linux-kernel - Re: [PATCH v4 4/6] io_uring: rsrc: avoid use of vmas parameter in pin_user

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Date:   Wed, 19 Apr 2023 10:59:27 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Lorenzo Stoakes <lstoakes@...il.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     Matthew Wilcox <willy@...radead.org>,
        David Hildenbrand <david@...hat.com>,
        Pavel Begunkov <asml.silence@...il.com>,
        io-uring@...r.kernel.org
Subject: Re: [PATCH v4 4/6] io_uring: rsrc: avoid use of vmas parameter in
 pin_user_pages()

On 4/19/23 10:35?AM, Jens Axboe wrote:
> On 4/18/23 9:49?AM, Lorenzo Stoakes wrote:
>> We are shortly to remove pin_user_pages(), and instead perform the required
>> VMA checks ourselves. In most cases there will be a single VMA so this
>> should caues no undue impact on an already slow path.
>>
>> Doing this eliminates the one instance of vmas being used by
>> pin_user_pages().
> 
> First up, please don't just send single patches from a series. It's
> really annoying when you are trying to get the full picture. Just CC the
> whole series, so reviews don't have to look it up separately.
> 
> So when you're doing a respin for what I'll mention below and the issue
> that David found, please don't just show us patch 4+5 of the series.

I'll reply here too rather than keep some of this conversaion
out-of-band.

I don't necessarily think that making io buffer registration dumber and
less efficient by needing a separate vma lookup after the fact is a huge
deal, as I would imagine most workloads register buffers at setup time
and then don't change them. But if people do switch sets at runtime,
it's not necessarily a slow path. That said, I suspect the other bits
that we do in here, like the GUP, is going to dominate the overhead
anyway.

My main question is, why don't we just have a __pin_user_pages or
something helper that still takes the vmas argument, and drop it from
pin_user_pages() only? That'd still allow the cleanup of the other users
that don't care about the vma at all, while retaining the bundled
functionality for the case/cases that do? That would avoid needing
explicit vma iteration in io_uring.

-- 
Jens Axboe