On Thu, Apr 22, 2021 at 1:07 PM Tarun Gupta (SW-GPU) <targupta@xxxxxxxxxx> wrote: > > > > On 4/22/2021 12:20 PM, Marc Zyngier wrote: > > External email: Use caution opening links or attachments > > > > > > On Thu, 22 Apr 2021 03:02:00 +0100, > > Gavin Shan <gshan@xxxxxxxxxx> wrote: > >> > >> Hi Marc, > >> > >> On 4/21/21 9:59 PM, Marc Zyngier wrote: > >>> On Wed, 21 Apr 2021 07:17:44 +0100, > >>> Keqian Zhu <zhukeqian1@xxxxxxxxxx> wrote: > >>>> On 2021/4/21 14:20, Gavin Shan wrote: > >>>>> On 4/21/21 12:59 PM, Keqian Zhu wrote: > >>>>>> On 2020/10/22 0:16, Santosh Shukla wrote: > >>>>>>> The Commit:6d674e28 introduces a notion to detect and handle the > >>>>>>> device mapping. The commit checks for the VM_PFNMAP flag is set > >>>>>>> in vma->flags and if set then marks force_pte to true such that > >>>>>>> if force_pte is true then ignore the THP function check > >>>>>>> (/transparent_hugepage_adjust()). > >>>>>>> > >>>>>>> There could be an issue with the VM_PFNMAP flag setting and checking. > >>>>>>> For example consider a case where the mdev vendor driver register's > >>>>>>> the vma_fault handler named vma_mmio_fault(), which maps the > >>>>>>> host MMIO region in-turn calls remap_pfn_range() and maps > >>>>>>> the MMIO's vma space. Where, remap_pfn_range implicitly sets > >>>>>>> the VM_PFNMAP flag into vma->flags. > >>>>>> Could you give the name of the mdev vendor driver that triggers this issue? > >>>>>> I failed to find one according to your description. Thanks. > >>>>>> > >>>>> > >>>>> I think it would be fixed in driver side to set VM_PFNMAP in > >>>>> its mmap() callback (call_mmap()), like vfio PCI driver does. > >>>>> It means it won't be delayed until page fault is issued and > >>>>> remap_pfn_range() is called. It's determined from the beginning > >>>>> that the vma associated the mdev vendor driver is serving as > >>>>> PFN remapping purpose. So the vma should be populated completely, > >>>>> including the VM_PFNMAP flag before it becomes visible to user > >>>>> space. > >>> > >>> Why should that be a requirement? Lazy populating of the VMA should be > >>> perfectly acceptable if the fault can only happen on the CPU side. > >>> Right. Hi keqian, You can refer to case http://lkml.iu.edu/hypermail/linux/kernel/2010.3/00952.html (Sorry Guys, I am not with nvidia, but My quick input.) > >> > >> It isn't a requirement and the drivers needn't follow strictly. I checked > >> several drivers before looking into the patch and found almost all the > >> drivers have VM_PFNMAP set at mmap() time. In drivers/vfio/vfio-pci.c, > >> there is a comment as below, but it doesn't reveal too much about why > >> we can't set VM_PFNMAP at fault time. > >> > >> static int vfio_pci_mmap(void *device_data, struct vm_area_struct *vma) > >> { > >> : > >> /* > >> * See remap_pfn_range(), called from vfio_pci_fault() but we can't > >> * change vm_flags within the fault handler. Set them now. > >> */ > >> vma->vm_flags |= VM_IO | VM_PFNMAP | VM_DONTEXPAND | VM_DONTDUMP; > >> vma->vm_ops = &vfio_pci_mmap_ops; > >> > >> return 0; > >> } > >> > >> To set these flags in advance does have advantages. For example, > >> VM_DONTEXPAND prevents the vma to be merged with another > >> one. VM_DONTDUMP make this vma isn't eligible for > >> coredump. Otherwise, the address space, which is associated with the > >> vma is accessed and unnecessary page faults are triggered on > >> coredump. VM_IO and VM_PFNMAP avoids to walk the page frames > >> associated with the vma since we don't have valid PFN in the > >> mapping. > > > > But PCI clearly isn't the case we are dealing with here, and not > > everything is VFIO either. I can *today* create a driver that > > implements a mmap+fault handler, call mmap() on it, pass the result to > > a memslot, and get to the exact same result Santosh describes. > > > > No PCI, no VFIO, just a random driver. We are *required* to handle > > that. > > Agree with Marc here, that kernel should be able to handle it without > VM_PFNMAP flag set in driver. > > For driver reference, you could check the V2 version of this patch that > got accepted upstream and has details as-to how this can be reproduced > using vfio-pci: https://www.spinics.net/lists/arm-kernel/msg848491.html > > > > > M. > > > > -- > > Without deviation from the norm, progress is not possible. > >
