On 5/2/23 10:57 AM, David Hildenbrand wrote: > On 02.05.23 15:35, Matthew Rosato wrote: >> On 5/2/23 9:04 AM, Christian Borntraeger wrote: >>> >>> >>> Am 02.05.23 um 14:54 schrieb Lorenzo Stoakes: >>>> On Tue, May 02, 2023 at 02:46:28PM +0200, Christian Borntraeger wrote: >>>>> Am 02.05.23 um 01:11 schrieb Lorenzo Stoakes: >>>>>> Writing to file-backed dirty-tracked mappings via GUP is inherently broken >>>>>> as we cannot rule out folios being cleaned and then a GUP user writing to >>>>>> them again and possibly marking them dirty unexpectedly. >>>>>> >>>>>> This is especially egregious for long-term mappings (as indicated by the >>>>>> use of the FOLL_LONGTERM flag), so we disallow this case in GUP-fast as >>>>>> we have already done in the slow path. >>>>> >>>>> Hmm, does this interfer with KVM on s390 and PCI interpretion of interrupt delivery? >>>>> It would no longer work with file backed memory, correct? >>>>> >>>>> See >>>>> arch/s390/kvm/pci.c >>>>> >>>>> kvm_s390_pci_aif_enable >>>>> which does have >>>>> FOLL_WRITE | FOLL_LONGTERM >>>>> to >>>>> >>>> >>>> Does this memory map a dirty-tracked file? It's kind of hard to dig into where >>>> the address originates from without going through a ton of code. In worst case >>>> if the fast code doesn't find a whitelist it'll fall back to slow path which >>>> explicitly checks for dirty-tracked filesystem. >>> >>> It does pin from whatever QEMU uses as backing for the guest. >>>> >>>> We can reintroduce a flag to permit exceptions if this is really broken, are you >>>> able to test? I don't have an s390 sat around :) >>> >>> Matt (Rosato on cc) probably can. In the end, it would mean having >>> <memoryBacking> >>> <source type="file"/> >>> </memoryBacking> >>> >>> In libvirt I guess. >> >> I am running with this series applied using a QEMU guest with memory-backend-file (using the above libvirt snippet) for a few different PCI device types and AEN forwarding (e.g. what is setup in kvm_s390_pci_aif_enable) is still working. >> > > That's ... unexpected. :) > > Either this series doesn't work as expected or you end up using a filesystem that is still compatible. But I guess most applicable filesystems (ext4, btrfs, xfs) all have a page_mkwrite callback and should, therefore, disallow long-term pinning with this series. > The memory backend file is on ext4 in my tests. A quick trace shows that pin_user_pages_fast(FOLL_WRITE | FOLL_LONGTERM) in kvm_s390_pci_aif_enable is still returning positive implying pages are being pinned.
