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OjbvubK7UHO2V9ImUwiaMRFrxIqs5/aWLldesQ4hfkObbDJxke0ee0F9ATmWOWgiGxEWtSF3ieD1CksVF83DL6mtY5wdNiagbHsX9JzAvz6c4q0i6F77+vDsIQ0pgoWqTDEGHTOPgqX83m0AXC4oFrnQCAuLSaTE9Lcb/Xb1DIKG7UgWu+DmrzfB8bT0sIs4GPxXPFSikfLMB/6+jQ1cm0JTEann3G5F7lH5t3QacGO1gpG1wFvo4/DdHmrXOEb+O2uQYzNrz/5HvKq4mp7DzECHxoiH4cIYft2P+VJ+3/tQQemanoGXbrhBNsJOju9Y29HUszLETRPLQylgiuTjAMxZw6zBitAvVK4FrovsQ0h4algSiOZmvLcjBKIU5ZzLRGoEOyAdVU0UP3Rngx0KqSlomyrmLtVdlshdl3tjd4uEV+Aa8zoy3NCy0lFTHwr5Xrr6h2W46RBoyh5t3HBYHMDB6rpfQGOqtgsWGirSTX5e4TUQEC3eXJzfYCXDawBEZiOmEdwzAFnQtKmy9l3aAMaU8E+piUTlCZDv3f4TgeJ3bCrny/I6EkLf3i6OoxpQbr07zxNydRHvPnWYZglcpTvs6BpI/oInqScRO5TMDaYPaINo= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 3/7/25 5:41 AM, Lorenzo Stoakes wrote: > On Fri, Mar 07, 2025 at 01:35:00PM +0000, Pedro Falcato wrote: >> On Fri, Mar 7, 2025 at 1:12 PM Lorenzo Stoakes >> wrote: >>> On Thu, Mar 06, 2025 at 01:49:48PM -0800, Yang Shi wrote: >>>> LKP reported 800% performance improvement for small-allocs benchmark >>>> from vm-scalability [1] with patch ("/dev/zero: make private mapping >>>> full anonymous mapping") [2], but the patch was nack'ed since it changes >>>> the output of smaps somewhat. >>> Yeah sorry about that, but unfortunately something we really do have to >>> think about (among other things, the VMA edge cases are always the source >>> of weirdness...) >>> >>>> The profiling shows one of the major sources of the performance >>>> improvement is the less contention to i_mmap_rwsem. >>> Great work tracking that down! Sorry I lost track of the other thread. >>> >>>> The small-allocs benchmark creates a lot of 40K size memory maps by >>>> mmap'ing private /dev/zero then triggers page fault on the mappings. >>>> When creating private mapping for /dev/zero, the anonymous VMA is >>>> created, but it has valid vm_file. Kernel basically assumes anonymous >>>> VMAs should have NULL vm_file, for example, mmap inserts VMA to the file >>>> rmap tree if vm_file is not NULL. So the private /dev/zero mapping >>>> will be inserted to the file rmap tree, this resulted in the contention >>>> to i_mmap_rwsem. But it is actually anonymous VMA, so it is pointless >>>> to insert it to file rmap tree. >>> Ughhhh god haha. >>> >>>> Skip anonymous VMA for this case. Over 400% performance improvement was >>>> reported [3]. >>> That's insane. Amazing work. >>> >> Ok, so the real question (to Yang) is: who are these /dev/zero users >> that require an insane degree of scalability, and why didn't they >> switch to regular MAP_ANONYMOUS? Are they in the room with us? > This could be said about a lot of benchmarks. > >>>> It is not on par with the 800% improvement from the original patch. It is >>>> because page fault handler needs to access some members of struct file >>>> if vm_file is not NULL, for example, f_mode and f_mapping. They are in >>>> the same cacheline with file refcount. When mmap'ing a file the file >>>> refcount is inc'ed and dec'ed, this caused bad cache false sharing >>>> problem. The further debug showed checking whether the VMA is anonymous >>>> or not can alleviate the problem. But I'm not sure whether it is the >>>> best way to handle it, maybe we should consider shuffle the layout of >>>> struct file. >>> Interesting, I guess you'll take a look at this also? >> ... And this is probably a non-issue in 99% of !/dev/zero mmaps unless >> it's something like libc.so.6 at an insane rate of execs/second. > But the cost of fixing this is...? > >> This seems like a patch in search of a problem and I really don't see >> why we should wart up the mmap code otherwise. Not that I have a huge >> problem with this patch, which is somewhat simple and obvious. >> It'd be great if there was a real workload driving this rather than >> useless synthetic benchmarks. > Disagree with first part. Disallowing a known-broken situation for very low > cost in the majority of cases, as well as documenting that such odd > creatures exist is valuable. Yes, I agree. This is one of the points of this patch too. The commit log may focus too much on the benchmark improvement. I will add more words about fixing the special odd behavior. > > Improving benchmarks, however synthetic they may be, also valuable. > > But on the latter bit, yes it'd be nice if we could get information on > real-life scenarios where this is an issue if you have it Yang. I wish I could. As I said in the reply to Pedro, creating anonymous mapping via mmap'ing private /dev/zero is an established way. So we can not rule out it is *NOT* used. Thanks, Yang > >> -- >> Pedro > The patch is fine as-is AFAIC, and I am very happy to reduce lock > contention on heavily contested locks wherever I can, especially when the > cost for doing so, in this case, is so low.