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

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Message-ID: <ead1bc0e-c9df-d590-3423-9cfa449167e7@redhat.com>
Date:   Fri, 26 Aug 2022 12:18:58 +0200
From:   David Hildenbrand <david@...hat.com>
To:     alexlzhu@...com, linux-mm@...ck.org
Cc:     willy@...radead.org, hannes@...xchg.org, akpm@...ux-foundation.org,
        riel@...riel.com, 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 25.08.22 23:30, alexlzhu@...com wrote:
> From: Alexander Zhu <alexlzhu@...com>
> 
> Currently, when /sys/kernel/mm/transparent_hugepage/enabled=always is set
> there are a large number of transparent hugepages that are almost entirely
> zero filled.  This is mentioned in a number of previous patchsets
> including:
> https://lore.kernel.org/all/20210731063938.1391602-1-yuzhao@google.com/
> https://lore.kernel.org/all/
> 1635422215-99394-1-git-send-email-ningzhang@...ux.alibaba.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. This is based off the previously mentioned patchset by Yu Zhao.
> However, we chose to free zero tail pages whenever they are encountered
> instead of only on reclaim or migration. We also add a self test to verify
> the RssAnon value to make sure zero pages are not remapped.
> 

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?

-- 
Thanks,

David / dhildenb