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

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Date:   Wed, 17 Apr 2019 14:54:36 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Boaz Harrosh <boaz@...xistor.com>
Cc:     Kent Overstreet <kent.overstreet@...il.com>,
        Jérôme Glisse <jglisse@...hat.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>, Jan Kara <jack@...e.cz>,
        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,
        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 Tue, Apr 16, 2019 at 12:28 PM Boaz Harrosh <boaz@...xistor.com> wrote:
>
> On 16/04/19 22:12, Dan Williams wrote:
> > On Tue, Apr 16, 2019 at 11:59 AM Kent Overstreet
> > <kent.overstreet@...il.com> wrote:
> <>
> > This all reminds of the failed attempt to teach the block layer to
> > operate without pages:
> >
> > https://lore.kernel.org/lkml/20150316201640.33102.33761.stgit@dwillia2-desk3.amr.corp.intel.com/
> >
>
> Exactly why I want to make sure it is just a [pointer | flag] and not any kind of pfn
> type. Let us please not go there again?
>
> >>
> >> Question though - why do we need a flag for whether a page is a GUP page or not?
> >> Couldn't the needed information just be determined by what range the pfn is not
> >> (i.e. whether or not it has a struct page associated with it)?
> >
> > That amounts to a pfn_valid() check which is a bit heavier than if we
> > can store a flag in the bv_pfn entry directly.
> >
> > I'd say create a new PFN_* flag, and make bv_pfn a 'pfn_t' rather than
> > an 'unsigned long'.
> >
>
> No, please please not. This is not a pfn and not a pfn_t. It is a page-ptr
> and a flag that says where/how to put_page it. IE I did a GUP on this page
> please do a PUP on this page instead of regular put_page. So no where do I mean
> pfn or pfn_t in this code. Then why?

If it's not a pfn then it shouldn't be an unsigned long named "bv_pfn".