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

David Hildenbrand
2024-07-17T14:17:18Z

On 16.07.24 21:01, Peter Xu wrote:
> On Tue, Jul 16, 2024 at 05:13:29PM +0800, Yan Zhao wrote:
>> 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.
>
> Right, neither vm_flags_set() nor io_remap_pfn_range() should be called in
> a fault handler. They should only be called in mmap() phase.
>
> When you mentioned ioremap(), it's only for the VGA device, right? I don't
> see it's used in the vfio-pci's fault handler.
>
>>
>>>
>>> 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().
>
> It can be conditionally injected. See Alex's commit (not yet posted
> anywhere, only used in our internal testing so far):
>
> https://github.com/xzpeter/linux/commit/f432e0e46c34e493943034be4cb9d6eb638f57d1
>
>>
>> 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?
>
> I must confess I'm not familiar with memtype stuff, and just started
> looking recently.
>
> Per my reading so far, we have these multiple ways of doing memtype
> reservations, and no matter which API we use (ioremap / track_pfn_remap /
> pci_iomap), the same memtype needs to be used otherwise the reservation
> will fail. Here ioremap / pci_iomap will involve one more vmap so that the
> virtual mapping will present already after the API returns.
>
> Then here IMHO track_pfn_remap() is the one that we should still use in
> page-level mmap() controls, because so far we don't want to map any MMIO
> range yet, however we want to say "hey this VMA maps something special", by
> reserving these memtype and set VM_PAT. We want the virtual mapping only
> later during a fault().
>
> In short, it looks to me the right thing we should do is to manually invoke
> track_pfn_remap() in vfio-pci's mmap() hook.
>
> if (!vdev->pm_runtime_engaged && __vfio_pci_memory_enabled(vdev))
> ret = remap_pfn_range(vma, vma->vm_start, pfn,
> req_len, vma->vm_page_prot);
> else
> // TODO: manually invoke track_pfn_remap() here
> vm_flags_set(vma, VM_IO | VM_PFNMAP |
> VM_DONTEXPAND | VM_DONTDUMP);
>
> Then the vma is registered, and untrack_pfn() should be automatically done
> when vma unmaps (and that relies on this patch to not do that too early).
> From that POV, I still think this patch does the right thing and should be
> merged, even if we probably have one more issue to fix as David Wang
> reported..

Especially if it only applies to OOT drivers. Do we have report from
in-tree drivers?

I'd be curious how a previous truncation on a file can tear down a
PFNMAP mapping -- and if there are simple ways to forbid that (if possible).

--
Cheers,

David / dhildenb