linux-kernel - Re: [PATCH v1] mm/ksm: update stale comment in write_protect

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Date:   Wed, 31 Aug 2022 21:15:09 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Yang Shi <shy828301@...il.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jason Gunthorpe <jgg@...dia.com>,
        John Hubbard <jhubbard@...dia.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Hugh Dickins <hughd@...gle.com>, Peter Xu <peterx@...hat.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH v1] mm/ksm: update stale comment in write_protect_page()

On 31.08.22 21:08, Yang Shi wrote:
> On Wed, Aug 31, 2022 at 11:29 AM David Hildenbrand <david@...hat.com> wrote:
>>
>> On 31.08.22 19:55, Yang Shi wrote:
>>> On Wed, Aug 31, 2022 at 1:30 AM David Hildenbrand <david@...hat.com> wrote:
>>>>
>>>> The comment is stale, because a TLB flush is no longer sufficient and
>>>> required to synchronize against concurrent GUP-fast. This used to be true
>>>> in the past, whereby a TLB flush would have implied an IPI on architectures
>>>> that support GUP-fast, resulting in GUP-fast that disables local interrupts
>>>> from completing before completing the flush.
>>>
>>> Hmm... it seems there might be problem for THP collapse IIUC. THP
>>> collapse clears and flushes pmd before doing anything on pte and
>>> relies on interrupt disable of fast GUP to serialize against fast GUP.
>>> But if TLB flush is no longer sufficient, then we may run into the
>>> below race IIUC:
>>>
>>>          CPU A                                                CPU B
>>> THP collapse                                             fast GUP
>>>
>>> gup_pmd_range() <-- see valid pmd
>>>
>>> gup_pte_range() <-- work on pte
>>> clear pmd and flush TLB
>>> __collapse_huge_page_isolate()
>>>     isolate page <-- before GUP bump refcount
>>>
>>>    pin the page
>>> __collapse_huge_page_copy()
>>>     copy data to huge page
>>>     clear pte (don't flush TLB)
>>> Install huge pmd for huge page
>>>
>>> return the obsolete page
>>
>> Hm, the is_refcount_suitable() check runs while the PTE hasn't been
>> cleared yet. And we don't check if the PMD changed once we're in
>> gup_pte_range().
> 
> Yes
> 
>>
>> The comment most certainly should be stale as well -- unless there is
>> some kind of an implicit IPI broadcast being done.
>>
>> 2667f50e8b81 mentions: "The RCU page table free logic coupled with an
>> IPI broadcast on THP split (which is a rare event), allows one to
>> protect a page table walker by merely disabling the interrupts during
>> the walk."
>>
>> I'm not able to quickly locate that IPI broadcast -- maybe there is one
>> being done here (in collapse) as well?
> 
> The TLB flush may call IPI. I'm supposed it is arch dependent, right?
> Some do use IPI, some may not.

Right, and the whole idea of the RCU GUP-fast was to support
architectures that don't do it. x86-64 does it. IIRC, powerpc doesn't do
it -- but maybe it does so for PMDs?

-- 
Thanks,

David / dhildenb