linux-kernel - Re: [PATCH v2 2/3] iov_iter: introduce iov_iter_pin_user

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Date:   Sun, 30 Aug 2020 21:17:05 +0100
From:   Al Viro <viro@...iv.linux.org.uk>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Christoph Hellwig <hch@...radead.org>,
        Ilya Dryomov <idryomov@...il.com>,
        Jens Axboe <axboe@...nel.dk>, linux-xfs@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-block@...r.kernel.org,
        linux-mm@...ck.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 2/3] iov_iter: introduce iov_iter_pin_user_pages*()
 routines

On Sat, Aug 29, 2020 at 01:08:52AM -0700, John Hubbard wrote:
> The new routines are:
>     iov_iter_pin_user_pages()
>     iov_iter_pin_user_pages_alloc()
> 
> and those correspond to these pre-existing routines:
>     iov_iter_get_pages()
>     iov_iter_get_pages_alloc()
> 
> Also, pipe_get_pages() and related are changed so as to pass
> down a "use_pup" (use pin_user_page() instead of get_page()) bool
> argument.
> 
> Unlike the iov_iter_get_pages*() routines, the
> iov_iter_pin_user_pages*() routines assert that only ITER_IOVEC or
> ITER_PIPE items are passed in. They then call pin_user_page*(), instead
> of get_user_pages_fast() or get_page().
> 
> Why: In order to incrementally change Direct IO callers from calling
> get_user_pages_fast() and put_page(), over to calling
> pin_user_pages_fast() and unpin_user_page(), there need to be mid-level
> routines that specifically call one or the other systems, for both page
> acquisition and page release.

Hmm...  Do you plan to kill iov_iter_get_pages* off, eventually getting
rid of that use_pup argument?