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

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Message-ID: <alpine.LSU.2.11.1907181457150.2510@eggly.anvils>
Date:   Thu, 18 Jul 2019 15:06:50 -0700 (PDT)
From:   Hugh Dickins <hughd@...gle.com>
To:     Vlastimil Babka <vbabka@...e.cz>
cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Yang Shi <yang.shi@...ux.alibaba.com>, 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 Thu, 18 Jul 2019, Vlastimil Babka wrote:
> 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.

Right, I had no further comment: Yang and I agreed one way round,
you thought the other way.  I was more persuaded by Yang's 4kB
example than by what you wrote; but not hugely excited either way.

Hugh