Re: [PATCH] mm: use correct VMA flags when freeing page-tables

[Hugh Dickins <hughd@google.com>]

On Thu, 21 Oct 2021, Nadav Amit wrote:

> From: Nadav Amit <namit@vmware.com>
> 
> Consistent use of the mmu_gather interface requires a call to
> tlb_start_vma() and tlb_end_vma() for each VMA. free_pgtables() does not
> follow this pattern.
> 
> Certain architectures need tlb_start_vma() to be called in order for
> tlb_update_vma_flags() to update the VMA flags (tlb->vma_exec and
> tlb->vma_huge), which are later used for the proper TLB flush to be
> issued. Since tlb_start_vma() is not called, this can lead to the wrong
> VMA flags being used when the flush is performed.
> 
> Specifically, the munmap syscall would call unmap_region(), which unmaps
> the VMAs and then frees the page-tables. A flush is needed after
> the page-tables are removed to prevent page-walk caches from holding
> stale entries, but this flush would use the flags of the VMA flags of
> the last VMA that was flushed. This does not appear to be right.
> 
> Use tlb_start_vma() and tlb_end_vma() to prevent this from happening.
> This might lead to unnecessary calls to flush_cache_range() on certain
> arch's. If needed, a new flag can be added to mmu_gather to indicate
> that the flush is not needed.
> 
> Cc: Andrea Arcangeli <aarcange@redhat.com>
> Cc: Andrew Cooper <andrew.cooper3@citrix.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Andy Lutomirski <luto@kernel.org>
> Cc: Dave Hansen <dave.hansen@linux.intel.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Will Deacon <will@kernel.org>
> Cc: Yu Zhao <yuzhao@google.com>
> Cc: Nick Piggin <npiggin@gmail.com>
> Cc: x86@kernel.org
> Signed-off-by: Nadav Amit <namit@vmware.com>
> ---
>  mm/memory.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index 12a7b2094434..056fbfdd3c1f 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -412,6 +412,8 @@ void free_pgtables(struct mmu_gather *tlb, struct vm_area_struct *vma,
>  		unlink_anon_vmas(vma);
>  		unlink_file_vma(vma);
>  
> +		tlb_start_vma(tlb, vma);
> +
>  		if (is_vm_hugetlb_page(vma)) {
>  			hugetlb_free_pgd_range(tlb, addr, vma->vm_end,
>  				floor, next ? next->vm_start : ceiling);
> @@ -429,6 +431,8 @@ void free_pgtables(struct mmu_gather *tlb, struct vm_area_struct *vma,
>  			free_pgd_range(tlb, addr, vma->vm_end,
>  				floor, next ? next->vm_start : ceiling);
>  		}
> +
> +		tlb_end_vma(tlb, vma);
>  		vma = next;
>  	}
>  }
> -- 
> 2.25.1

No.

This is an experiment to see whether reviewers look at a wider context
than is seen in the patch itself?  Let's take a look:

		tlb_start_vma(tlb, vma);

		if (is_vm_hugetlb_page(vma)) {
			hugetlb_free_pgd_range(tlb, addr, vma->vm_end,
				floor, next ? next->vm_start : ceiling);
		} else {
			/*
			 * Optimization: gather nearby vmas into one call down
			 */
			while (next && next->vm_start <= vma->vm_end + PMD_SIZE
			       && !is_vm_hugetlb_page(next)) {
				vma = next;
				next = vma->vm_next;
				unlink_anon_vmas(vma);
				unlink_file_vma(vma);
			}
			free_pgd_range(tlb, addr, vma->vm_end,
				floor, next ? next->vm_start : ceiling);
		}

		tlb_end_vma(tlb, vma);
		vma = next;

So, the vma may well have changed in between the new tlb_start_vma()
and tlb_end_vma(): which defeats the intent of the patch.

And I doubt that optimization should be dropped to suit the patch:
you admit to limited understanding of those architectures which need
tlb_start_vma(), me too; but they seem to have survived many years
without it there in free_pgtables(), and I think that tlb_start_vma()
is for when freeing pages, not for when freeing page tables.  Surely
all architectures have to accommodate the fact that a single page
table can be occupied by many different kinds of vma.

(Sorry, I'm being totally unresponsive at present, needing to focus
on something else; but thought I'd better get these comments in before
mmotm's mm-use-correct-vma-flags-when-freeing-page-tables.patch goes to 5.16.)

Hugh