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X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: * Nikita Kalyazin [250918 13:16]: ... > > > > > > Nevertheless, let's step back for a second and instead focus on the problem > > > these patches are trying to solve, which is to allow guest_memfd implement > > > UFFD_CONTINUE (or minor fault in other terminology). > > > > Well, this is about modularizing memory types, but the first user is > > supposed to be the guest-memfd support. > > > > > > > > This means uffd should be able to map a folio that's already in > > > guest_memfd page cache to the faulted address. Obviously, the page table > > > update happens in uffd. But it still has to find what to map and we need > > > some way to let guest_memfd tell that to uffd. > > > > > > So we need a hook somewhere that will return a folio matching pgoff in > > > vma->file->inode. > > > > > > Do you see a way to implement it otherwise? > > > > I must be missing something. > > > > UFFDIO_CONTINUE currently enters through an ioctl that calls > > userfaultfd_continue() -> mfill_atomic_continue()... mfill_atomic() gets > > and uses the folio to actually do the work. Right now, we don't hand > > out the folio, so what is different here? > > > > I am under the impression that we don't need to return the folio, but > > may need to do work on it. That is, we can give the mm side what it > > needs to call the related memory type functions to service the request. > > > > For example, one could pass in the inode, pgoff, and memory type and the > > mm code could then call the fault handler for that memory type? > > > > I didn't think Nikita had a folio returned in his first three patches > > [1], but then they built on other patches and it was difficult to follow > > along. Is it because that interface was agreed on in a call on 23 Jan > > 2025 [2], as somewhat unclearly stated in [1]? > > I believe you can safely ignore what was discussed in [2] as it is > irrelevant to this discussion. That was just reasoning why it was possible > to use UserfaultFD for guest_memfd as opposed to inventing an alternative > solution to handling faults in userspace. > > Regarding returning a folio, [1] was calling vm_ops->fault() in UserfaultFD > code. The fault() itself gets a folio (at least in guest_memfd > implementation [3]). Does it look like a preferable solution to you? I think this answers my question.. but I want to be sure. Does that mean you were getting the folio and doing the work in uffd without returning the uffd? I tried to get those patches, but they didn't apply for me. What I want to do is limit the "memory type" that we support by restricting what is done to service the fault, and handle that in mm code (mm/uffd.c or whatever). What we get is more people using the same fault handler and thus more eyes and testing. Less code duplication. Unless there is a technical reason we need more flexibility? > > The other patches it I was building on top were mmap support in guest_memfd > [4], which is currently merged in kvm/next, and also part of [3]. Can we process it in the mm without returning the folio like the ioctl does today, or is there a technical reason that won't work? Thanks, Liam