linux-kernel - Re: [PATCH v1 12/15] fs/direct-io: keep track of wether a page is coming from GUP or not

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Date:   Thu, 11 Apr 2019 20:08:46 -0400
From:   Jerome Glisse <jglisse@...hat.com>
To:     Dave Chinner <david@...morbit.com>
Cc:     linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-block@...r.kernel.org, linux-mm@...ck.org,
        John Hubbard <jhubbard@...dia.com>, Jan Kara <jack@...e.cz>,
        Dan Williams <dan.j.williams@...el.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Johannes Thumshirn <jthumshirn@...e.de>,
        Christoph Hellwig <hch@....de>, Jens Axboe <axboe@...nel.dk>,
        Ming Lei <ming.lei@...hat.com>, Jason Gunthorpe <jgg@...pe.ca>,
        Matthew Wilcox <willy@...radead.org>,
        Ernesto A . Fernández 
        <ernesto.mnd.fernandez@...il.com>, Jeff Moyer <jmoyer@...hat.com>
Subject: Re: [PATCH v1 12/15] fs/direct-io: keep track of wether a page is
 coming from GUP or not

On Fri, Apr 12, 2019 at 09:14:43AM +1000, Dave Chinner wrote:
> On Thu, Apr 11, 2019 at 05:08:31PM -0400, jglisse@...hat.com wrote:
> > From: Jérôme Glisse <jglisse@...hat.com>
> > 
> > We want to keep track of how we got a reference on page when doing DIO,
> > ie wether the page was reference through GUP (get_user_page*) or not.
> > For that this patch rework the way page reference is taken and handed
> > over between DIO code and BIO. Instead of taking a reference for page
> > that have been successfuly added to a BIO we just steal the reference
> > we have when we lookup the page (either through GUP or for ZERO_PAGE).
> > 
> > So this patch keep track of wether the reference has been stolen by the
> > BIO or not. This avoids a bunch of get_page()/put_page() so this limit
> > the number of atomic operations.
> 
> Is the asme set of changes appropriate for the fs/iomap.c direct IO
> path (i.e. XFS)?

Yes and it is part of this patchset AFAICT iomap use bio_iov_iter_get_pages()
which is updated to pass down wether page are coming from GUP or not. The
bio you get out of that is then release through iomap_dio_bio_end_io() which
calls bvec_put_page() which will use put_user_page() for GUPed page.

I may have miss a case and review are welcome.

Note that while the convertion is happening put_user_page is exactly the same
as put_page() in fact the implementation just call put_page() with nothing
else.

The tricky part is making sure that before we diverge with a put_user_page()
that does something else that put_page() we will need to be sure that we did
not left a path that do GUP but does call put_page() and not put_user_page().
We have some plan to catch that in debug build.

In any case i believe we will be very careful when the times come to change
put_user_page() to something different.

Cheers,
Jérôme