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

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Date:   Wed, 31 Aug 2022 20:29:09 +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 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().

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?

-- 
Thanks,

David / dhildenb