linux-kernel - Re: [RFC 2/3] mm: changes to split_huge_page() to free zero filled tail pages

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Date:   Mon, 29 Aug 2022 09:17:07 -0400
From:   Rik van Riel <riel@...riel.com>
To:     David Hildenbrand <david@...hat.com>, alexlzhu@...com,
        linux-mm@...ck.org
Cc:     willy@...radead.org, hannes@...xchg.org, akpm@...ux-foundation.org,
        kernel-team@...com, linux-kernel@...r.kernel.org
Subject: Re: [RFC 2/3] mm: changes to split_huge_page() to free zero filled
 tail pages

On Mon, 2022-08-29 at 12:02 +0200, David Hildenbrand wrote:
> On 26.08.22 23:18, Rik van Riel wrote:
> > On Fri, 2022-08-26 at 12:18 +0200, David Hildenbrand wrote:
> > > On 25.08.22 23:30, alexlzhu@...com wrote:
> > > > From: Alexander Zhu <alexlzhu@...com>
> > 
> > I could see wanting to maybe consolidate the scanning between
> > KSM and this thing at some point, if it could be done without
> > too much complexity, but keeping this change to split_huge_page
> > looks like it might make sense even when KSM is enabled, since
> > it will get rid of the unnecessary memory much faster than KSM
> > could.
> > 
> > Keeping a hundred MB of unnecessary memory around for longer
> > would simply result in more THPs getting split up, and more
> > memory pressure for a longer time than we need.
> 
> Right. I was wondering if we want to map the shared zeropage instead
> of
> the "detected to be zero" page, similar to how KSM would do it. For
> example, with userfaultfd there would be an observable difference.
> 
> (maybe that's already done in this patch set)
> 
The patch does not currently do that, but I suppose it could?

What exactly are the userfaultfd differences here, and how does
dropping 4kB pages break things vs. using the shared zeropage?

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