linux-kernel - Re: [PATCH v7 0/6] ksm: support tracking KSM-placed zero-pages

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Date:   Mon, 17 Apr 2023 10:03:00 +0200
From:   David Hildenbrand <david@...hat.com>
To:     yang.yang29@....com.cn, akpm@...ux-foundation.org
Cc:     imbrenda@...ux.ibm.com, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, ran.xiaokai@....com.cn, xu.xin.sc@...il.com,
        xu.xin16@....com.cn, Stefan Roesch <shr@...kernel.io>
Subject: Re: [PATCH v7 0/6] ksm: support tracking KSM-placed zero-pages

On 13.04.23 07:46, yang.yang29@....com.cn wrote:
> From: xu xin <xu.xin16@....com.cn>
> 
> The core idea of this patch set is to enable users to perceive the number
> of any pages merged by KSM, regardless of whether use_zero_page switch has
> been turned on, so that users can know how much free memory increase is
> really due to their madvise(MERGEABLE) actions. But the problem is, when
> enabling use_zero_pages, all empty pages will be merged with kernel zero
> pages instead of with each other as use_zero_pages is disabled, and then
> these zero-pages are no longer monitored by KSM.
> 
> The motivations to do this is seen at:
> https://lore.kernel.org/lkml/202302100915227721315@zte.com.cn/
> 
> In one word, we hope to implement the support for KSM-placed zero pages
> tracking without affecting the feature of use_zero_pages, so that app
> developer can also benefit from knowing the actual KSM profit by getting
> KSM-placed zero pages to optimize applications eventually when
> /sys/kernel/mm/ksm/use_zero_pages is enabled.
> 

Thanks for the update!

> the patch uses pte_mkdirty (related with architecture) to mark KSM-placed
> zero pages. Some architecture(like sparc64) treat R/O dirty PTEs as
> writable, which will break KSM pages state (wrprotect) and affect

With [1] that should be resolved and we should be able to enable it 
unconditionally.

Further, ideally this should get based on [2], such that we can include 
the zeropages in the ksm and per-mm profit calculation.

Last but not least, I realized that we also have to handle the case when 
khugepaged replaces a shared zeropage by a THP. I think that should be 
easy by adjusting the counters in the the is_zero_pfn() handling in 
mm/khugepaged.c:__collapse_huge_page_copy().

> the KSM functionality. For safety, we restrict this feature only to the
> tested and known-working architechtures (ARM, ARM64, and X86) fow now.
> 
> Change log
> ----------
> v6->v7:
> This is an all-newed version which is different from v6 which relys on KSM's
> rmap_item. The patch series don't rely on rmap_item but pte_dirty, so the
> general handling of tracking KSM-placed zero-pages is simplified a lot.
> 
> For safety, we restrict this feature only to the tested and known-working
> architechtures (ARM, ARM64, and X86) fow now.

Yeah, with [1] this can be further simplified.


I'll be on vacation starting on Thursday for ~1.5 weeks, not sure if I 
get to review before that. But it's unlikely that we'll make the 
upcoming merge windows, so I guess we still have time (especially, for 
[1] and [2] to land)


[1] https://lkml.kernel.org/r/20230411142512.438404-4-david@redhat.com
[2] https://lkml.kernel.org/r/20230413233115.1878303-1-shr@devkernel.io

-- 
Thanks,

David / dhildenb