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

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Message-ID: <87fshj3jrv.fsf@yhuang6-desk2.ccr.corp.intel.com>
Date:   Fri, 26 Aug 2022 08:56:04 +0800
From:   "Huang, Ying" <ying.huang@...el.com>
To:     Alistair Popple <apopple@...dia.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>,
        David Hildenbrand <david@...hat.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

Alistair Popple <apopple@...dia.com> writes:

> "Huang, Ying" <ying.huang@...el.com> writes:
>
>> Alistair Popple <apopple@...dia.com> writes:
>>
>>> 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);
>>
>> It appears that we can increase "unmapped" only if ptep_get_and_clear()
>> is used?
>
> In other words you mean we only need to increase unmapped if pte_present
> && !anon_exclusive?
>
> Agree, that's a good optimisation to make. However I'm just trying to
> solve a data corruption issue (not dirtying the page) here, so will post
> that as a separate optimisation patch. Thanks.

OK.  Then the patch looks good to me.  Feel free to add my,

Reviewed-by: "Huang, Ying" <ying.huang@...el.com>

Best Regards,
Huang, Ying

>>
>>> +	arch_leave_lazy_mmu_mode();
>>> +	pte_unmap_unlock(ptep - 1, ptl);
>>> +
>>>  	return 0;
>>>  }
>>>
>>>
>>> base-commit: ffcf9c5700e49c0aee42dcba9a12ba21338e8136