linux-kernel - Re: [PATCH v4 2/2] mm/gup/writeback: add callbacks for inaccessible pages

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Date:   Thu, 16 Apr 2020 12:02:54 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Claudio Imbrenda <imbrenda@...ux.ibm.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>, linux-next@...r.kernel.org,
        akpm@...ux-foundation.org, jack@...e.cz, kirill@...temov.name,
        "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        borntraeger@...ibm.com, david@...hat.com, aarcange@...hat.com,
        linux-mm@...ck.org, frankja@...ux.ibm.com, sfr@...b.auug.org.au,
        jhubbard@...dia.com, linux-kernel@...r.kernel.org,
        linux-s390@...r.kernel.org, Will Deacon <will@...nel.org>,
        "Williams, Dan J" <dan.j.williams@...el.com>
Subject: Re: [PATCH v4 2/2] mm/gup/writeback: add callbacks for inaccessible
 pages

On 4/16/20 9:34 AM, Claudio Imbrenda wrote:
>> Ahh, so this is *just* intended to precede I/O done on the page, when
>> a non-host entity is touching the memory?
> yep

OK, so we've got to do an action that precedes *all* I/O to a page.
That's not too bad.

I still don't understand how this could work generally, though  There
are lots of places where I/O is done to a page without either going
through __test_set_page_writeback() or gup() with FOLL_PIN set.

sendfile() is probably the best example of this:

	fd = open("/normal/ext4/file", O_RDONLY);
	sendfile(socket_fd, fd, &off, count);

There's no gup in sight since the file doesn't have an address and it's
not being written to so there's no writeback.

How does sendfile work?