linux-kernel - Re: [PATCH v2 4/7] iov_iter: new iov_iter_pin

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Date:   Thu, 22 Sep 2022 07:36:16 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Jan Kara <jack@...e.cz>, Christoph Hellwig <hch@...radead.org>,
        John Hubbard <jhubbard@...dia.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jens Axboe <axboe@...nel.dk>,
        Miklos Szeredi <miklos@...redi.hu>,
        "Darrick J . Wong" <djwong@...nel.org>,
        Trond Myklebust <trond.myklebust@...merspace.com>,
        Anna Schumaker <anna@...nel.org>,
        David Hildenbrand <david@...hat.com>,
        Logan Gunthorpe <logang@...tatee.com>,
        linux-block@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-xfs@...r.kernel.org, linux-nfs@...r.kernel.org,
        linux-mm@...ck.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 4/7] iov_iter: new iov_iter_pin_pages*() routines

On Tue, Sep 20, 2022 at 06:02:11AM +0100, Al Viro wrote:
> nvme target: nvme read requests end up with somebody allocating and filling
> sglist, followed by reading from file into it (using ITER_BVEC).  Then the
> pages are sent out, presumably

Yes.

> .  I would be very surprised if it turned out
> to be anything other than anon pages allocated by the driver, but I'd like
> to see that confirmed by nvme folks.  Probably doesn't need pinning.

They are anon pages allocated by the driver using sgl_alloc().

> drivers/target/target_core_file.c:292:  iov_iter_bvec(&iter, is_write, aio_cmd->bvecs, sgl_nents, len);

Same as nvme target.

> The picture so far looks like we mostly need to take care of pinning when
> we obtain the references from iov_iter_get_pages().  What's more, it looks
> like ITER_BVEC/ITER_XARRAY/ITER_PIPE we really don't need to pin anything on
> get_pages/pin_pages - they are already protected (or, in case of ITER_PIPE,
> allocated by iov_iter itself and not reachable by anybody outside).

That's what I've been trying to say for a while..