linux-kernel - Re: [PATCH 5/7] io_uring: rsrc: use FOLL_SAME_FILE on pin_user

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Date:   Mon, 17 Apr 2023 16:24:04 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Lorenzo Stoakes <lstoakes@...il.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Matthew Wilcox <willy@...radead.org>,
        David Hildenbrand <david@...hat.com>,
        Jens Axboe <axboe@...nel.dk>,
        Pavel Begunkov <asml.silence@...il.com>,
        io-uring@...r.kernel.org
Subject: Re: [PATCH 5/7] io_uring: rsrc: use FOLL_SAME_FILE on
 pin_user_pages()

On Mon, Apr 17, 2023 at 08:00:48PM +0100, Lorenzo Stoakes wrote:

> So I don't think this route is plausible unless you were thinking of
> somehow offloading to a thread?

ah, fair enough
 
> In any case, if we institute the FOLL_ALLOW_BROKEN_FILE_MAPPINGS flag we
> can just drop FOLL_ANON altogether right, as this will be implied and
> hugetlb should work here too?

Well.. no, as I said read-only access to the pages works fine, so GUP
should not block that. It is only write that has issues

> Separately, I find the semantics of access_remote_vm() kind of weird, and
> with a possible mmap_lock-free future it does make me wonder whether
> something better could be done there.

Yes, it is very weird, kthread_use_mm is much nicer.
 
> (Section where I sound like I might be going mad) Perhaps having some means
> of context switching into the kernel portion of the remote process as if
> were a system call or soft interrupt handler and having that actually do
> the uaccess operation could be useful here?

This is the kthread_use_mm() approach, that is basically what it
does. You are suggesting to extend it to kthreads that already have a
process attached...

access_remote_vm is basically copy_to/from_user built using kmap and
GUP.

even a simple step of localizing FOLL_ANON to __access_remote_vm,
since it must have the VMA nyhow, would be an improvement.

Jason