netdev - Re: [PATCH 1/2] docs: mm/gup: pin_user

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Date:   Fri, 12 Jun 2020 12:24:26 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        "Michael S . Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Jérôme Glisse <jglisse@...hat.com>,
        Jan Kara <jack@...e.cz>, Dave Chinner <david@...morbit.com>,
        Souptick Joarder <jrdr.linux@...il.com>,
        Jonathan Corbet <corbet@....net>, linux-doc@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, kvm@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org
Subject: Re: [PATCH 1/2] docs: mm/gup: pin_user_pages.rst: add a "case 5"

On Fri, May 29, 2020 at 04:43:08PM -0700, John Hubbard wrote:
> +CASE 5: Pinning in order to write to the data within the page
> +-------------------------------------------------------------
> +Even though neither DMA nor Direct IO is involved, just a simple case of "pin,
> +access page's data, unpin" can cause a problem. Case 5 may be considered a
> +superset of Case 1, plus Case 2, plus anything that invokes that pattern. In
> +other words, if the code is neither Case 1 nor Case 2, it may still require
> +FOLL_PIN, for patterns like this:
> +
> +Correct (uses FOLL_PIN calls):
> +    pin_user_pages()
> +    access the data within the pages
> +    set_page_dirty_lock()
> +    unpin_user_pages()
> +
> +INCORRECT (uses FOLL_GET calls):
> +    get_user_pages()
> +    access the data within the pages
> +    set_page_dirty_lock()
> +    put_page()

Why does this case need to pin?  Why can't it just do ...

	get_user_pages()
	lock_page(page);
	... modify the data ...
	set_page_dirty(page);
	unlock_page(page);