linux-kernel - Re: [PATCH] mm: make PR_SET_THP

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Date:   Fri, 2 Jun 2017 22:55:12 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Linux API <linux-api@...r.kernel.org>,
        Michal Hocko <mhocko@...nel.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Arnd Bergmann <arnd@...db.de>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Pavel Emelyanov <xemul@...tuozzo.com>,
        linux-mm <linux-mm@...ck.org>,
        lkml <linux-kernel@...r.kernel.org>,
        Michal Hocko <mhocko@...e.com>
Subject: Re: [PATCH] mm: make PR_SET_THP_DISABLE immediately active

On 06/02/2017 10:40 PM, Andrew Morton wrote:
> On Fri, 2 Jun 2017 22:31:47 +0200 Vlastimil Babka <vbabka@...e.cz> wrote:
>>> Perhaps we should be adding new prctl modes to select this new
>>> behaviour and leave the existing PR_SET_THP_DISABLE behaviour as-is?
>>
>> I think we can reasonably assume that most users of the prctl do just
>> the fork() & exec() thing, so they will be unaffected.
> 
> That sounds optimistic.  Perhaps people are using the current behaviour
> to set on particular mapping to MMF_DISABLE_THP, with
> 
> 	prctl(PR_SET_THP_DISABLE)
> 	mmap()
> 	prctl(PR_CLR_THP_DISABLE)
> 
> ?
> 
> Seems a reasonable thing to do.

Using madvise(MADV_NOHUGEPAGE) seems reasonabler to me, with the same
effect. And it's older (2.6.38).

> But who knows - people do all sorts of
> inventive things.

Yeah :( but we can hope they don't even know that the prctl currently
behaves they way it does - man page doesn't suggest it would, and most
of us in this thread found it surprising.

>> And as usual, if
>> somebody does complain in the end, we revert and try the other way?
> 
> But by then it's too late - the new behaviour will be out in the field.

Revert in stable then?
But I don't think this patch should go to stable. I understand right
that CRIU will switch to the UFFDIO_COPY approach and doesn't need the
prctl change/new madvise anymore?