linux-kernel - Re: [PATCH v2] proc/ksm: add ksm stats to /proc/pid/smaps

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Message-ID: <ad33c7f1-8c7c-27b6-7c2e-adbb349f2dff@redhat.com>
Date:   Wed, 16 Aug 2023 11:39:52 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Stefan Roesch <shr@...kernel.io>
Cc:     kernel-team@...com, akpm@...ux-foundation.org,
        linux-fsdevel@...r.kernel.org, hannes@...xchg.org,
        riel@...riel.com, linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v2] proc/ksm: add ksm stats to /proc/pid/smaps

On 15.08.23 19:10, Stefan Roesch wrote:
> 
> David Hildenbrand <david@...hat.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@...kernel.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@....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.

That's probably the right thing to do: make the stat return the same 
value independent of the usage of the shared zeropage.

>      I'm not sure what to do about smaps_pmd_entry in that case. We
>      probably don't care about compund pages.

No, KSM only places the shared zeropage for PTEs, no need to handle PMDs.

-- 
Cheers,

David / dhildenb