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Date:   Mon, 5 Sep 2022 10:02:19 +0200
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Subject: Re: [PATCH v1] mm/gup: adjust stale comment for RCU GUP-fast
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To:     Aneesh Kumar K V <aneesh.kumar@linux.ibm.com>,
        Peter Xu <peterx@redhat.com>
Cc:     linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        Sasha Levin <sasha.levin@oracle.com>,
        "Aneesh Kumar K . V" <aneesh.kumar@linux.vnet.ibm.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Jerome Marchand <jmarchan@redhat.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Hugh Dickins <hughd@google.com>,
        Jason Gunthorpe <jgg@nvidia.com>,
        John Hubbard <jhubbard@nvidia.com>,
        Yang Shi <shy828301@gmail.com>
References: <20220901072119.37588-1-david@redhat.com>
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 <bb46d28c-57e3-4cfd-b5b3-e4c6db1f003f@linux.ibm.com>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
In-Reply-To: <bb46d28c-57e3-4cfd-b5b3-e4c6db1f003f@linux.ibm.com>
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On 04.09.22 18:49, Aneesh Kumar K V wrote:
> On 9/1/22 10:04 PM, David Hildenbrand wrote:
>> On 01.09.22 18:28, Peter Xu wrote:
>>> On Thu, Sep 01, 2022 at 09:21:19AM +0200, David Hildenbrand wrote:
>>>> commit 4b471e8898c3 ("mm, thp: remove infrastructure for handling splitting
>>>> PMDs") didn't remove all details about the THP split requirements for
>>>> RCU GUP-fast.
>>>>
>>>> IPI broeadcasts on THP split are no longer required.
>>>>
>>>> Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
>>>> Cc: Sasha Levin <sasha.levin@oracle.com>
>>>> Cc: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
>>>> Cc: Vlastimil Babka <vbabka@suse.cz>
>>>> Cc: Jerome Marchand <jmarchan@redhat.com>
>>>> Cc: Andrea Arcangeli <aarcange@redhat.com>
>>>> Cc: Hugh Dickins <hughd@google.com>
>>>> Cc: Jason Gunthorpe <jgg@nvidia.com>
>>>> Cc: John Hubbard <jhubbard@nvidia.com>
>>>> Cc: Peter Xu <peterx@redhat.com>
>>>> Cc: Yang Shi <shy828301@gmail.com>
>>>> Signed-off-by: David Hildenbrand <david@redhat.com>
>>>> ---
>>>>   mm/gup.c | 5 ++---
>>>>   1 file changed, 2 insertions(+), 3 deletions(-)
>>>>
>>>> diff --git a/mm/gup.c b/mm/gup.c
>>>> index 5abdaf487460..cfe71f422787 100644
>>>> --- a/mm/gup.c
>>>> +++ b/mm/gup.c
>>>> @@ -2309,9 +2309,8 @@ EXPORT_SYMBOL(get_user_pages_unlocked);
>>>>    *
>>>>    * Another way to achieve this is to batch up page table containing pages
>>>>    * belonging to more than one mm_user, then rcu_sched a callback to free those
>>>> - * pages. Disabling interrupts will allow the fast_gup walker to both block
>>>> - * the rcu_sched callback, and an IPI that we broadcast for splitting THPs
>>>> - * (which is a relatively rare event). The code below adopts this strategy.
>>>> + * pages. Disabling interrupts will allow the fast_gup walker to block the
>>>> + * rcu_sched callback.
>>>
>>> This is the comment for fast-gup in general but not only for thp split.
>>
>> "an IPI that we broadcast for splitting THP" is about splitting THP.
>>
>>>
>>> I can understand that we don't need IPI for thp split, but isn't the IPIs
>>> still needed for thp collapse (aka pmdp_collapse_flush)?
>>
>> That was, unfortunately, never documented -- and as discussed in the
>> other thread, arm64 doesn't do that IPI before collapse and might need
>> fixing. We'll most probably end up getting rid of that
>> (undocumented/forgotten) IPI requirement and fix it in GUP-fast by
>> re-rechecking if the PMD changed.
>>
> 
> Can you point to the other thread ?

Sure

see

https://lkml.kernel.org/r/CAHbLzkqeDAnCdt3q4E2RZw64QEzVaO_pseR3VaoHUhB+rZFcZQ@mail.gmail.com

and a resulting patch from that discussion

https://lkml.kernel.org/r/20220901222707.477402-1-shy828301@gmail.com

-- 
Thanks,

David / dhildenb