Re: [RFC PATCH 5/5] mm: shmem: add anonymous share mTHP counters

[Baolin Wang]

On 2024/4/23 19:39, Ryan Roberts wrote:
> On 23/04/2024 02:46, Baolin Wang wrote:
> > On 2024/4/23 09:17, Barry Song wrote:
> > > On Mon, Apr 22, 2024 at 3:03 PM Baolin Wang
> > > <baolin.wang@xxxxxxxxxxxxxxxxx> wrote:
> > > > Signed-off-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
> > > > ---
> > > >    include/linux/huge_mm.h | 2 ++
> > > >    mm/huge_memory.c        | 4 ++++
> > > >    mm/shmem.c              | 5 ++++-
> > > >    3 files changed, 10 insertions(+), 1 deletion(-)
> > > >
> > > > diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> > > > index 26b6fa98d8ac..67b9c1acad31 100644
> > > > --- a/include/linux/huge_mm.h
> > > > +++ b/include/linux/huge_mm.h
> > > > @@ -270,6 +270,8 @@ enum mthp_stat_item {
> > > >           MTHP_STAT_ANON_SWPOUT,
> > > >           MTHP_STAT_ANON_SWPOUT_FALLBACK,
> > > >           MTHP_STAT_ANON_SWPIN_REFAULT,
> > > > +       MTHP_STAT_SHMEM_ANON_ALLOC,
> > > > +       MTHP_STAT_SHMEM_ANON_ALLOC_FALLBACK,
> > >
> > > not quite sure about this. for 2MB pmd-mapped THP shmem, we count them
> > > as FILE_THP.
> > > here we are counting as SHMEM_ANON. To me, SHMEM_ANON is more correct but
> > > it doesn't align with pmd-mapped THP. David, Ryan, what do you think?
> >
> > Thanks for reviewing.
> >
> > IMO, I think both approaches are acceptable, which also reflects the dual nature
> > of anonymous shared pages: on the one hand they are anonymous pages, and on the
> > other hand, they are backed by a pseudo file. From the user's perspective, I
> > prefer to use the term "anonymous shmem", which can be distinguished from the
> > real file-backed THP.
> >
> > Anyway, let's see what others think.
>
>  From a quick look at the code, it looks like the shmem alloc/fallback/charge
> events are all lumped in with FILE_THP. But the instantaneous "how many are
> allocated" and "how many are mapped" have their own NR_SHMEM_THPS and
> NR_SHMEM_PMDMAPPED counters? So its a bit inconsistent today.
>
> My preference would be to add these to be consistent with the anon stats:
>
> MTHP_STAT_SHMEM_FAULT_ALLOC,
> MTHP_STAT_SHMEM_FAULT_FALLBACK,
> MTHP_STAT_SHMEM_FAULT_FALLBACK_CHARGE,
>
> But it looks like these aren't always allocated due to faults? So perhaps:
>
> MTHP_STAT_SHMEM_ALLOC,
> MTHP_STAT_SHMEM_FALLBACK,
> MTHP_STAT_SHMEM_FALLBACK_CHARGE,

This looks good to me.

> If I've understood the code correctly (I know nothing about shmem), the
> allocation can be for both mmap(SHARED|ANON) and for tmpfs? So "SHMEM_ANON"

This is allowed, but the 'fd' for tmpfs will be ignored (see 
ksys_mmap_pgoff()), which is same with anonymous shmem.

> probably isn't quite right?
>
>
> > > >           __MTHP_STAT_COUNT
> > > >    };
> > > >
> > > > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > > > index 9e52c0db7580..dc15240c1ab3 100644
> > > > --- a/mm/huge_memory.c
> > > > +++ b/mm/huge_memory.c
> > > > @@ -557,6 +557,8 @@ DEFINE_MTHP_STAT_ATTR(anon_alloc_fallback,
> > > > MTHP_STAT_ANON_ALLOC_FALLBACK);
> > > >    DEFINE_MTHP_STAT_ATTR(anon_swpout, MTHP_STAT_ANON_SWPOUT);
> > > >    DEFINE_MTHP_STAT_ATTR(anon_swpout_fallback, MTHP_STAT_ANON_SWPOUT_FALLBACK);
> > > >    DEFINE_MTHP_STAT_ATTR(anon_swpin_refault, MTHP_STAT_ANON_SWPIN_REFAULT);
> > > > +DEFINE_MTHP_STAT_ATTR(shmem_anon_alloc, MTHP_STAT_SHMEM_ANON_ALLOC);
> > > > +DEFINE_MTHP_STAT_ATTR(shmem_anon_alloc_fallback,
> > > > MTHP_STAT_SHMEM_ANON_ALLOC_FALLBACK);
> > > >
> > > >    static struct attribute *stats_attrs[] = {
> > > >           &anon_alloc_attr.attr,
> > > > @@ -564,6 +566,8 @@ static struct attribute *stats_attrs[] = {
> > > >           &anon_swpout_attr.attr,
> > > >           &anon_swpout_fallback_attr.attr,
> > > >           &anon_swpin_refault_attr.attr,
> > > > +       &shmem_anon_alloc_attr.attr,
> > > > +       &shmem_anon_alloc_fallback_attr.attr,
> > > >           NULL,
> > > >    };
> > > >
> > > > diff --git a/mm/shmem.c b/mm/shmem.c
> > > > index 8b009e7040b2..4a0aa75ab29c 100644
> > > > --- a/mm/shmem.c
> > > > +++ b/mm/shmem.c
> > > > @@ -1706,11 +1706,14 @@ static struct folio *shmem_alloc_and_add_folio(struct
> > > > vm_fault *vmf,
> > > >                           pages = 1 << order;
> > > >                           index = round_down(index, pages);
> > > >                           folio = shmem_alloc_hugefolio(gfp, info, index, order);
> > > > -                       if (folio)
> > > > +                       if (folio) {
> > > > +                               count_mthp_stat(order,
> > > > MTHP_STAT_SHMEM_ANON_ALLOC);
>
> is there any reason why this can't go next to the existing PMD-size stat?

No, will move to the existing PMD-size stat.