Re: [PATCH v2 0/3] Create 'old' ptes for faultaround mappings on arm64 with hardware access flag
On Tue, Jan 12, 2021 at 1:48 PM Will Deacon wrote: > > > And then if somebody actually needs to pass in "modified fault state" > > (ie that whole "I'm doing fault-around, so I'll use multiple > > addresses") they'd never modify the address in the fault info, they'd > > just pass the address as an explicit argument (like most cases already > > do - the "change addr or flags in vmf" is actually already _fairly_ > > rare). > > Alright then, I'll take another crack at this for v3 and see how far I > get. But please do keep it as a separate patch, and if it turns out to be ugly for whatever reason, treat it as just a "maybe that would help" suggestion rather than anything stronger.. Linus
Re: [PATCH v2 0/3] Create 'old' ptes for faultaround mappings on arm64 with hardware access flag
On Mon, Jan 11, 2021 at 11:25:37AM -0800, Linus Torvalds wrote: > On Mon, Jan 11, 2021 at 6:24 AM Kirill A. Shutemov > wrote: > > > > I wonder if it would be acceptable to pass down to faultaround a copy > > of vmf, so it mess with it without risking to corrupt the original one? > > I'd almost prefer to split vmf into two parts: the 'this is the fault > info' part and the 'this is the fault handling state' part. > > So the first one would be filled in by the actual page faulter (or > GUP) - and then be 'const' during the lookup, while the second one > would be set up by handle_mm_fault() and would contain that "this is > the current state of my fault state machine" and contain things like > that ->pte thing. > > And then if somebody actually needs to pass in "modified fault state" > (ie that whole "I'm doing fault-around, so I'll use multiple > addresses") they'd never modify the address in the fault info, they'd > just pass the address as an explicit argument (like most cases already > do - the "change addr or flags in vmf" is actually already _fairly_ > rare). Alright then, I'll take another crack at this for v3 and see how far I get. Will
Re: [PATCH v2 0/3] Create 'old' ptes for faultaround mappings on arm64 with hardware access flag
On Mon, Jan 11, 2021 at 01:03:29PM -0800, Hugh Dickins wrote: > On Mon, 11 Jan 2021, Will Deacon wrote: > > On Fri, Jan 08, 2021 at 11:34:08AM -0800, Linus Torvalds wrote: > > > On Fri, Jan 8, 2021 at 9:15 AM Will Deacon wrote: > > > > > > > > The big difference in this version is that I have reworked it based on > > > > Kirill's patch which he posted as a follow-up to the original. However, > > > > I can't tell where we've landed on that -- Linus seemed to like it, but > > > > Hugh was less enthusiastic. > > > > > > Yeah, I like it, but I have to admit that it had a disturbingly high > > > number of small details wrong for several versions. I hope you picked > > > up the final version of the code. > > > > I picked the version from here: > > > > https://lore.kernel.org/r/20201229132819.najtavneutnf7ajp@box > > > > and actually, I just noticed that willy spotted a typo in a comment, so > > I'll fix that locally as well as adding the above to a 'Link:' tag for > > reference. > > > > > At the same time, I do think that the "disturbingly high number of > > > issues" was primarily exactly _because_ the old code was so > > > incomprehensible, and I think the end result is much cleaner, so I > > > still like it. > > Just to report that I gave this v2 set a spin on a few (x86_64 and i386) > machines, and found nothing objectionable this time around. Thanks, Hugh. > And the things that I'm unenthusiastic about are exactly those details > that you and Kirill and Linus find unsatisfactory, but awkward to > eliminate: expect no new insights from me! Well, I'll keep you on CC for v3 -- just in case! Will
Re: [PATCH v2 0/3] Create 'old' ptes for faultaround mappings on arm64 with hardware access flag
On Mon, 11 Jan 2021, Will Deacon wrote: > On Fri, Jan 08, 2021 at 11:34:08AM -0800, Linus Torvalds wrote: > > On Fri, Jan 8, 2021 at 9:15 AM Will Deacon wrote: > > > > > > The big difference in this version is that I have reworked it based on > > > Kirill's patch which he posted as a follow-up to the original. However, > > > I can't tell where we've landed on that -- Linus seemed to like it, but > > > Hugh was less enthusiastic. > > > > Yeah, I like it, but I have to admit that it had a disturbingly high > > number of small details wrong for several versions. I hope you picked > > up the final version of the code. > > I picked the version from here: > > https://lore.kernel.org/r/20201229132819.najtavneutnf7ajp@box > > and actually, I just noticed that willy spotted a typo in a comment, so > I'll fix that locally as well as adding the above to a 'Link:' tag for > reference. > > > At the same time, I do think that the "disturbingly high number of > > issues" was primarily exactly _because_ the old code was so > > incomprehensible, and I think the end result is much cleaner, so I > > still like it. Just to report that I gave this v2 set a spin on a few (x86_64 and i386) machines, and found nothing objectionable this time around. And the things that I'm unenthusiastic about are exactly those details that you and Kirill and Linus find unsatisfactory, but awkward to eliminate: expect no new insights from me! Hugh
