Re: [PATCH v4] mmu_gather: move tlb flush for VM_PFNMAP/VM_MIXEDMAP vmas into free_pgtables()

[Hugh Dickins <hughd@google.com>]

On Mon, 27 Jan 2025, Roman Gushchin wrote:

> Commit b67fbebd4cf9 ("mmu_gather: Force tlb-flush VM_PFNMAP vmas")
> added a forced tlbflush to tlb_vma_end(), which is required to avoid a
> race between munmap() and unmap_mapping_range(). However it added some
> overhead to other paths where tlb_vma_end() is used, but vmas are not
> removed, e.g. madvise(MADV_DONTNEED).
> 
> Fix this by moving the tlb flush out of tlb_end_vma() into
> free_pgtables(), somewhat similar to the stable version of the
> original commit: e.g. stable commit 895428ee124a ("mm: Force TLB flush
> for PFNMAP mappings before unlink_file_vma()").
> 
> Note, that if tlb->fullmm is set, no flush is required, as the whole
> mm is about to be destroyed.
> 
> Signed-off-by: Roman Gushchin <roman.gushchin@linux.dev>
> Cc: Jann Horn <jannh@google.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Will Deacon <will@kernel.org>
> Cc: "Aneesh Kumar K.V" <aneesh.kumar@kernel.org>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Nick Piggin <npiggin@gmail.com>
> Cc: Hugh Dickins <hughd@google.com>
> Cc: linux-arch@vger.kernel.org
> Cc: linux-mm@kvack.org
> 
> ---
> 
> v4:
>   - naming/comments update (by Peter Z.)
>   - check vma->vma->vm_flags in tlb_free_vma() (by Peter Z.)

Let me just put on record: you were absolutely right not to extend to
this the Ack I gave to v3, this v4 is silly (tlb_free_vma() and its
multiple calls, necessary only because of the unnecessary extra test);
but I don't see it as doing any actual damage, so I'll stop short of
NAKking it.

Hugh