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Date: Fri, 23 Dec 2016 14:46:43 -0800 (PST) From: David Rientjes <rientjes@...gle.com> To: Michal Hocko <mhocko@...nel.org> cc: Andrew Morton <akpm@...ux-foundation.org>, Jonathan Corbet <corbet@....net>, "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>, Vlastimil Babka <vbabka@...e.cz>, Mel Gorman <mgorman@...hsingularity.net>, linux-kernel@...r.kernel.org, linux-mm@...ck.org Subject: Re: [patch] mm, thp: always direct reclaim for MADV_HUGEPAGE even when deferred On Fri, 23 Dec 2016, Michal Hocko wrote: > > We have no way to compact memory for users who are not using > > MADV_HUGEPAGE, > > yes we have. it is defrag=always. If you do not want direct compaction > and the resulting allocation stalls then you have to rely on kcompactd > which is something we should work longterm. > No, the point of madvise(MADV_HUGEPAGE) is for applications to tell the kernel that they really want hugepages. Really. Everybody else either never did direct compaction or did a substantially watered down version of it. Now, we have a situation where you can either do direct compaction for MADV_HUGEPAGE and nothing for anybody else, or direct compaction for everybody. In our usecase, we want everybody to kick off background compaction because order=9 gfp_mask & __GFP_KSWAPD_RECLAIM is the only thing that is going to trigger background compaction but are unable to do so without still incurring lengthy pagefaults for non MADV_HUGEPAGE users. > > which is some customers, others require MADV_HUGEPAGE for > > .text segment remap while loading their binary, without defrag=always or > > defrag=defer. The problem is that we want to demand direct compact for > > MADV_HUGEPAGE: they _really_ want hugepages, it's the point of the > > madvise. > > and that is the point of defrag=madvise to give them this direct > compaction. > Do you see the problem by first suggesting defrag=always at the top of your reply and then defrag=madvise now? We cannot set both at once, it's the entire problem with the tristate and now quadstate setting. We want a combination: EVERYBODY kicks off background compaction and applications that really want hugepages and are fine with incuring lengthy page fault, such as those (for the third time) remapping .text segment and doing madvise(MADV_HUGEPAGE) before fault, can use the madvise. > > We have no setting, without this patch, to ask for background > > compaction for everybody so that their fault does not have long latency > > and for some customers to demand compaction. > > that is true and what I am trying to say is that we should aim to give > this background compaction for everybody via kcompactd because there are > more users than THP who might benefit from low latency high order pages > availability. My patch does that, we _defer_ for everybody unless you're using madvise(MADV_HUGEPAGE) and really want hugepages. Forget defrag=never exists, it's not important in the discussion. Forget defrag=always exists because all apps, like batch jobs, don't want lengthy pagefaults. We have two options remaining: - defrag=defer: everybody kicks off background compaction, _nobody_ does direct compaction - defrag=madvise: madvise(MADV_HUGEPAGE) does direct compaction, everybody else does nothing The point you're missing is that we _want_ defrag=defer. We really do. We don't want to stall in the page allocator to get thp, but we want to try to make it available in the short term. However, apps that do madvise(MADV_HUGEPAGE), like remapping your .text segment and wanting your text backed by hugepages and incurring the expense up front, or a database, or a vm, _want_ hugepages now and don't care about lengthy page faults. The point is that I HAVE NO SETTING to get that behavior and defrag=madvise is _not_ a solution because it requires the presence of an app that is doing madvise(MADV_HUGEPAGE) AND faulting memory to get any order=9 compaction. > > ?????? Why does the admin care if a user's page fault wants to reclaim to > > get high order memory? > > Because the whole point of the defrag knob is to allow _administrator_ > control how much we try to fault in THP. And the primary motivation were > latencies. The whole point of introducing defer option was to _never_ > stall in the page fault while it still allows to kick the background > compaction. If you really want to tweak any option then madvise would be > more appropriate IMHO because the semantic would be still clear. Use > direct compaction for MADV_HUGEPAGE vmas and kick in kswapd/kcompactd > for others. > You want defrag=madvise to start doing background compaction for everybody, which was never done before for existing users of defrag=madvise? That might be possible, I don't really care, I just think it's riskier because there are existing users of defrag=madvise who are opting in to new behavior because of the kernel change. This patch changes defrag=defer because it's the new option and people setting the mode know what they are getting. I disagree with your description of what the defrag setting is intended for. The setting of thp defrag is to optimize for apps that truly want transparent behavior, i.e. they aren't doing madvise(MADV_HUGEPAGE). Are they willing to incur lengthy pagefaults for thp when not doing any madvise(2)? defrag=defer should not mean that users of madvise(MADV_HUGEPAGE) that have clearly specified their intent should not be allowed to try compacting memory themselves because they have indicated they are fine with such an expense by doing the madvise(2). This is obviously fine for Kirill, and I have users who remap their .text segment and do madvise(MADV_DONTNEED) because they really want hugepages when they are exec'd, so I'd kindly ask you to consider the real-world use cases that require background compaction to make hugepages available for everybody but allow apps to opt-in to take the expense of compaction on themselves rather than your own theory of what users want.
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