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

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Date:   Thu, 1 Sep 2022 08:58:47 +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 01.09.22 00:18, Yang Shi wrote:
> On Wed, Aug 31, 2022 at 12:43 PM Yang Shi <shy828301@...il.com> wrote:
>>
>> On Wed, Aug 31, 2022 at 12:36 PM David Hildenbrand <david@...hat.com> wrote:
>>>
>>> On 31.08.22 21:34, Yang Shi wrote:
>>>> On Wed, Aug 31, 2022 at 12:15 PM David Hildenbrand <david@...hat.com> 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?
>>>>
>>>> It looks powerpc does issue IPI for pmd flush. But arm64 doesn't IIRC.
>>>>
>>>> So maybe we should implement pmdp_collapse_flush() for those arches to
>>>> issue IPI.
>>>
>>> ... or find another way to detect and handle this in GUP-fast?
>>>
>>> Not sure if, for handling PMDs, it could be sufficient to propagate the
>>> pmdp pointer + value and double check that the values didn't change.
>>
>> Should work too, right before pinning the page.
> 
> I actually mean the same place for checking pte. So, something like:
> 
> diff --git a/mm/gup.c b/mm/gup.c
> index 5abdaf487460..2b0703403902 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -2392,7 +2392,8 @@ static int gup_pte_range(pmd_t pmd, unsigned
> long addr, unsigned long end,
>                         goto pte_unmap;
>                 }
> 
> -               if (unlikely(pte_val(pte) != pte_val(*ptep))) {
> +               if (unlikely(pmd_val(pmd) != pmd_val(*pmdp)) ||
> +                   unlikely(pte_val(pte) != pte_val(*ptep))) {
>                         gup_put_folio(folio, 1, flags);
>                         goto pte_unmap;
>                 }
> 
> It doesn't build, just shows the idea.

Exactly what I had in mind. We should add a comment spelling out that
this is for handling huge PMD collapse.


-- 
Thanks,

David / dhildenb