linux-kernel - Re: [PATCH v4] ksm: use range-walk function to jump over holes in scan_get_next_rmap

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Message-ID: <07cd9c09-2a4f-48d5-845f-0c958b7b45d0@gmail.com>
Date: Wed, 22 Oct 2025 22:48:52 -0300
From: Pedro Demarchi Gomes <pedrodemargomes@...il.com>
To: David Hildenbrand <david@...hat.com>,
 Andrew Morton <akpm@...ux-foundation.org>
Cc: Xu Xin <xu.xin16@....com.cn>, Chengming Zhou <chengming.zhou@...ux.dev>,
 craftfever <craftfever@...mail.cc>, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4] ksm: use range-walk function to jump over holes in
 scan_get_next_rmap_item

On 10/22/25 17:52, David Hildenbrand wrote:

> On 22.10.25 22:31, Andrew Morton wrote:
>> On Wed, 22 Oct 2025 12:30:59 -0300 Pedro Demarchi Gomes <pedrodemargomes@...il.com> wrote:
>>
>>> Currently, scan_get_next_rmap_item() walks every page address in a VMA
>>> to locate mergeable pages. This becomes highly inefficient when scanning
>>> large virtual memory areas that contain mostly unmapped regions.
>>>
>>> This patch replaces the per-address lookup with a range walk using
>>> walk_page_range(). The range walker allows KSM to skip over entire
>>> unmapped holes in a VMA, avoiding unnecessary lookups.
>>> This problem was previously discussed in [1].
>>>
>>> [1] https://lore.kernel.org/linux-mm/423de7a3-1c62-4e72-8e79-19a6413e420c@redhat.com/
>>>
>>
>> Thanks.  It would be helpful of the changelog were to tell people how
>> significant this change is for our users.
>>
>>> Reported-by: craftfever <craftfever@...mail.cc>
>>> Closes: https://lkml.kernel.org/r/020cf8de6e773bb78ba7614ef250129f11a63781@murena.io
>>
>> Buried in here is a claim that large amount of CPU are being used, but
>> nothing quantitative.
>>
>> So is there something we can tell people who are looking at this patch
>> in Feb 2026 and wondering "hm, should I add that to our kernel"?
>>
>>> Suggested-by: David Hildenbrand <david@...hat.com>
>>> Co-developed-by: David Hildenbrand <david@...hat.com>
>>> Signed-off-by: David Hildenbrand <david@...hat.com>
>>> Fixes: 31dbd01f3143 ("ksm: Kernel SamePage Merging")
>>
>> If the observed runtime problem is bad enough then a cc:stable might be
>> justified.  But a description of that observed runtime behavior would
>> be needed for that, please.
>
> Agreed.
>
> With the following simple program
>
> #include <unistd.h>
> #include <stdio.h>
> #include <sys/mman.h>
>
> /* 32 TiB */
> const size_t size = 32ul * 1024 * 1024 * 1024 * 1024;
>
> int main() {
>         char *area = mmap(NULL, size, PROT_READ | PROT_WRITE,
>                           MAP_NORESERVE | MAP_PRIVATE | MAP_ANON, -1, 0);
>
>         if (area == MAP_FAILED) {
>                 perror("mmap() failed\n");
>                 return -1;
>         }
>
>         /* Populate a single page such that we get an anon_vma. */
>         *area = 0;
>
>         /* Enable KSM. */
>         madvise(area, size, MADV_MERGEABLE);
>         pause();
>         return 0;
> }
>
> $ ./ksm-sparse  &
> $ echo 1 > /sys/kernel/mm/ksm/run
>
> ksmd goes to 100% for quite a long time.
>
> Now imagine if a cloud user spins up a couple of these programs.
>
> KSM in the system is essentially deadlocked not able to deduplicate
> anything of value.
>
> @Pedro, can you incorporate all that in the patch description? 

Ok, I will send a v5 with a better changelog and patch description.
Thanks!