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: <f42b7b67-7de4-4a7b-8074-25ca87a952a6@redhat.com>
Date: Wed, 22 Oct 2025 22:52:31 +0200
From: David Hildenbrand <david@...hat.com>
To: Andrew Morton <akpm@...ux-foundation.org>,
 Pedro Demarchi Gomes <pedrodemargomes@...il.com>
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 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?

-- 
Cheers

David / dhildenb