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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 v3] proc/ksm: add ksm stats to /proc/pid/smaps
Date:   Tue, 22 Aug 2023 11:03:16 -0700
In-reply-to: <886b6a56-8acb-e975-b5f3-d8098a2285ab@redhat.com>
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David Hildenbrand <david@redhat.com> writes:

> On 17.08.23 18:23, 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. The returned value for KSM is
>> independent of the use of the shared zeropage.
>> 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 | 4 ++++
>>   fs/proc/task_mmu.c                 | 5 +++++
>>   2 files changed, 9 insertions(+)
>> diff --git a/Documentation/filesystems/proc.rst
>> b/Documentation/filesystems/proc.rst
>> index 7897a7dafcbc..d5bdfd59f5b0 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 +502,9 @@ 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. The
>> +value is independent of the use of shared zeropage.
>> +
>>   "LazyFree" shows the amount of memory which is marked by madvise(MADV_FREE).
>>   The memory isn't freed immediately with madvise(). It's freed in memory
>>   pressure if the memory is clean. Please note that the printed value might
>> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
>> index 51315133cdc2..f591c750ffda 100644
>> --- a/fs/proc/task_mmu.c
>> +++ b/fs/proc/task_mmu.c
>> @@ -396,6 +396,7 @@ struct mem_size_stats {
>>   	unsigned long swap;
>>   	unsigned long shared_hugetlb;
>>   	unsigned long private_hugetlb;
>> +	unsigned long ksm;
>>   	u64 pss;
>>   	u64 pss_anon;
>>   	u64 pss_file;
>> @@ -452,6 +453,9 @@ static void smaps_account(struct mem_size_stats *mss, struct page *page,
>>   			mss->lazyfree += size;
>>   	}
>>   +	if (PageKsm(page))
>> +		mss->ksm += size;
>> +
>
> Did you accidentally not include handling of the KSM-shared zeropage?
>
> Or did I misinterpret "independent of the use of the shared zeropage."

I think I misunderstood your review comment. I'll address it in the next version.