From mboxrd@z Thu Jan  1 00:00:00 1970
From: Pavel Emelyanov <xemul@virtuozzo.com>
Subject: Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE
Date: Wed, 24 May 2017 17:28:17 +0300
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To: Vlastimil Babka <vbabka@suse.cz>, Mike Rapoport <rppt@linux.vnet.ibm.com>
Cc: Michal Hocko <mhocko@kernel.org>, "Kirill A. Shutemov" <kirill@shutemov.name>, Andrew Morton <akpm@linux-foundation.org>, Arnd Bergmann <arnd@arndb.de>, "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>, Andrea Arcangeli <aarcange@redhat.com>, linux-mm <linux-mm@kvack.org>, lkml <linux-kernel@vger.kernel.org>, Linux API <linux-api@vger.kernel.org>
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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.

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.

-- Pavel

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