linux-kernel - Re: [PATCH 1/2] mm: introduce put_user

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Message-ID: <20181217195922.GQ10600@bombadil.infradead.org>
Date:   Mon, 17 Dec 2018 11:59:22 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     Jerome Glisse <jglisse@...hat.com>
Cc:     Dave Chinner <david@...morbit.com>, Jan Kara <jack@...e.cz>,
        John Hubbard <jhubbard@...dia.com>,
        Dan Williams <dan.j.williams@...el.com>,
        John Hubbard <john.hubbard@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux MM <linux-mm@...ck.org>, tom@...pey.com,
        Al Viro <viro@...iv.linux.org.uk>, benve@...co.com,
        Christoph Hellwig <hch@...radead.org>,
        Christopher Lameter <cl@...ux.com>,
        "Dalessandro, Dennis" <dennis.dalessandro@...el.com>,
        Doug Ledford <dledford@...hat.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Michal Hocko <mhocko@...nel.org>, mike.marciniszyn@...el.com,
        rcampbell@...dia.com,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH 1/2] mm: introduce put_user_page*(), placeholder versions

On Mon, Dec 17, 2018 at 02:54:08PM -0500, Jerome Glisse wrote:
> On Mon, Dec 17, 2018 at 11:51:51AM -0800, Matthew Wilcox wrote:
> > On Mon, Dec 17, 2018 at 02:48:00PM -0500, Jerome Glisse wrote:
> > > On Mon, Dec 17, 2018 at 10:34:43AM -0800, Matthew Wilcox wrote:
> > > > No.  The solution John, Dan & I have been looking at is to take the
> > > > dirty page off the LRU while it is pinned by GUP.  It will never be
> > > > found for writeback.
> > > 
> > > With the solution you are proposing we loose GUP fast and we have to
> > > allocate a structure for each page that is under GUP, and the LRU
> > > changes too. Moreover by not writing back there is a greater chance
> > > of data loss.
> > 
> > Why can't you store the hmm_data in a side data structure?  Why does it
> > have to be in struct page?
> 
> hmm_data is not even the issue here, we can have a pincount without
> moving things around. So i do not see the need to complexify any of
> the existing code to add new structure and consume more memory for
> no good reasons. I do not see any benefit in that.

You said "we have to allocate a structure for each page that is under
GUP".  The only reason to do that is if we want to keep hmm_data in
struct page.  If we ditch hmm_data, there's no need to allocate a
structure, and we don't lose GUP fast either.