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

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Date:   Wed, 31 Aug 2022 21:33:31 +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 21:15, David Hildenbrand wrote:
> 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?

Looking into the details (and the outdated comment for gup_pte_range()
we should fixup), THP splitting used in the past pmdp_splitting_flush()
for triggering an IPI broadcast.

However, that has been removed in 4b471e8898c3 ("mm, thp: remove
infrastructure for handling splitting PMDs") due to refcount handling
changes that no longer require it.

Consequently, I don't think we can expect an IPI broadcast to sync with
GUP-fast at that point ...

-- 
Thanks,

David / dhildenb