linux-kernel - Re: [PATCH] mm: introduce MADV_CLR

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Date:   Wed, 24 May 2017 16:54:38 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Pavel Emelyanov <xemul@...tuozzo.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>
Cc:     Michal Hocko <mhocko@...nel.org>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Arnd Bergmann <arnd@...db.de>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        linux-mm <linux-mm@...ck.org>,
        lkml <linux-kernel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>
Subject: Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE

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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