0700 Yang Shi <yang.shi@...ux.alibaba.com> wrote:
> 
>>> Sorry for replying rather late, and not in the v2 thread, but unlike
>>> Hugh I'm not convinced that we should include vma size/alignment in the
>>> test for reporting THPeligible, which was supposed to reflect
>>> administrative settings and madvise hints. I guess it's mostly a matter
>>> of personal feeling. But one objective distinction is that the admin
>>> settings and madvise do have an exact binary result for the whole VMA,
>>> while this check is more fuzzy - only part of the VMA's span might be
>>> properly sized+aligned, and THPeligible will be 1 for the whole VMA.
>>
>> I think THPeligible is used to tell us if the vma is suitable for 
>> allocating THP. Both anonymous and shmem THP checks vma size/alignment 
>> to decide to or not to allocate THP.
>>
>> And, if vma size/alignment is not checked, THPeligible may show "true" 
>> for even 4K mapping. This doesn't make too much sense either.
> 
> This discussion seems rather inconclusive.  I'll merge up the patchset
> anyway.  Vlastimil, if you think some changes are needed here then
> please let's get them sorted out over the next few weeks?

Well, Hugh did ack it, albeit without commenting on this part. I don't
feel strongly enough about this for a nack.