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From:   Stefan Roesch <shr@devkernel.io>
To:     David Hildenbrand <david@redhat.com>
Cc:     kernel-team@fb.com, akpm@linux-foundation.org,
        linux-fsdevel@vger.kernel.org, hannes@cmpxchg.org,
        riel@surriel.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH v2] proc/ksm: add ksm stats to /proc/pid/smaps
Date:   Tue, 15 Aug 2023 10:10:55 -0700
In-reply-to: <43d64aee-4bd9-bba0-9434-55cec26bd9dc@redhat.com>
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David Hildenbrand <david@redhat.com> writes:

> Sorry for the late reply, Gmail once again decided to classify your mails as
> spam (for whatever reason).
>
> On 11.08.23 18:28, Stefan Roesch wrote:
>> With madvise and prctl KSM can be enabled for different VMA's. Once it
>> is enabled we can query how effective KSM is overall. However we cannot
>> easily query if an individual VMA benefits from KSM.
>> This commit adds a KSM section to the /prod/<pid>/smaps file. It reports
>> how many of the pages are KSM pages.
>> Here is a typical output:
>> 7f420a000000-7f421a000000 rw-p 00000000 00:00 0
>> Size:             262144 kB
>> KernelPageSize:        4 kB
>> MMUPageSize:           4 kB
>> Rss:               51212 kB
>> Pss:                8276 kB
>> Shared_Clean:        172 kB
>> Shared_Dirty:      42996 kB
>> Private_Clean:       196 kB
>> Private_Dirty:      7848 kB
>> Referenced:        15388 kB
>> Anonymous:         51212 kB
>> KSM:               41376 kB
>> LazyFree:              0 kB
>> AnonHugePages:         0 kB
>> ShmemPmdMapped:        0 kB
>> FilePmdMapped:         0 kB
>> Shared_Hugetlb:        0 kB
>> Private_Hugetlb:       0 kB
>> Swap:             202016 kB
>> SwapPss:            3882 kB
>> Locked:                0 kB
>> THPeligible:    0
>> ProtectionKey:         0
>> ksm_state:          0
>> ksm_skip_base:      0
>> ksm_skip_count:     0
>> VmFlags: rd wr mr mw me nr mg anon
>> This information also helps with the following workflow:
>> - First enable KSM for all the VMA's of a process with prctl.
>> - Then analyze with the above smaps report which VMA's benefit the most
>> - Change the application (if possible) to add the corresponding madvise
>> calls for the VMA's that benefit the most
>> Signed-off-by: Stefan Roesch <shr@devkernel.io>
>> ---
>>   Documentation/filesystems/proc.rst | 3 +++
>>   fs/proc/task_mmu.c                 | 5 +++++
>>   2 files changed, 8 insertions(+)
>> diff --git a/Documentation/filesystems/proc.rst
>> b/Documentation/filesystems/proc.rst
>> index 7897a7dafcbc..4ef3c0bbf16a 100644
>> --- a/Documentation/filesystems/proc.rst
>> +++ b/Documentation/filesystems/proc.rst
>> @@ -461,6 +461,7 @@ Memory Area, or VMA) there is a series of lines such as the following::
>>       Private_Dirty:         0 kB
>>       Referenced:          892 kB
>>       Anonymous:             0 kB
>> +    KSM:                   0 kB
>>       LazyFree:              0 kB
>>       AnonHugePages:         0 kB
>>       ShmemPmdMapped:        0 kB
>> @@ -501,6 +502,8 @@ accessed.
>>   a mapping associated with a file may contain anonymous pages: when MAP_PRIVATE
>>   and a page is modified, the file page is replaced by a private anonymous copy.
>>   +"KSM" shows the amount of anonymous memory that has been de-duplicated.
>
>
> How do we want to treat memory that has been deduplicated into the shared
> zeropage?
>
> It would also match this description.
>
> See in mm-stable:
>
> commit 30ff6ed9a65c7e73545319fc15f7bcf9c52457eb
> Author: xu xin <xu.xin16@zte.com.cn>
> Date:   Tue Jun 13 11:09:28 2023 +0800
>
>     ksm: support unsharing KSM-placed zero pages
>
>     Patch series "ksm: support tracking KSM-placed zero-pages", v10.

I see two approaches how to deal with zero page:
 - If zero page is not enabled, it works as is
 - If enabled
    - Document that zero page is accounted for the current vma or
    - Pass in the pte from smaps_pte_entry() to smaps_account() so we can
    determine if this is a zero page.
    I'm not sure what to do about smaps_pmd_entry in that case. We
    probably don't care about compund pages.