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Message-ID: <519fd6a7-072e-43a2-a9a8-2467ee783524@redhat.com>
Date: Fri, 8 Mar 2024 10:34:21 +0100
From: David Hildenbrand <david@...hat.com>
To: Barry Song <21cnbao@...il.com>, akpm@...ux-foundation.org,
linux-mm@...ck.org
Cc: minchan@...nel.org, fengwei.yin@...el.com, linux-kernel@...r.kernel.org,
mhocko@...e.com, peterx@...hat.com, ryan.roberts@....com,
shy828301@...il.com, songmuchun@...edance.com, wangkefeng.wang@...wei.com,
xiehuan09@...il.com, zokeefe@...gle.com, chrisl@...nel.org,
yuzhao@...gle.com, Barry Song <v-songbaohua@...o.com>,
Lance Yang <ioworker0@...il.com>
Subject: Re: [PATCH v2] mm: prohibit the last subpage from reusing the entire
large folio
On 08.03.24 10:27, Barry Song wrote:
> From: Barry Song <v-songbaohua@...o.com>
>
> In a Copy-on-Write (CoW) scenario, the last subpage will reuse the entire
> large folio, resulting in the waste of (nr_pages - 1) pages. This wasted
> memory remains allocated until it is either unmapped or memory
> reclamation occurs.
>
> The following small program can serve as evidence of this behavior
>
> main()
> {
> #define SIZE 1024 * 1024 * 1024UL
> void *p = malloc(SIZE);
> memset(p, 0x11, SIZE);
> if (fork() == 0)
> _exit(0);
> memset(p, 0x12, SIZE);
> printf("done\n");
> while(1);
> }
>
> For example, using a 1024KiB mTHP by:
> echo always > /sys/kernel/mm/transparent_hugepage/hugepages-1024kB/enabled
>
> (1) w/o the patch, it takes 2GiB,
>
> Before running the test program,
> / # free -m
> total used free shared buff/cache available
> Mem: 5754 84 5692 0 17 5669
> Swap: 0 0 0
>
> / # /a.out &
> / # done
>
> After running the test program,
> / # free -m
> total used free shared buff/cache available
> Mem: 5754 2149 3627 0 19 3605
> Swap: 0 0 0
>
> (2) w/ the patch, it takes 1GiB only,
>
> Before running the test program,
> / # free -m
> total used free shared buff/cache available
> Mem: 5754 89 5687 0 17 5664
> Swap: 0 0 0
>
> / # /a.out &
> / # done
>
> After running the test program,
> / # free -m
> total used free shared buff/cache available
> Mem: 5754 1122 4655 0 17 4632
> Swap: 0 0 0
>
> This patch migrates the last subpage to a small folio and immediately
> returns the large folio to the system. It benefits both memory availability
> and anti-fragmentation.
It might be controversial optimization, and as Ryan said, there, are
likely other cases where we'd want to migrate off-of a thp if possible
earlier.
But I like that it just handles large folios now in a consistent way for
the time being.
Acked-by: David Hildenbrand <david@...hat.com>
--
Cheers,
David / dhildenb
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