Date: Sat, 03 Jun 2017 13:40:20 +0300
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Subject: Re: [PATCH] mm: make PR_SET_THP_DISABLE immediately active
To: Andrew Morton <akpm@linux-foundation.org>
CC: Linux API <linux-api@vger.kernel.org>,
        Michal Hocko <mhocko@kernel.org>, Vlastimil Babka <vbabka@suse.cz>,
        Andrea Arcangeli <aarcange@redhat.com>, Arnd Bergmann <arnd@arndb.de>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Pavel Emelyanov <xemul@virtuozzo.com>, linux-mm <linux-mm@kvack.org>,
        lkml <linux-kernel@vger.kernel.org>, Michal Hocko <mhocko@suse.com>
From: Mike Rapoprt <rppt@linux.vnet.ibm.com>
Message-Id: <495443D6-654F-4751-8279-FBB96E3D90B3@linux.vnet.ibm.com>
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On June 2, 2017 10:50:59 PM GMT+03:00, Andrew Morton <akpm@linux-foundation.org> wrote:
>On Fri,  2 Jun 2017 18:03:22 +0300 "Mike Rapoport"
><rppt@linux.vnet.ibm.com> wrote:
>
>> PR_SET_THP_DISABLE has a rather subtle semantic. It doesn't affect
>any
>> existing mapping because it only updated mm->def_flags which is a
>template
>> for new mappings. The mappings created after
>prctl(PR_SET_THP_DISABLE) have
>> VM_NOHUGEPAGE flag set.  This can be quite surprising for all those
>> applications which do not do prctl(); fork() & exec() and want to
>control
>> their own THP behavior.
>> 
>> Another usecase when the immediate semantic of the prctl might be
>useful is
>> a combination of pre- and post-copy migration of containers with
>CRIU.  In
>> this case CRIU populates a part of a memory region with data that was
>saved
>> during the pre-copy stage. Afterwards, the region is registered with
>> userfaultfd and CRIU expects to get page faults for the parts of the
>region
>> that were not yet populated. However, khugepaged collapses the pages
>and
>> the expected page faults do not occur.
>> 
>> In more general case, the prctl(PR_SET_THP_DISABLE) could be used as
>a
>> temporary mechanism for enabling/disabling THP process wide.
>> 
>> Implementation wise, a new MMF_DISABLE_THP flag is added. This flag
>is
>> tested when decision whether to use huge pages is taken either during
>page
>> fault of at the time of THP collapse.
>> 
>> It should be noted, that the new implementation makes
>PR_SET_THP_DISABLE
>> master override to any per-VMA setting, which was not the case
>previously.
>>
>> Fixes: a0715cc22601 ("mm, thp: add VM_INIT_DEF_MASK and
>PRCTL_THP_DISABLE")
>
>"Fixes" is a bit strong.  I'd say "alters".  And significantly altering
>the runtime behaviour of a three-year-old interface is rather a worry,
>no?

Well, there are people that consider current behavior as bug :)
One can argue we alter the implementation​details and users should not rely on that...

>Perhaps we should be adding new prctl modes to select this new
>behaviour and leave the existing PR_SET_THP_DISABLE behaviour as-is?


-- 
Sincerely yours,
Mike.