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

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Date:   Wed, 24 Aug 2022 22:26:22 +1000
From:   Alistair Popple <apopple@...dia.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     linux-mm@...ck.org, akpm@...ux-foundation.org,
        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


David Hildenbrand <david@...hat.com> writes:

> 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?

Correct.