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Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE

From Vlastimil Babka <vbabka@suse.cz>
Newsgroups linux.kernel
Subject Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE
Date 2017-05-24 17:00 +0200
Message-ID <tKFVv-3Cg-5@gated-at.bofh.it> (permalink)
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On 05/24/2017 04:28 PM, Pavel Emelyanov wrote:
> On 05/24/2017 02:31 PM, Vlastimil Babka wrote:
>> On 05/24/2017 12:39 PM, Mike Rapoport wrote:
>>>> Hm so the prctl does:
>>>>
>>>>                 if (arg2)
>>>>                         me->mm->def_flags |= VM_NOHUGEPAGE;
>>>>                 else
>>>>                         me->mm->def_flags &= ~VM_NOHUGEPAGE;
>>>>
>>>> That's rather lazy implementation IMHO. Could we change it so the flag
>>>> is stored elsewhere in the mm, and the code that decides to (not) use
>>>> THP will check both the per-vma flag and the per-mm flag?
>>>
>>> I afraid I don't understand how that can help.
>>> What we need is an ability to temporarily disable collapse of the pages in
>>> VMAs that do not have VM_*HUGEPAGE flags set and that after we re-enable
>>> THP, the vma->vm_flags for those VMAs will remain intact.
>>
>> That's what I'm saying - instead of implementing the prctl flag via
>> mm->def_flags (which gets permanently propagated to newly created vma's
>> but e.g. doesn't affect already existing ones), it would be setting a
>> flag somewhere in mm, which khugepaged (and page faults) would check in
>> addition to the per-vma flags.
> 
> I do not insist, but this would make existing paths (checking for flags) be 
> 2 times slower -- from now on these would need to check two bits (vma flags
> and mm flags) which are 100% in different cache lines.

I'd expect you already have mm struct cached during a page fault. And
THP-eligible page fault is just one per pmd, the overhead should be
practically zero.

> What Mike is proposing is the way to fine-tune the existing vma flags. This
> would keep current paths as fast (or slow ;) ) as they are now. All the
> complexity would go to rare cases when someone needs to turn thp off for a
> while and then turn it back on.

Yeah but it's extending user-space API for a corner case. We should do
that only when there's no other option.

> -- Pavel
> 
> --
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Thread

[PATCH] mm: introduce MADV_CLR_HUGEPAGE "Mike Rapoport" <rppt@linux.vnet.ibm.com> - 2017-05-22 08:20 +0200
  Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE Anshuman Khandual <khandual@linux.vnet.ibm.com> - 2017-05-22 09:30 +0200
    Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE Mike Rapoport <rppt@linux.vnet.ibm.com> - 2017-05-22 10:20 +0200
  Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE "Kirill A. Shutemov" <kirill@shutemov.name> - 2017-05-22 13:50 +0200
    Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE Mike Rapoport <rppt@linux.vnet.ibm.com> - 2017-05-22 15:40 +0200
      Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE "Kirill A. Shutemov" <kirill@shutemov.name> - 2017-05-22 15:50 +0200
      Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE Michal Hocko <mhocko@kernel.org> - 2017-05-22 16:00 +0200
        Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE Mike Rapoport <rppt@linux.vnet.ibm.com> - 2017-05-22 16:30 +0200
          Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE Vlastimil Babka <vbabka@suse.cz> - 2017-05-22 18:00 +0200
            Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE Mike Rapoport <rppt@linux.vnet.ibm.com> - 2017-05-22 20:00 +0200
            Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE Mike Rapoport <rppt@linux.vnet.ibm.com> - 2017-05-24 10:00 +0200
              Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE Vlastimil Babka <vbabka@suse.cz> - 2017-05-24 10:10 +0200
                Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE Mike Rapoport <rppt@linux.vnet.ibm.com> - 2017-05-24 12:50 +0200
                Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE Michal Hocko <mhocko@kernel.org> - 2017-05-24 13:20 +0200
                Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE Mike Rapoport <rppt@linux.vnet.ibm.com> - 2017-05-24 16:30 +0200
                Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE Andrea Arcangeli <aarcange@redhat.com> - 2017-05-24 17:30 +0200
                Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE Michal Hocko <mhocko@kernel.org> - 2017-05-30 09:50 +0200
                Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE Mike Rapoport <rppt@linux.vnet.ibm.com> - 2017-05-30 12:20 +0200
                Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE Michal Hocko <mhocko@kernel.org> - 2017-05-30 12:50 +0200
                Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE Andrea Arcangeli <aarcange@redhat.com> - 2017-05-30 16:10 +0200
                Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE Michal Hocko <mhocko@kernel.org> - 2017-05-30 16:50 +0200
                Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE Michal Hocko <mhocko@kernel.org> - 2017-05-30 17:00 +0200
                Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE Andrea Arcangeli <aarcange@redhat.com> - 2017-05-30 18:10 +0200
                Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE Vlastimil Babka <vbabka@suse.cz> - 2017-05-31 08:40 +0200
                Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE Michal Hocko <mhocko@kernel.org> - 2017-05-31 10:30 +0200
                Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE Mike Rapoport <rppt@linux.vnet.ibm.com> - 2017-05-31 11:30 +0200
                Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE Michal Hocko <mhocko@kernel.org> - 2017-05-31 12:30 +0200
                Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE Michal Hocko <mhocko@kernel.org> - 2017-05-31 12:30 +0200
                Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE Mike Rapoport <rppt@linux.vnet.ibm.com> - 2017-06-01 13:10 +0200
                Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE Michal Hocko <mhocko@kernel.org> - 2017-06-01 14:30 +0200
                Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE Andrea Arcangeli <aarcange@redhat.com> - 2017-05-30 17:50 +0200
                Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE Michal Hocko <mhocko@kernel.org> - 2017-05-31 14:10 +0200
                Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE Mike Rapoprt <rppt@linux.vnet.ibm.com> - 2017-05-31 14:40 +0200
                Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE Andrea Arcangeli <aarcange@redhat.com> - 2017-05-31 16:20 +0200
                Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE Michal Hocko <mhocko@kernel.org> - 2017-05-31 16:40 +0200
                Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE Andrea Arcangeli <aarcange@redhat.com> - 2017-05-31 17:50 +0200
                Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE Mike Rapoport <rppt@linux.vnet.ibm.com> - 2017-06-01 09:00 +0200
                Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE Michal Hocko <mhocko@kernel.org> - 2017-05-31 16:20 +0200
                Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE Mike Rapoport <rppt@linux.vnet.ibm.com> - 2017-06-01 09:00 +0200
                Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE Michal Hocko <mhocko@kernel.org> - 2017-06-01 10:10 +0200
                Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE Mike Rapoport <rppt@linux.vnet.ibm.com> - 2017-06-01 10:40 +0200
                Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE Andrea Arcangeli <aarcange@redhat.com> - 2017-06-01 15:50 +0200
                Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE Mike Rapoport <rppt@linux.vnet.ibm.com> - 2017-06-02 11:20 +0200
                Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE Mike Rapoport <rppt@linux.vnet.ibm.com> - 2017-05-31 11:10 +0200
                Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE Michal Hocko <mhocko@kernel.org> - 2017-05-31 14:10 +0200
                Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE Mike Rapoprt <rppt@linux.vnet.ibm.com> - 2017-05-31 14:30 +0200
                Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE Vlastimil Babka <vbabka@suse.cz> - 2017-05-24 13:40 +0200
                Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE Vlastimil Babka <vbabka@suse.cz> - 2017-05-24 17:00 +0200
                Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE Mike Rapoport <rppt@linux.vnet.ibm.com> - 2017-05-24 17:20 +0200

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