linux-kernel - Re: [PATCH v2] mmu_gather: move tlb flush for VM_PFNMAP/VM_MIXEDMAP vmas into free

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Message-ID: <cdc9cb25-c43c-e829-5483-2f28d57f609f@google.com>
Date: Thu, 23 Jan 2025 00:06:51 -0800 (PST)
From: Hugh Dickins <hughd@...gle.com>
To: Roman Gushchin <roman.gushchin@...ux.dev>
cc: Hugh Dickins <hughd@...gle.com>, linux-kernel@...r.kernel.org, 
    linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>, 
    Jann Horn <jannh@...gle.com>, Peter Zijlstra <peterz@...radead.org>, 
    Will Deacon <will@...nel.org>, 
    "Aneesh Kumar K.V" <aneesh.kumar@...nel.org>, 
    Nick Piggin <npiggin@...il.com>, linux-arch@...r.kernel.org
Subject: Re: [PATCH v2] mmu_gather: move tlb flush for VM_PFNMAP/VM_MIXEDMAP
 vmas into free_pgtables()

On Wed, 22 Jan 2025, Hugh Dickins wrote:
> On Wed, 22 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.
> > 
> > v2:
> >   - moved vma_pfn flag handling into tlb.h (by Peter Z.)
> >   - added comments (by Peter Z.)
> >   - fixed the vma_pfn flag setting (by Hugh D.)

And in v3, that changelog should be after the ---, not in the commit.