linux-kernel - Re: [PATCH v3 3/5] hugetlb: only set HPageMigratable for migratable hstates

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Message-Id: <20210128141531.17fec31e6075fe0eb6e33683@linux-foundation.org>
Date:   Thu, 28 Jan 2021 14:15:31 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Mike Kravetz <mike.kravetz@...cle.com>
Cc:     Oscar Salvador <osalvador@...e.de>, Michal Hocko <mhocko@...e.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
        Muchun Song <songmuchun@...edance.com>,
        David Hildenbrand <david@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Miaohe Lin <linmiaohe@...wei.com>
Subject: Re: [PATCH v3 3/5] hugetlb: only set HPageMigratable for migratable
 hstates

On Thu, 28 Jan 2021 14:00:29 -0800 Mike Kravetz <mike.kravetz@...cle.com> wrote:

> On 1/28/21 1:37 PM, Andrew Morton wrote:
> > On Thu, 28 Jan 2021 06:52:21 +0100 Oscar Salvador <osalvador@...e.de> wrote:
> > 
> >> On Wed, Jan 27, 2021 at 03:36:41PM -0800, Mike Kravetz wrote:
> >>> Yes, this patch is somewhat optional.  It should be a minor improvement
> >>> in cases where we are dealing with hpages in a non-migratable hstate.
> >>> Although, I do not believe this is the common case.
> >>>
> >>> The real reason for even looking into this was a comment by Oscar.  With
> >>> the name change to HPageMigratable, it implies that the page is migratable.
> >>> However, this is not the case if the page's hstate does not support migration.
> >>> So, if we check the hstate when setting the flag we can eliminate those
> >>> cases where the page is certainly not migratable.
> >>>
> >>> I don't really love this patch.  It has minimal functional value.
> >>>
> >>> Oscar, what do you think about dropping this?
> >>
> >> Yeah, I remember this topic arose during a discussion of patch#2 in the
> >> early versions, about whether the renaming to HPageMigratable made
> >> sense.
> >>
> >> Back then I thought that we could have this in one place at fault-path [1],
> >> which should have made this prettier, but it is not the case.
> >> True is that the optimization is little, so I am fine with dropping this
> >> patch.
> > 
> > I've dropped it.
> 
> Thanks Andrew.
> 
> Michal suggested that comments describing synchronization be added for each
> flag.  Since I did 'one patch per flag', that would be an update to each patch.
> Or, I could simply add a patch to update the comment block based on what you
> already have.
> 
> Let me know what is best/easiest for you.

I guess just one patch is best for reviewers.  Then I'll split up into
a sprinkle of -fix patches if I'm feeling energetic ;)