Re: [v2 PATCH] mm: thp: fix false negative of shmem vma's THP eligibility

[Yang Shi]

On 6/7/19 3:57 AM, Hugh Dickins wrote:
> On Thu, 6 Jun 2019, Yang Shi wrote:
> > On 5/7/19 10:10 AM, Yang Shi wrote:
> > > On 5/7/19 3:47 AM, Michal Hocko wrote:
> > > > [Hmm, I thought, Hugh was CCed]
> > > >
> > > > On Mon 06-05-19 16:37:42, Yang Shi wrote:
> > > > > On 4/28/19 12:13 PM, Yang Shi wrote:
> > > > > > On 4/23/19 10:52 AM, Michal Hocko wrote:
> > > > > > > On Wed 24-04-19 00:43:01, Yang Shi wrote:
> > > > > > > > The commit 7635d9cbe832 ("mm, thp, proc: report THP eligibility
> > > > > > > > for each
> > > > > > > > vma") introduced THPeligible bit for processes' smaps. But, when
> > > > > > > > checking
> > > > > > > > the eligibility for shmem vma, __transparent_hugepage_enabled()
> > > > > > > > is
> > > > > > > > called to override the result from shmem_huge_enabled(). It may
> > > > > > > > result
> > > > > > > > in the anonymous vma's THP flag override shmem's. For example,
> > > > > > > > running a
> > > > > > > > simple test which create THP for shmem, but with anonymous THP
> > > > > > > > disabled,
> > > > > > > > when reading the process's smaps, it may show:
> > > > > > > >
> > > > > > > > 7fc92ec00000-7fc92f000000 rw-s 00000000 00:14 27764 /dev/shm/test
> > > > > > > > Size:ÂÂÂÂÂÂÂÂÂÂÂÂÂÂ 4096 kB
> > > > > > > > ...
> > > > > > > > [snip]
> > > > > > > > ...
> > > > > > > > ShmemPmdMapped:ÂÂÂÂ 4096 kB
> > > > > > > > ...
> > > > > > > > [snip]
> > > > > > > > ...
> > > > > > > > THPeligible:ÂÂÂ 0
> > > > > > > >
> > > > > > > > And, /proc/meminfo does show THP allocated and PMD mapped too:
> > > > > > > >
> > > > > > > > ShmemHugePages:ÂÂÂÂ 4096 kB
> > > > > > > > ShmemPmdMapped:ÂÂÂÂ 4096 kB
> > > > > > > >
> > > > > > > > This doesn't make too much sense. The anonymous THP flag should
> > > > > > > > not
> > > > > > > > intervene shmem THP. Calling shmem_huge_enabled() with checking
> > > > > > > > MMF_DISABLE_THP sounds good enough. And, we could skip stack and
> > > > > > > > dax vma check since we already checked if the vma is shmem
> > > > > > > > already.
> > > > > > > Kirill, can we get a confirmation that this is really intended
> > > > > > > behavior
> > > > > > > rather than an omission please? Is this documented? What is a
> > > > > > > global
> > > > > > > knob to simply disable THP system wise?
> > > > > > Hi Kirill,
> > > > > >
> > > > > > Ping. Any comment?
> > > > > Talked with Kirill at LSFMM, it sounds this is kind of intended
> > > > > behavior
> > > > > according to him. But, we all agree it looks inconsistent.
> > > > >
> > > > > So, we may have two options:
> > > > >  ÂÂÂÂ - Just fix the false negative issue as what the patch does
> > > > >  ÂÂÂÂ - Change the behavior to make it more consistent
> > > > >
> > > > > I'm not sure whether anyone relies on the behavior explicitly or
> > > > > implicitly
> > > > > or not.
> > > > Well, I would be certainly more happy with a more consistent behavior.
> > > > Talked to Hugh at LSFMM about this and he finds treating shmem objects
> > > > separately from the anonymous memory. And that is already the case
> > > > partially when each mount point might have its own setup. So the primary
> > > > question is whether we need a one global knob to controll all THP
> > > > allocations. One argument to have that is that it might be helpful to
> > > > for an admin to simply disable source of THP at a single place rather
> > > > than crawling over all shmem mount points and remount them. Especially
> > > > in environments where shmem points are mounted in a container by a
> > > > non-root. Why would somebody wanted something like that? One example
> > > > would be to temporarily workaround high order allocations issues which
> > > > we have seen non trivial amount of in the past and we are likely not at
> > > > the end of the tunel.
> > > Shmem has a global control for such use. Setting shmem_enabled to "force"
> > > or "deny" would enable or disable THP for shmem globally, including non-fs
> > > objects, i.e. memfd, SYS V shmem, etc.
> > >
> > > > That being said I would be in favor of treating the global sysfs knob to
> > > > be global for all THP allocations. I will not push back on that if there
> > > > is a general consensus that shmem and fs in general are a different
> > > > class of objects and a single global control is not desirable for
> > > > whatever reasons.
> > > OK, we need more inputs from Kirill, Hugh and other folks.
> > [Forgot cc to mailing lists]
> >
> > Hi guys,
> >
> > How should we move forward for this one? Make the sysfs knob
> > (/sys/kernel/mm/transparent_hugepage/enabled) to be global for both anonymous
> > and tmpfs? Or just treat shmem objects separately from anon memory then fix
> > the false-negative of THP eligibility by this patch?
> Sorry for not getting back to you sooner on this.
>
> I don't like to drive design by smaps. I agree with the word "mess" used
> several times of THP tunings in this thread, but it's too easy to make
> that mess worse by unnecessary changes, so I'm very cautious here.
>
> The addition of "THPeligible" without an "Anon" in its name was
> unfortunate. I suppose we're two releases too late to change that.

The smaps shows it is anon vma or shmem vma for the most cases.

> Applying process (PR_SET_THP_DISABLE) and mm (MADV_*HUGEPAGE)
> limitations to shared filesystem objects doesn't work all that well.

The THP eligibility indicator is per vma, it just reports whether THP is 
eligible for a specific vma. So, I'm supposed it should keep consistent 
with MMF_DISABLE_THP and MADV_*HUGEPAGE setting.

The current implementation in shmem and kuhugepaged also checks these.

> I recommend that you continue to treat shmem objects separately from
> anon memory, and just make the smaps "THPeligible" more often accurate.
>
> Is your v2 patch earlier in this thread the best for that?

The v2 patch treats shmem objects separately from anon memory and it 
makes the "THPeligible" more often accurate.

> No answer tonight, I'll re-examine later in the day.
>
> Hugh