linux-kernel - Re: [PATCH v1 00/15] Keep track of GUPed pages in fs and block

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Message-ID: <CAPcyv4iSyM2r5fv=p4B=h=1sR8Zok3gxb1BVsQOy6FHmQtjGCg@mail.gmail.com>
Date:   Thu, 18 Apr 2019 11:03:16 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Jan Kara <jack@...e.cz>
Cc:     Jerome Glisse <jglisse@...hat.com>,
        Kent Overstreet <kent.overstreet@...il.com>,
        Boaz Harrosh <boaz@...xistor.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-block@...r.kernel.org, Linux MM <linux-mm@...ck.org>,
        John Hubbard <jhubbard@...dia.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>,
        Steve French <sfrench@...ba.org>, linux-cifs@...r.kernel.org,
        samba-technical@...ts.samba.org, Yan Zheng <zyan@...hat.com>,
        Sage Weil <sage@...hat.com>, Ilya Dryomov <idryomov@...il.com>,
        Alex Elder <elder@...nel.org>, ceph-devel@...r.kernel.org,
        Eric Van Hensbergen <ericvh@...il.com>,
        Latchesar Ionkov <lucho@...kov.net>,
        Mike Marshall <hubcap@...ibond.com>,
        Martin Brandenburg <martin@...ibond.com>,
        devel@...ts.orangefs.org,
        Dominique Martinet <asmadeus@...ewreck.org>,
        v9fs-developer@...ts.sourceforge.net, Coly Li <colyli@...e.de>,
        linux-bcache@...r.kernel.org,
        Ernesto A. Fernández 
        <ernesto.mnd.fernandez@...il.com>
Subject: Re: [PATCH v1 00/15] Keep track of GUPed pages in fs and block

On Thu, Apr 18, 2019 at 3:42 AM Jan Kara <jack@...e.cz> wrote:
> > Except that this solution (biasing everyone in bio) would _more complex_
> > it is only conceptualy appealing. The changes are on the other hand much
> > deeper and much riskier but you decided to ignore that and focus on some-
> > thing i was just giving as an example.
>
> Yeah, after going and reading several places like fs/iomap.c, fs/mpage.c,
> drivers/md/dm-io.c I agree with you. The places that are not doing direct
> IO usually just don't hold any page reference that could be directly
> attributed to the bio (and they don't drop it when bio finishes). They
> rather use other means (like PageLocked, PageWriteback) to make sure the
> page stays alive so mandating gup-pin reference for all pages attached to a
> bio would require a lot of reworking of places that are not related to our
> problem and currently work just fine. So I withdraw my suggestion. Nice in
> theory, too much work in practice ;).

Is it though? We already have BIO_NO_PAGE_REF, so it seems it would be
a useful cleanup to have all locations that don't participate in page
references use that existing flag and then teach all other locations
to use gup-pinned pages.