linux-kernel - Re: [PATCH v3 1/3] mm/migrate_device.c: Flush TLB while holding PTL

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Message-ID: <86b0622d-06eb-cfab-2ff1-8a0eaf823f8a@redhat.com>
Date:   Wed, 24 Aug 2022 10:21:54 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Alistair Popple <apopple@...dia.com>, linux-mm@...ck.org,
        akpm@...ux-foundation.org
Cc:     Peter Xu <peterx@...hat.com>, Nadav Amit <nadav.amit@...il.com>,
        huang ying <huang.ying.caritas@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        "Sierra Guiza, Alejandro (Alex)" <alex.sierra@....com>,
        Felix Kuehling <Felix.Kuehling@....com>,
        Jason Gunthorpe <jgg@...dia.com>,
        John Hubbard <jhubbard@...dia.com>,
        Ralph Campbell <rcampbell@...dia.com>,
        Matthew Wilcox <willy@...radead.org>,
        Karol Herbst <kherbst@...hat.com>,
        Lyude Paul <lyude@...hat.com>, Ben Skeggs <bskeggs@...hat.com>,
        Logan Gunthorpe <logang@...tatee.com>, paulus@...abs.org,
        linuxppc-dev@...ts.ozlabs.org, stable@...r.kernel.org
Subject: Re: [PATCH v3 1/3] mm/migrate_device.c: Flush TLB while holding PTL

On 24.08.22 05:03, Alistair Popple wrote:
> When clearing a PTE the TLB should be flushed whilst still holding the
> PTL to avoid a potential race with madvise/munmap/etc. For example
> consider the following sequence:
> 
>   CPU0                          CPU1
>   ----                          ----
> 
>   migrate_vma_collect_pmd()
>   pte_unmap_unlock()
>                                 madvise(MADV_DONTNEED)
>                                 -> zap_pte_range()
>                                 pte_offset_map_lock()
>                                 [ PTE not present, TLB not flushed ]
>                                 pte_unmap_unlock()
>                                 [ page is still accessible via stale TLB ]
>   flush_tlb_range()
> 
> In this case the page may still be accessed via the stale TLB entry
> after madvise returns. Fix this by flushing the TLB while holding the
> PTL.
> 
> Signed-off-by: Alistair Popple <apopple@...dia.com>
> Reported-by: Nadav Amit <nadav.amit@...il.com>
> Fixes: 8c3328f1f36a ("mm/migrate: migrate_vma() unmap page from vma while collecting pages")
> Cc: stable@...r.kernel.org
> 
> ---
> 
> Changes for v3:
> 
>  - New for v3
> ---
>  mm/migrate_device.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/migrate_device.c b/mm/migrate_device.c
> index 27fb37d..6a5ef9f 100644
> --- a/mm/migrate_device.c
> +++ b/mm/migrate_device.c
> @@ -254,13 +254,14 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
>  		migrate->dst[migrate->npages] = 0;
>  		migrate->src[migrate->npages++] = mpfn;
>  	}
> -	arch_leave_lazy_mmu_mode();
> -	pte_unmap_unlock(ptep - 1, ptl);
>  
>  	/* Only flush the TLB if we actually modified any entries */
>  	if (unmapped)
>  		flush_tlb_range(walk->vma, start, end);
>  
> +	arch_leave_lazy_mmu_mode();
> +	pte_unmap_unlock(ptep - 1, ptl);
> +
>  	return 0;
>  }
>  
> 
> base-commit: ffcf9c5700e49c0aee42dcba9a12ba21338e8136

I'm not a TLB-flushing expert, but this matches my understanding (and a
TLB flushing Linux documentation I stumbled over some while ago but
cannot quickly find).

In the ordinary try_to_migrate_one() path, flushing would happen via
ptep_clear_flush() (just like we do for the anon_exclusive case here as
well), correct?

-- 
Thanks,

David / dhildenb