linux-kernel - Re: [PATCH v1] mm/gup: adjust stale comment for RCU GUP-fast

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Date:   Thu, 1 Sep 2022 18:30:04 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Sasha Levin <sasha.levin@...cle.com>,
        "Aneesh Kumar K . V" <aneesh.kumar@...ux.vnet.ibm.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Jerome Marchand <jmarchan@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Hugh Dickins <hughd@...gle.com>,
        John Hubbard <jhubbard@...dia.com>,
        Peter Xu <peterx@...hat.com>, Yang Shi <shy828301@...il.com>
Subject: Re: [PATCH v1] mm/gup: adjust stale comment for RCU GUP-fast

On 01.09.22 18:12, Jason Gunthorpe 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@...ux.intel.com>
>> Cc: Sasha Levin <sasha.levin@...cle.com>
>> Cc: Aneesh Kumar K.V <aneesh.kumar@...ux.vnet.ibm.com>
>> Cc: Vlastimil Babka <vbabka@...e.cz>
>> Cc: Jerome Marchand <jmarchan@...hat.com>
>> Cc: Andrea Arcangeli <aarcange@...hat.com>
>> Cc: Hugh Dickins <hughd@...gle.com>
>> Cc: Jason Gunthorpe <jgg@...dia.com>
>> Cc: John Hubbard <jhubbard@...dia.com>
>> Cc: Peter Xu <peterx@...hat.com>
>> Cc: Yang Shi <shy828301@...il.com>
>> Signed-off-by: David Hildenbrand <david@...hat.com>
>> ---
>>  mm/gup.c | 5 ++---
>>  1 file changed, 2 insertions(+), 3 deletions(-)
> 
> The comment a bit above seems to need touching to:
> 
>  * protected from page table pages being freed from under it, and should
>  * block any THP splits.

Ah right. Will drop it as well -- thanks!

> 
> What is the current situation for THP splits anyhow? Is there are
> comment in the fast pmd code explaining it?

Not aware of a comment, I think it just works naturally by always
re-routing references taken on tail pages to the head page refcount.

Before that, we had "Tail page refcounting", which -- as I understand --
was a mess.

ddc58f27f9eee9117219936f77e90ad5b2e00e96 contains some comments.

> 
> But this seems OK too
> 
> Reviewed-by: Jason Gunthorpe <jgg@...dia.com>
> 
> Jason
> 


-- 
Thanks,

David / dhildenb