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

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Message-ID: <YxDZ86XW3hPcJtPX@nvidia.com>
Date:   Thu, 1 Sep 2022 13:12:35 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     David Hildenbrand <david@...hat.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 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.

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

But this seems OK too

Reviewed-by: Jason Gunthorpe <jgg@...dia.com>

Jason