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

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Date:   Thu, 20 Apr 2023 14:57:57 +0100
From:   Lorenzo Stoakes <lstoakes@...il.com>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     Matthew Wilcox <willy@...radead.org>, Jens Axboe <axboe@...nel.dk>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.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 Wed, Apr 19, 2023 at 08:22:51PM -0300, Jason Gunthorpe wrote:
> On Wed, Apr 19, 2023 at 07:45:06PM +0100, Lorenzo Stoakes wrote:
>
> > For example, imagine if a user (yes it'd be weird) mlock'd some pages in a
> > buffer and not others, then we'd break their use case. Also (perhaps?) more
> > feasibly, a user might mix hugetlb and anon pages. So I think that'd be too
> > restrictive here.
>
> Yeah, I agree we should not add a broad single-vma restriction to
> GUP. It turns any split of a VMA into a potentially uABI breaking
> change and we just don't need that headache in the mm..
>
> > I do like the idea of a FOLL_SINGLE_VMA for other use cases though, the
> > majority of which want one and one page only. Perhaps worth taking the
> > helper added in this series (get_user_page_vma_remote() from [1]) and
> > replacing it with an a full GUP function which has an interface explicitly
> > for this common single page/vma case.
>
> Like I showed in another thread a function signature that can only do
> one page and also returns the VMA would force it to be used properly
> and we don't need a FOLL flag.
>

Indeed the latest spin of the series uses this. The point is by doing so we
can use per-VMA locks for a common case, I was thinking perhaps as a
separate function call (or perhaps just reusing the wrapper).

This would be entirely separate to all the other work.

> Jason