From: Michal Hocko <mhocko@kernel.org>
To: Yang Shi <yang.shi@linux.alibaba.com>
Cc: vbabka@suse.cz, rientjes@google.com, kirill@shutemov.name,
akpm@linux-foundation.org, linux-mm@kvack.org,
linux-kernel@vger.kernel.org
Subject: Re: [v2 PATCH] mm: thp: fix false negative of shmem vma's THP eligibility
Date: Wed, 24 Apr 2019 09:58:34 +0200 [thread overview]
Message-ID: <20190424075834.GB12751@dhcp22.suse.cz> (raw)
In-Reply-To: <dace50e0-b72c-33db-5624-bf7449552ff8@linux.alibaba.com>
On Tue 23-04-19 17:22:36, Yang Shi wrote:
>
>
> On 4/23/19 11:34 AM, 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: 4096 kB
> > > > ...
> > > > [snip]
> > > > ...
> > > > ShmemPmdMapped: 4096 kB
> > > > ...
> > > > [snip]
> > > > ...
> > > > THPeligible: 0
> > > >
> > > > And, /proc/meminfo does show THP allocated and PMD mapped too:
> > > >
> > > > ShmemHugePages: 4096 kB
> > > > ShmemPmdMapped: 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?
> > >
> > > I have to say that the THP tuning API is one giant mess :/
> > >
> > > Btw. this patch also seem to fix khugepaged behavior because it
> > > previously
> > > ignored both VM_NOHUGEPAGE and MMF_DISABLE_THP.
>
> Second look shows this is not ignored. hugepage_vma_check() would check this
> for both anonymous vma and shmem vma before scanning. It is called before
> shmem_huge_enabled().
Right. I have missed the earlier check. The main question remains
though.
--
Michal Hocko
SUSE Labs
next prev parent reply other threads:[~2019-04-24 7:58 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-04-23 16:43 [v2 PATCH] mm: thp: fix false negative of shmem vma's THP eligibility Yang Shi
2019-04-23 17:52 ` Michal Hocko
2019-04-23 18:34 ` Yang Shi
2019-04-24 0:22 ` Yang Shi
2019-04-24 7:58 ` Michal Hocko [this message]
2019-04-28 19:13 ` Yang Shi
2019-05-06 23:37 ` Yang Shi
2019-05-07 10:47 ` Michal Hocko
2019-05-07 17:10 ` Yang Shi
2019-06-06 18:59 ` Yang Shi
2019-06-07 10:57 ` Hugh Dickins
2019-06-07 14:25 ` Michal Hocko
2019-06-07 18:51 ` Yang Shi
2019-04-24 13:10 ` Vlastimil Babka
2019-04-24 15:47 ` Yang Shi
2019-04-24 16:17 ` Vlastimil Babka
2019-04-25 16:44 ` Yang Shi
2019-06-08 3:58 ` Hugh Dickins
2019-06-10 17:33 ` Yang Shi
2019-06-12 18:44 ` Hugh Dickins
2019-06-12 19:59 ` Yang Shi
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