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On Tue, Jun 17, 2025 at 09:34:16AM +0100, Pedro Falcato wrote: > On Mon, Jun 16, 2025 at 10:41:20PM +0200, David Hildenbrand wrote: > > On 16.06.25 22:24, David Hildenbrand wrote: > > > Hi Lorenzo, > > > > > > as discussed offline, there is a lot going on an this is rather ... a > > > lot of code+complexity for something that is more a corner cases. :) > > > > > > Corner-case as in: only select user space will benefit from this, which > > > is really a shame. > > > > > > After your presentation at LSF/MM, I thought about this further, and I > > > was wondering whether: > > > > > > (a) We cannot make this semi-automatic, avoiding flags. > > > > > > (b) We cannot simplify further by limiting it to the common+easy cases > > > first. > > > > > > I think you already to some degree did b) as part of this non-RFC, which > > > is great. > > > > > > > > > So before digging into the details, let's discuss the high level problem > > > briefly. > > > > > > I think there are three parts to it: > > > > > > (1) Detecting whether it is safe to adjust the folio->index (small > > > folios) > > > > > > (2) Performance implications of doing so > > > > > > (3) Detecting whether it is safe to adjust the folio->index (large PTE- > > > mapped folios) > > > > > > > > > Regarding (1), if we simply track whether a folio was ever used for > > > COW-sharing, it would be very easy: and not only for present folios, but > > > for any anon folios that are referenced by swap/migration entries. > > > Skimming over patch #1, I think you apply a similar logic, which is good. > > > > > > Regarding (2), it would apply when we mremap() anon VMAs and they happen > > > to reside next to other anon VMAs. Which workloads are we concerned > > > about harming by implementing this optimization? I recall that the most > > > common use case for mremap() is actually for file mappings, but I might > > realloc() for mmapped allocations commonly calls mremap(), FYI (at least for > glibc, and musl; can't bother to look at the rest). > > > > be wrong. In any case, we could just have a different way to enable this > > > optimization than for each and every mremap() invocation in a process. > > /me thinks of prctl > > :P God please :P > > > FWIW, with regards to the whole feature: While I do understand it's purpose ( > relocating anon might be too much for most workloads, but great for some), I'm > uncomfortable with the amount of internals we're exposing here. Who's to say > this is how mm rmap looks in 20 years? And we're stuck maintaining the userspace > ABI until then. I'm not sure what internals exactly we're exposing... if we have a future (I am working on it...) where anon rmap works better then these flags become no-ops right? Unless you just mean the name? I am open to changing the name but I think I already changed it based on what I thought David might say :P > > Personally, I would prefer if we just had a flag 'MREMAP_HARDER' that would > vaguely be documented as "mremap but harder, even if have to do a little more > work". Then we could move things around without promising RELOCATE_ANON makes > conceptual sense, and userspace wouldn't have to think through the implications > of such a flag by reading Lorenzo's great book. Well thanks for book plug ;) I think that's far too vague though. I don't think users would have any idea what that's supposed to help. And then how does the MREMAP_MUST_RELOCATE_ANON flag work here? Naming is hard, basically. > > -- > Pedro >