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

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Date:   Wed, 24 May 2017 13:31:12 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     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>,
        Pavel Emelyanov <xemul@...tuozzo.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 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.


> --
> Sincerely yours,
> Mike.
> 
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