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

[David Hildenbrand]

On 15.08.23 19:10, Stefan Roesch wrote:
> David Hildenbrand <david@xxxxxxxxxx> 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@xxxxxxxxxxxx>
> > > ---
> > >    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@xxxxxxxxxx>
> > 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