linux-kernel - Re: [PATCH v3 1/5] mm: introduce MADV

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Date:   Tue, 9 Jul 2019 11:19:45 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Minchan Kim <minchan@...nel.org>
Cc:     Dave Hansen <dave.hansen@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-mm <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>, linux-api@...r.kernel.org,
        Johannes Weiner <hannes@...xchg.org>,
        Tim Murray <timmurray@...gle.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Daniel Colascione <dancol@...gle.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Sonny Rao <sonnyrao@...gle.com>, oleksandr@...hat.com,
        hdanton@...a.com, lizeb@...gle.com,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH v3 1/5] mm: introduce MADV_COLD

On Mon 01-07-19 16:35:00, Minchan Kim wrote:
> >From 39df9f94e6204b8893f3f3feb692745657392657 Mon Sep 17 00:00:00 2001
> From: Minchan Kim <minchan@...nel.org>
> Date: Fri, 24 May 2019 13:47:54 +0900
> Subject: [PATCH v3 1/5] mm: introduce MADV_COLD
> 
> When a process expects no accesses to a certain memory range, it could
> give a hint to kernel that the pages can be reclaimed when memory pressure
> happens but data should be preserved for future use.  This could reduce
> workingset eviction so it ends up increasing performance.
> 
> This patch introduces the new MADV_COLD hint to madvise(2) syscall.
> MADV_COLD can be used by a process to mark a memory range as not expected
> to be used in the near future. The hint can help kernel in deciding which
> pages to evict early during memory pressure.
> 
> It works for every LRU pages like MADV_[DONTNEED|FREE]. IOW, It moves
> 
> 	active file page -> inactive file LRU
> 	active anon page -> inacdtive anon LRU
> 
> Unlike MADV_FREE, it doesn't move active anonymous pages to inactive
> file LRU's head because MADV_COLD is a little bit different symantic.
> MADV_FREE means it's okay to discard when the memory pressure because
> the content of the page is *garbage* so freeing such pages is almost zero
> overhead since we don't need to swap out and access afterward causes just
> minor fault. Thus, it would make sense to put those freeable pages in
> inactive file LRU to compete other used-once pages. It makes sense for
> implmentaion point of view, too because it's not swapbacked memory any
> longer until it would be re-dirtied. Even, it could give a bonus to make
> them be reclaimed on swapless system. However, MADV_COLD doesn't mean
> garbage so reclaiming them requires swap-out/in in the end so it's bigger
> cost. Since we have designed VM LRU aging based on cost-model, anonymous
> cold pages would be better to position inactive anon's LRU list, not file
> LRU. Furthermore, it would help to avoid unnecessary scanning if system
> doesn't have a swap device. Let's start simpler way without adding
> complexity at this moment. However, keep in mind, too that it's a caveat
> that workloads with a lot of pages cache are likely to ignore MADV_COLD
> on anonymous memory because we rarely age anonymous LRU lists.
> 
> * man-page material
> 
> MADV_COLD (since Linux x.x)
> 
> Pages in the specified regions will be treated as less-recently-accessed
> compared to pages in the system with similar access frequencies.
> In contrast to MADV_FREE, the contents of the region are preserved
> regardless of subsequent writes to pages.
> 
> MADV_COLD cannot be applied to locked pages, Huge TLB pages, or VM_PFNMAP
> pages.
> 
> * v2
>  * add up the warn with lots of page cache workload - mhocko
>  * add man page stuff - dave
> 
> * v1
>  * remove page_mapcount filter - hannes, mhocko
>  * remove idle page handling - joelaf
> 
> * RFCv2
>  * add more description - mhocko
> 
> * RFCv1
>  * renaming from MADV_COOL to MADV_COLD - hannes
> 
> * internal review
>  * use clear_page_youn in deactivate_page - joelaf
>  * Revise the description - surenb
>  * Renaming from MADV_WARM to MADV_COOL - surenb
> 
> Signed-off-by: Minchan Kim <minchan@...nel.org>

OK, looks reasonable to me. THP part still gives me a head spin but it
is consistent with madv_free part so I will trust that all weird corner
cases are already caught there.

Acked-by: Michal Hocko <mhocko@...e.com>

Thanks!
-- 
Michal Hocko
SUSE Labs