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

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Message-ID: <YxDdycTur733hMgt@xz-m1.local>
Date:   Thu, 1 Sep 2022 12:28:57 -0400
From:   Peter Xu <peterx@...hat.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>,
        Jason Gunthorpe <jgg@...dia.com>,
        John Hubbard <jhubbard@...dia.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(-)
> 
> 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.

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)?

-- 
Peter Xu