linux-kernel - Re: [PATCH] mm: split thp synchronously on MADV

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Date:   Thu, 25 Nov 2021 18:09:19 +0800
From:   Peter Xu <peterx@...hat.com>
To:     Shakeel Butt <shakeelb@...gle.com>
Cc:     David Hildenbrand <david@...hat.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Yang Shi <shy828301@...il.com>, Zi Yan <ziy@...dia.com>,
        Matthew Wilcox <willy@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: split thp synchronously on MADV_DONTNEED

On Tue, Nov 23, 2021 at 09:28:34AM -0800, Shakeel Butt wrote:
> On Tue, Nov 23, 2021 at 9:26 AM David Hildenbrand <david@...hat.com> wrote:
> >
> > On 23.11.21 18:24, Shakeel Butt wrote:
> > > On Tue, Nov 23, 2021 at 9:20 AM David Hildenbrand <david@...hat.com> wrote:
> > >>
> > >> On 23.11.21 18:17, Shakeel Butt wrote:
> > >>> On Tue, Nov 23, 2021 at 8:57 AM David Hildenbrand <david@...hat.com> wrote:
> > >>>>
> > >>> [...]
> > >>>>>>
> > >>>>>> I do wonder which these locking contexts are exactly, and if we could
> > >>>>>> also do the same thing on ordinary munmap -- because I assume it can be
> > >>>>>> similarly problematic for some applications.
> > >>>>>
> > >>>>> This is a good question regarding munmap. One main difference is
> > >>>>> munmap takes mmap_lock in write mode and usually performance critical
> > >>>>> applications avoid such operations.
> > >>>>
> > >>>> Maybe we can extend it too most page zapping, if that makes things simpler.
> > >>>>
> > >>>
> > >>> Do you mean doing sync THP split for most of page zapping functions
> > >>> (but only if that makes things simpler)?
> > >>>
> > >>
> > >> Yes -- if there are no downsides.
> > >>
> > >
> > > I will try. At the moment the assumption of "Not null zap_details
> > > implies leave swap entries" is giving me a headache.
> >
> > Not only you, did you stumble over
> >
> > https://lkml.kernel.org/r/20211115134951.85286-1-peterx@redhat.com

(thanks for raising this, David)

> >
> > already?
> >
> 
> Oh thanks for the pointer. I missed that. I will take a look. Thanks again.

Hi, Shakeel, 

I saw your v2 has started to pass in zap_details, then we need know the side
effect on that skip-swap-entry thing because with your v2 code munmap() will
start to skip swap entry too (while it was not before).

I saw that you didn't mention this in v2 patch either in commit message or
code, not sure whether you digged that up.  I think it needs some double check
(or feel free to start this digging by reviewing my small patch above :).

Thanks,

-- 
Peter Xu