On Fri, Mar 17, 2023 at 3:41 PM Alex Williamson <alex.williamson@xxxxxxxxxx> wrote: > > On Fri, 17 Mar 2023 12:08:32 -0700 > Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote: > > > On Tue, Mar 14, 2023 at 1:11 PM Alex Williamson > > <alex.williamson@xxxxxxxxxx> wrote: > > > > > > On Thu, 26 Jan 2023 11:37:45 -0800 > > > Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote: > > > > > > > This patchset was originally published as a part of per-VMA locking [1] and > > > > was split after suggestion that it's viable on its own and to facilitate > > > > the review process. It is now a preprequisite for the next version of per-VMA > > > > lock patchset, which reuses vm_flags modifier functions to lock the VMA when > > > > vm_flags are being updated. > > > > > > > > VMA vm_flags modifications are usually done under exclusive mmap_lock > > > > protection because this attrubute affects other decisions like VMA merging > > > > or splitting and races should be prevented. Introduce vm_flags modifier > > > > functions to enforce correct locking. > > > > > > > > The patchset applies cleanly over mm-unstable branch of mm tree. > > > > > > With this series, vfio-pci developed a bunch of warnings around not > > > holding the mmap_lock write semaphore while calling > > > io_remap_pfn_range() from our fault handler, vfio_pci_mmap_fault(). > > > > > > I suspect vdpa has the same issue for their use of remap_pfn_range() > > > from their fault handler, JasonW, MST, FYI. > > > > > > It also looks like gru_fault() would have the same issue, Dimitri. > > > > > > In all cases, we're preemptively setting vm_flags to what > > > remap_pfn_range_notrack() uses, so I thought we were safe here as I > > > specifically remember trying to avoid changing vm_flags from the > > > fault handler. But apparently that doesn't take into account > > > track_pfn_remap() where VM_PAT comes into play. > > > > > > The reason for using remap_pfn_range() on fault in vfio-pci is that > > > we're mapping device MMIO to userspace, where that MMIO can be disabled > > > and we'd rather zap the mapping when that occurs so that we can sigbus > > > the user rather than allow the user to trigger potentially fatal bus > > > errors on the host. > > > > > > Peter Xu has suggested offline that a non-lazy approach to reinsert the > > > mappings might be more inline with mm expectations relative to touching > > > vm_flags during fault. What's the right solution here? Can the fault > > > handling be salvaged, is proactive remapping the right approach, or is > > > there something better? Thanks, > > > > Hi Alex, > > If in your case it's safe to change vm_flags without holding exclusive > > mmap_lock, maybe you can use __vm_flags_mod() the way I used it in > > https://lore.kernel.org/all/20230126193752.297968-7-surenb@xxxxxxxxxx, > > while explaining why this should be safe? > > Hi Suren, > > Thanks for the reply, but I'm not sure I'm following. Are you > suggesting a bool arg added to io_remap_pfn_range(), or some new > variant of that function to conditionally use __vm_flags_mod() in place > of vm_flags_set() across the call chain? Thanks, I think either way could work but after taking a closer look, both ways would be quite ugly. If we could somehow identify that we are handling a page fault and use __vm_flags_mod() without additional parameters it would be more palatable IMHO... Peter's suggestion to avoid touching vm_flags during fault would be much cleaner but I'm not sure how easily that can be done. > > Alex > > -- > To unsubscribe from this group and stop receiving emails from it, send an email to kernel-team+unsubscribe@xxxxxxxxxxx. >