Re: [PATCH v2 0/3] Create 'old' ptes for faultaround mappings on arm64 with hardware access flag
On Mon, Jan 11, 2021 at 6:24 AM Kirill A. Shutemov wrote: > > I wonder if it would be acceptable to pass down to faultaround a copy > of vmf, so it mess with it without risking to corrupt the original one? I'd almost prefer to split vmf into two parts: the 'this is the fault info' part and the 'this is the fault handling state' part. So the first one would be filled in by the actual page faulter (or GUP) - and then be 'const' during the lookup, while the second one would be set up by handle_mm_fault() and would contain that "this is the current state of my fault state machine" and contain things like that ->pte thing. And then if somebody actually needs to pass in "modified fault state" (ie that whole "I'm doing fault-around, so I'll use multiple addresses") they'd never modify the address in the fault info, they'd just pass the address as an explicit argument (like most cases already do - the "change addr or flags in vmf" is actually already _fairly_ rare). Linus
Re: [PATCH v2 0/3] Create 'old' ptes for faultaround mappings on arm64 with hardware access flag
On Fri, Jan 08, 2021 at 11:34:08AM -0800, Linus Torvalds wrote: > I still dislike how we basically randomly modify the information in > that 'vmf' thing. I wounder if it would be acceptable to pass down to faultaround a copy of vmf, so it mess with it without risking to corrupt the original one? We would need to transfer back prealloc_pte, but the rest can be safely discarded, I believe. But it's somewhat wasteful of stack. sizeof(vmf) == 96 on x86-64. -- Kirill A. Shutemov
Re: [PATCH v2 0/3] Create 'old' ptes for faultaround mappings on arm64 with hardware access flag
On Fri, Jan 08, 2021 at 11:42:39AM -0800, Linus Torvalds wrote: > On Fri, Jan 8, 2021 at 11:34 AM Linus Torvalds > wrote: > > > > Yeah, I think that's a side effect of "now the code really makes a lot > > more sense". Your subsequent patches 2-3 certainly are much simpler > > now > > On that note - they could be simpler still if this was just done > entirely unconditionally.. > > I'm taking your word for "it makes sense", but when you say > > On CPUs with hardware AF/DBM, initialising prefaulted PTEs as 'old' > improves vmscan behaviour and does not appear to introduce any overhead. > > in the description for patch 3, it makes me wonder how noticeable the > overhead is on the hardware that _does_ take a fault on old pte's.. > > IOW, it would be lovely to see numbers if you have any like that.. [Vinayak -- please chime in if I miss anything here, as you've posted these numbers before] The initial posting in 2016 had some numbers based on a 3.18 kernel, which didn't have support for hardware AF/DBM: https://lore.kernel.org/lkml/fdc23a2a-b42a-f0af-d403-41ea4e755...@codeaurora.org (note that "Kirill's-fix" in the last column was a quick hack and didn't make the faulting pte young) So yes, for the cases we care about in Android (where the vmscan behaviour seems to be the important thing), then this patch makes sense for non-hardware AF/DBM CPUs too. In either case, we see ~80% reduction in direct reclaim time according to mmtests [1] and double-digit percentage reductions in app launch latency (some of this is mentioned in the link above). The actual fault cost isn't especially relevant. *However...* For machines with lots of memory, the increased fault cost when hardware AF/DBM is not available may well be measurable, and I suspect it would hurt unixbench (which was the reason behind reverting this on x86 [2], although I must admit that the diagnosis wasn't particularly satisfactory [3]). We could run those numbers on arm64 but, due to the wide diversity of micro-architectures we have to deal with, I would like to keep our options open to detecting this dynamically anyway, just in case somebody builds a CPU which struggles in this area. Cheers, Will [1] https://github.com/gormanm/mmtests [2] https://lore.kernel.org/lkml/20160613125248.ga30...@black.fi.intel.com/ [3] https://lore.kernel.org/lkml/20160616151049.gm6...@dhcp22.suse.cz/
Re: [PATCH v2 0/3] Create 'old' ptes for faultaround mappings on arm64 with hardware access flag
