From mboxrd@z Thu Jan 1 00:00:00 1970 From: Michal Hocko Subject: Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE Date: Thu, 1 Jun 2017 14:27:04 +0200 Message-ID: <20170601122703.GB9091@dhcp22.suse.cz> References: <20170530074408.GA7969@dhcp22.suse.cz> <20170530101921.GA25738@rapoport-lnx> <20170530103930.GB7969@dhcp22.suse.cz> <20170530140456.GA8412@redhat.com> <20170530143941.GK7969@dhcp22.suse.cz> <20170530145632.GL7969@dhcp22.suse.cz> <20170530160610.GC8412@redhat.com> <20170531082414.GB27783@dhcp22.suse.cz> <20170601110048.GE30495@rapoport-lnx> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <20170601110048.GE30495@rapoport-lnx> Sender: owner-linux-mm@kvack.org To: Mike Rapoport Cc: Vlastimil Babka , Andrea Arcangeli , "Kirill A. Shutemov" , Andrew Morton , Arnd Bergmann , "Kirill A. Shutemov" , Pavel Emelyanov , linux-mm , lkml , Linux API List-Id: linux-api@vger.kernel.org On Thu 01-06-17 14:00:48, Mike Rapoport wrote: > On Wed, May 31, 2017 at 10:24:14AM +0200, Michal Hocko wrote: > > On Wed 31-05-17 08:30:08, Vlastimil Babka wrote: > > > 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. > > > > Exactly. Something like the below (not even compile tested). > > I did a quick go with the patch, compiles just fine :) > It worked for my simple examples, the THP is enabled/disabled as expected > and the vma->vm_flags are indeed unaffected. > > > > We could consider whether MADV_HUGEPAGE should be > > > able to override the prctl or not. > > > > This should be a master override to any per vma setting. > > Here you've introduced a change to the current behaviour. Consider the > following sequence: > > { > prctl(PR_SET_THP_DISABLE); > address = mmap(...); > madvise(address, len, MADV_HUGEPAGE); > } > > Currently, for the vma that backs the address > transparent_hugepage_enabled(vma) will return true, and after your patch it > will return false. > The new behaviour may be more correct, I just wanted to bring the change to > attention. The system wide disable should override any VMA specific setting IMHO. Why would we disable the THP for the whole process otherwise? Anyway this needs to be discussed at linux-api mailing list. I will try to make my change into a proper patch and post it there. -- Michal Hocko SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org