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

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Date:   Fri, 26 Aug 2022 17:18:48 -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 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>
> > 
> > Currently, split_huge_page() does not have a way to identify zero
> > filled
> > pages within the THP. Thus these zero pages get remapped and
> > continue to
> > create memory waste. In this patch, we identify and free tail pages
> > that
> > are zero filled in split_huge_page(). In this way, we avoid mapping
> > these
> > pages back into page table entries and can free up unused memory
> > within
> > THPs. 
> > 
> 
> Isn't this to some degree splitting the THP (PMDs->PTEs + dissolve
> compound page) and then letting KSM replace the zero-filled page by
> the
> shared zeropage?
> 
Many systems do not run KSM, though, and even on the systems
where it does, KSM only covers a subset of the memory in the
system.

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.

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