Re: [PATCH] mm/x86/pat: Only untrack the pfn range if unmap region

Yan Zhao
2024-07-16T09:15:08Z

On Mon, Jul 15, 2024 at 10:29:59PM +0800, Peter Xu wrote:
> On Mon, Jul 15, 2024 at 03:08:58PM +0800, Yan Zhao wrote:
> > On Fri, Jul 12, 2024 at 10:42:44AM -0400, Peter Xu wrote:
> > > This patch is one patch of an old series [1] that got reposted standalone
> > > here, with the hope to fix some reported untrack_pfn() issues reported
> > > recently [2,3], where there used to be other fix [4] but unfortunately
> > > which looks like to cause other issues. The hope is this patch can fix it
> > > the right way.
> > >
> > > X86 uses pfn tracking to do pfnmaps. AFAICT, the tracking should normally
> > > start at mmap() of device drivers, then untracked when munmap(). However
> > > in the current code the untrack is done in unmap_single_vma(). This might
> > > be problematic.
> > >
> > > For example, unmap_single_vma() can be used nowadays even for zapping a
> > > single page rather than the whole vmas. It's very confusing to do whole
> > > vma untracking in this function even if a caller would like to zap one
> > > page. It could simply be wrong.
> > >
> > > Such issue won't be exposed by things like MADV_DONTNEED won't ever work
> > > for pfnmaps and it'll fail the madvise() already before reaching here.
> > > However looks like it can be triggered like what was reported where invoked
> > > from an unmap request from a file vma.
> > >
> > > There's also work [5] on VFIO (merged now) to allow tearing down MMIO
> > > pgtables before an munmap(), in which case we may not want to untrack the
> > > pfns if we're only tearing down the pgtables. IOW, we may want to keep the
> > > pfn tracking information as those pfn mappings can be restored later with
> > > the same vma object. Currently it's not an immediate problem for VFIO, as
> > > VFIO uses UC- by default, but it looks like there's plan to extend that in
> > > the near future.
> > >
> > > IIUC, this was overlooked when zap_page_range_single() was introduced,
> > > while in the past it was only used in the munmap() path which wants to
> > > always unmap the region completely. E.g., commit f5cc4eef9987 ("VM: make
> > > zap_page_range() callers that act on a single VMA use separate helper") is
> > > the initial commit that introduced unmap_single_vma(), in which the chunk
> > > of untrack_pfn() was moved over from unmap_vmas().
> > >
> > > Recover that behavior to untrack pfnmap only when unmap regions.
> > >
> > > [1] https://lore.kernel.org/r/20240523223745.395337-1-peterx@redhat.com
> > > [2] https://groups.google.com/g/syzkaller-bugs/c/FeQZvSbqWbQ/m/tHFmoZthAAAJ
> > > [3] https://lore.kernel.org/r/20240712131931.20207-1-00107082@163.com
> > > [4] https://lore.kernel.org/all/20240710-bug12-v1-1-0e5440f9b8d3@gmail.com/
> > > [5] https://lore.kernel.org/r/20240523195629.218043-1-alex.williamson@redhat.com
> > >
> > > Cc: Alex Williamson <alex.williamson@redhat.com>
> > > Cc: Jason Gunthorpe <jgg@nvidia.com>
> > > Cc: Al Viro <viro@zeniv.linux.org.uk>
> > > Cc: Dave Hansen <dave.hansen@linux.intel.com>
> > > Cc: Andy Lutomirski <luto@kernel.org>
> > > Cc: Peter Zijlstra <peterz@infradead.org>
> > > Cc: Thomas Gleixner <tglx@linutronix.de>
> > > Cc: Ingo Molnar <mingo@redhat.com>
> > > Cc: Borislav Petkov <bp@alien8.de>
> > > Cc: Kirill A. Shutemov <kirill@shutemov.name>
> > > Cc: x86@kernel.org
> > > Cc: Yan Zhao <yan.y.zhao@intel.com>
> > > Cc: Kevin Tian <kevin.tian@intel.com>
> > > Cc: Pei Li <peili.dev@gmail.com>
> > > Cc: David Hildenbrand <david@redhat.com>
> > > Cc: David Wang <00107082@163.com>
> > > Cc: Bert Karwatzki <spasswolf@web.de>
> > > Cc: Sergey Senozhatsky <senozhatsky@chromium.org>
> > > Signed-off-by: Peter Xu <peterx@redhat.com>
> > > ---
> > >
> > > NOTE: I massaged the commit message comparing to the rfc post [1], the
> > > patch itself is untouched. Also removed rfc tag, and added more people
> > > into the loop. Please kindly help test this patch if you have a reproducer,
> > > as I can't reproduce it myself even with the syzbot reproducer on top of
> > > mm-unstable. Instead of further check on the reproducer, I decided to send
> > > this out first as we have a bunch of reproducers on the list now..
> > > ---
> > > mm/memory.c | 5 ++---
> > > 1 file changed, 2 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/mm/memory.c b/mm/memory.c
> > > index 4bcd79619574..f57cc304b318 100644
> > > --- a/mm/memory.c
> > > +++ b/mm/memory.c
> > > @@ -1827,9 +1827,6 @@ static void unmap_single_vma(struct mmu_gather *tlb,
> > > if (vma->vm_file)
> > > uprobe_munmap(vma, start, end);
> > >
> > > - if (unlikely(vma->vm_flags & VM_PFNMAP))
> > > - untrack_pfn(vma, 0, 0, mm_wr_locked);
> > > -
> > Specifically to VFIO's case, looks it doesn't matter if untrack_pfn() is
> > called here, since remap_pfn_range() is not called in mmap() and fault
> > handler, and therefore (vma->vm_flags & VM_PAT) is always 0.
>
> Right when with current repo, but I'm thinking maybe we should have VM_PAT
> there..
Yes, I agree.

But, currently for VFIO, it cannot call io_remap_pfn_range() in the fault
handler since vm_flags_set() requires mmap lock held for write while
the fault handler can only hold mmap lock for read.
So, it relies on ioremap()/iounmap() to reserve/de-reserve memtypes,
without VM_PAT being set in vma.

>
> See what reserve_pfn_range() does right now: it'll make sure only one owner
> reserve this area, e.g. memtype_reserve() will fail if it has already been
> reserved. Then when succeeded as the first one to reserve the region,
> it'll make sure this mem type to also be synchronized to the kernel map
> (memtype_kernel_map_sync).
>
> So I feel like when MMIO disabled for a VFIO card, we may need to call
> reserve_pfn_range(), telling the kernel the mem type VFIO uses, even if the
> pgtable will be empty, and even if currently it's always UC- for now:
>
> vfio_pci_core_mmap():
> vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);
>
> Then the memtype will be reserved even if it cannot be used. Otherwise I
> don't know whether there's side effects of kernel identity mapping where it
> mismatch later with what's mapped in the userspace via the vma, when MMIO
> got enabled again: pgtable started to be injected with a different memtype
> that specified only in the vma's pgprot.
Current VFIO relies on pci_iomap() to reserve MMIO pfns as UC-, thus
no VM_PAT in vmas.

Calling remap_pfn_range() in the mmap() will cause problem to support
dynamic faulting. Looks there's still a window even with an immediate
unmap after mmap().

Also, calling remap_pfn_range() in mmap() may not meet the requirement of
drivers to dynamic switch backend resources, e.g. as what's in
cxl_mmap_fault(), since one remap_pfn_range() cannot reserve memtypes for
all backend resources at once.

So, is there any way for the driver to reserve memtypes and set VM_PAT in
fault handler?