Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE

From	Vlastimil Babka <vbabka@suse.cz>
Newsgroups	linux.kernel
Subject	Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE
Date	2017-05-31 08:40 +0200
Message-ID	<tN5st-1db-13@gated-at.bofh.it> (permalink)
References	(6 earlier) <tMMSS-6dU-23@gated-at.bofh.it> <tMQ0q-8oA-17@gated-at.bofh.it> <tMQD7-ar-3@gated-at.bofh.it> <tMQMO-dW-9@gated-at.bofh.it> <tMRSy-16W-19@gated-at.bofh.it>
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On 05/30/2017 06:06 PM, Andrea Arcangeli wrote:
> 
> I'm not sure if it should be considered a bug, the prctl is intended
> to use normally by wrappers so it looks optimal as implemented this
> way: affecting future vmas only, which will all be created after
> execve executed by the wrapper.
> 
> What's the point of messing with the prctl so it mangles over the
> wrapper process own vmas before exec? Messing with those vmas is pure
> wasted CPUs for the wrapper use case which is what the prctl was
> created for.
> 
> Furthermore there would be the risk a program that uses the prctl not
> as a wrapper and then calls the prctl to clear VM_NOHUGEPAGE from
> def_flags assuming the current kABI. The program could assume those
> vmas that were instantiated before disabling the prctl are still with
> VM_NOHUGEPAGE set (they would not after the change you propose).
> 
> Adding a scan of all vmas to PR_SET_THP_DISABLE to clear VM_NOHUGEPAGE
> on existing vmas looks more complex too and less finegrined so
> probably more complex for userland to manage

I would expect the prctl wouldn't iterate all vma's, nor would it modify
def_flags anymore. It would just set a flag somewhere in mm struct that
would be considered in addition to the per-vma flags when deciding
whether to use THP. We could consider whether MADV_HUGEPAGE should be
able to override the prctl or not.

> but ignoring all above
> considerations it would be a functional alternative for CRIU's
> needs. However if you didn't like the complexity of the new madvise
> which is functionally a one-liner equivalent to MADV_NORMAL, I
> wouldn't expect you to prefer to make the prctl even more complex with
> a loop over all vmas that despite being fairly simple it'll still be
> more than a trivial one liner.
> 
> Thanks,
> Andrea
> 
> --
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Thread

Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE Michal Hocko <mhocko@kernel.org> - 2017-05-30 09:50 +0200
  Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE Mike Rapoport <rppt@linux.vnet.ibm.com> - 2017-05-30 12:20 +0200
    Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE Michal Hocko <mhocko@kernel.org> - 2017-05-30 12:50 +0200
      Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE Andrea Arcangeli <aarcange@redhat.com> - 2017-05-30 16:10 +0200
        Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE Michal Hocko <mhocko@kernel.org> - 2017-05-30 16:50 +0200
          Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE Michal Hocko <mhocko@kernel.org> - 2017-05-30 17:00 +0200
            Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE Andrea Arcangeli <aarcange@redhat.com> - 2017-05-30 18:10 +0200
              Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE Vlastimil Babka <vbabka@suse.cz> - 2017-05-31 08:40 +0200
                Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE Michal Hocko <mhocko@kernel.org> - 2017-05-31 10:30 +0200
                Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE Mike Rapoport <rppt@linux.vnet.ibm.com> - 2017-05-31 11:30 +0200
                Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE Michal Hocko <mhocko@kernel.org> - 2017-05-31 12:30 +0200
                Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE Michal Hocko <mhocko@kernel.org> - 2017-05-31 12:30 +0200
                Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE Mike Rapoport <rppt@linux.vnet.ibm.com> - 2017-06-01 13:10 +0200
                Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE Michal Hocko <mhocko@kernel.org> - 2017-06-01 14:30 +0200
          Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE Andrea Arcangeli <aarcange@redhat.com> - 2017-05-30 17:50 +0200
            Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE Michal Hocko <mhocko@kernel.org> - 2017-05-31 14:10 +0200
              Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE Mike Rapoprt <rppt@linux.vnet.ibm.com> - 2017-05-31 14:40 +0200
                Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE Andrea Arcangeli <aarcange@redhat.com> - 2017-05-31 16:20 +0200
                Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE Michal Hocko <mhocko@kernel.org> - 2017-05-31 16:40 +0200
                Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE Andrea Arcangeli <aarcange@redhat.com> - 2017-05-31 17:50 +0200
                Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE Mike Rapoport <rppt@linux.vnet.ibm.com> - 2017-06-01 09:00 +0200
                Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE Michal Hocko <mhocko@kernel.org> - 2017-05-31 16:20 +0200
          Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE Mike Rapoport <rppt@linux.vnet.ibm.com> - 2017-06-01 09:00 +0200
            Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE Michal Hocko <mhocko@kernel.org> - 2017-06-01 10:10 +0200
              Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE Mike Rapoport <rppt@linux.vnet.ibm.com> - 2017-06-01 10:40 +0200
              Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE Andrea Arcangeli <aarcange@redhat.com> - 2017-06-01 15:50 +0200
                Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE Mike Rapoport <rppt@linux.vnet.ibm.com> - 2017-06-02 11:20 +0200
      Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE Mike Rapoport <rppt@linux.vnet.ibm.com> - 2017-05-31 11:10 +0200
        Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE Michal Hocko <mhocko@kernel.org> - 2017-05-31 14:10 +0200
          Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE Mike Rapoprt <rppt@linux.vnet.ibm.com> - 2017-05-31 14:30 +0200

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