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

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Message-ID: <ZEB3y0V2GSDcUMc2@nvidia.com>
Date:   Wed, 19 Apr 2023 20:22:51 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Lorenzo Stoakes <lstoakes@...il.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 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.

Jason