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

[Yan Zhao]

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@xxxxxxxxxx
> [2] https://groups.google.com/g/syzkaller-bugs/c/FeQZvSbqWbQ/m/tHFmoZthAAAJ
> [3] https://lore.kernel.org/r/20240712131931.20207-1-00107082@xxxxxxx
> [4] https://lore.kernel.org/all/20240710-bug12-v1-1-0e5440f9b8d3@xxxxxxxxx/
> [5] https://lore.kernel.org/r/20240523195629.218043-1-alex.williamson@xxxxxxxxxx
> 
> Cc: Alex Williamson <alex.williamson@xxxxxxxxxx>
> Cc: Jason Gunthorpe <jgg@xxxxxxxxxx>
> Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
> Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> Cc: Andy Lutomirski <luto@xxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Borislav Petkov <bp@xxxxxxxxx>
> Cc: Kirill A. Shutemov <kirill@xxxxxxxxxxxxx>
> Cc: x86@xxxxxxxxxx
> Cc: Yan Zhao <yan.y.zhao@xxxxxxxxx>
> Cc: Kevin Tian <kevin.tian@xxxxxxxxx>
> Cc: Pei Li <peili.dev@xxxxxxxxx>
> Cc: David Hildenbrand <david@xxxxxxxxxx>
> Cc: David Wang <00107082@xxxxxxx>
> Cc: Bert Karwatzki <spasswolf@xxxxxx>
> Cc: Sergey Senozhatsky <senozhatsky@xxxxxxxxxxxx>
> Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
> ---
> 
> 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.

>  	if (start != end) {
>  		if (unlikely(is_vm_hugetlb_page(vma))) {
>  			/*
> @@ -1894,6 +1891,8 @@ void unmap_vmas(struct mmu_gather *tlb, struct ma_state *mas,
>  		unsigned long start = start_addr;
>  		unsigned long end = end_addr;
>  		hugetlb_zap_begin(vma, &start, &end);
> +		if (unlikely(vma->vm_flags & VM_PFNMAP))
> +			untrack_pfn(vma, 0, 0, mm_wr_locked);
Same here.

>  		unmap_single_vma(tlb, vma, start, end, &details,
>  				 mm_wr_locked);
>  		hugetlb_zap_end(vma, &details);
> -- 
> 2.45.0
>