On Fri, Jan 08, 2021 at 11:34:08AM -0800, Linus Torvalds wrote: > On Fri, Jan 8, 2021 at 9:15 AM Will Deacon wrote: > > > > The big difference in this version is that I have reworked it based on > > Kirill's patch which he posted as a follow-up to the original. However, > > I can't tell where we've landed on that -- Linus seemed to like it, but > > Hugh was less enthusiastic. > > Yeah, I like it, but I have to admit that it had a disturbingly high > number of small details wrong for several versions. I hope you picked > up the final version of the code. I picked the version from here: https://lore.kernel.org/r/20201229132819.najtavneutnf7ajp@box and actually, I just noticed that willy spotted a typo in a comment, so I'll fix that locally as well as adding the above to a 'Link:' tag for reference. > At the same time, I do think that the "disturbingly high number of > issues" was primarily exactly _because_ the old code was so > incomprehensible, and I think the end result is much cleaner, so I > still like it. > > >I think that my subsequent patches are an > > awful lot cleaner after the rework > > Yeah, I think that's a side effect of "now the code really makes a lot > more sense". Your subsequent patches 2-3 certainly are much simpler > now, although I'd be inclined to add an argument to "do_set_pte()" > that has the "write" and "pretault" bits in it, instead of having to > modify the 'vmf' structure. I played with a few different ways of doing this, but I can't say I prefer them over what I ended up posting. Having a bunch of 'bool' arguments makes the callers hard to read and brings into question what exactly vmf->flags is for. I also tried adding a separate 'address' parameter so that vmf->address is always the real faulting address, but 'address' is the thing to use for the pte (i.e. prefault is when 'address != vmf->address'). That wasn't too bad, but it made the normal finish_fault() case look weird. So I think I'll leave it as-is and see if anybody wants to change it later on. Will
Re: [PATCH v2 0/3] Create 'old' ptes for faultaround mappings on arm64 with hardware access flag
On Fri, Jan 8, 2021 at 11:34 AM Linus Torvalds wrote: > > Yeah, I think that's a side effect of "now the code really makes a lot > more sense". Your subsequent patches 2-3 certainly are much simpler > now On that note - they could be simpler still if this was just done entirely unconditionally.. I'm taking your word for "it makes sense", but when you say On CPUs with hardware AF/DBM, initialising prefaulted PTEs as 'old' improves vmscan behaviour and does not appear to introduce any overhead. in the description for patch 3, it makes me wonder how noticeable the overhead is on the hardware that _does_ take a fault on old pte's.. IOW, it would be lovely to see numbers if you have any like that.. Both ways, actually. Because I also wonder how noticeable the vmscan improvement is. You say there's no measurable overhead for platforms with hardware dirty/accessed bits, but maybe there's not a lot of measurable improvements from a more exact accessed bit either? Linus
Re: [PATCH v2 0/3] Create 'old' ptes for faultaround mappings on arm64 with hardware access flag
On Fri, Jan 8, 2021 at 9:15 AM Will Deacon wrote: > > The big difference in this version is that I have reworked it based on > Kirill's patch which he posted as a follow-up to the original. However, > I can't tell where we've landed on that -- Linus seemed to like it, but > Hugh was less enthusiastic. Yeah, I like it, but I have to admit that it had a disturbingly high number of small details wrong for several versions. I hope you picked up the final version of the code. At the same time, I do think that the "disturbingly high number of issues" was primarily exactly _because_ the old code was so incomprehensible, and I think the end result is much cleaner, so I still like it. >I think that my subsequent patches are an > awful lot cleaner after the rework Yeah, I think that's a side effect of "now the code really makes a lot more sense". Your subsequent patches 2-3 certainly are much simpler now, although I'd be inclined to add an argument to "do_set_pte()" that has the "write" and "pretault" bits in it, instead of having to modify the 'vmf' structure. I still dislike how we basically randomly modify the information in that 'vmf' thing. That said, now it's just a small detail - not really objectionable, just a "this could be cleaner, I think". I think it was Kirill who pointed out that we sadly cannot make 'vmf' read-only anyway, because it does also contain those pre-allocation details etc (vmf->pte etc) that are very much about what the current "state" of the fault is. So while I would hope it could be more read-only than it is, my wish that it could _actually_ be 'const' is clearly just an irrelevant dream. Linus
