linux-kernel - Re: [PATCH 4/4] mm/gup: remove get_user_pages

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Message-ID: <37484a02-9e64-ea20-314a-a65759b138d2@nvidia.com>
Date:   Mon, 31 Jan 2022 12:01:48 -0800
From:   John Hubbard <jhubbard@...dia.com>
To:     Jan Kara <jack@...e.cz>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Peter Xu <peterx@...hat.com>, Jason Gunthorpe <jgg@...pe.ca>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Alex Williamson <alex.williamson@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Jérôme Glisse <jglisse@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org
Subject: Re: [PATCH 4/4] mm/gup: remove get_user_pages_locked()

On 1/31/22 04:05, Jan Kara wrote:
> On Sun 30-01-22 21:17:52, John Hubbard wrote:
>> Unraveling the rat's nest set of APIs in mm/gup.c a bit more.
>> get_user_pages_locked() was not helping at all, so remove it.
>>
>> Also, lookup_node() has only a single caller, but it is still worth
>> having a clearer locking policy there. Changing it so that the caller
>> both takes and releases the mmap_lock, thus leaving lookup_node() with
>> the sole job of translating a virtual address into a numa node ID.
>>
>> Signed-off-by: John Hubbard <jhubbard@...dia.com>
> 
> Well, the point of _locked() GUP variants is that we can unlock mmap_sem
> when reading a page from the disk during a page fault (hidden behind
> VM_FAULT_RETRY). So as such _locked() variants are about reducing mmap_sem
> latency rather than code readability.  In this particular case, I don't
> think using _locked() variant in lookup_node() is very beneficial
> (generally I would not expect to take a fault there) but at least a
> justification in the commit message should be different :).
> 
> 								Honza

I'll rewrite this commit description to cover this point properly.
Jason also suggested using gup-fast, which I like.
  thanks,
-- 
John Hubbard
NVIDIA