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

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Message-Id: <20170602125059.66209870607085b84c257593@linux-foundation.org>
Date:   Fri, 2 Jun 2017 12:50:59 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     "Mike Rapoport" <rppt@...ux.vnet.ibm.com>
Cc:     Linux API <linux-api@...r.kernel.org>,
        Michal Hocko <mhocko@...nel.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        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 Fri,  2 Jun 2017 18:03:22 +0300 "Mike Rapoport" <rppt@...ux.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?

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