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Date:   Sun, 26 May 2019 04:06:31 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     john.hubbard@...il.com
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        Jason Gunthorpe <jgg@...pe.ca>,
        LKML <linux-kernel@...r.kernel.org>, linux-rdma@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, John Hubbard <jhubbard@...dia.com>,
        Doug Ledford <dledford@...hat.com>,
        Mike Marciniszyn <mike.marciniszyn@...el.com>,
        Dennis Dalessandro <dennis.dalessandro@...el.com>,
        Christian Benvenuti <benve@...co.com>, Jan Kara <jack@...e.cz>,
        Ira Weiny <ira.weiny@...el.com>,
        Jérôme Glisse <jglisse@...hat.com>,
        Jason Gunthorpe <jgg@...lanox.com>,
        Christoph Hellwig <hch@...radead.org>
Subject: Re: [PATCH v2] infiniband/mm: convert put_page() to put_user_page*()

On Fri, May 24, 2019 at 06:45:22PM -0700, john.hubbard@...il.com wrote:
> For infiniband code that retains pages via get_user_pages*(),
> release those pages via the new put_user_page(), or
> put_user_pages*(), instead of put_page()

I have no objection to this particular patch, but ...

> This is a tiny part of the second step of fixing the problem described
> in [1]. The steps are:
> 
> 1) Provide put_user_page*() routines, intended to be used
>    for releasing pages that were pinned via get_user_pages*().
> 
> 2) Convert all of the call sites for get_user_pages*(), to
>    invoke put_user_page*(), instead of put_page(). This involves dozens of
>    call sites, and will take some time.
> 
> 3) After (2) is complete, use get_user_pages*() and put_user_page*() to
>    implement tracking of these pages. This tracking will be separate from
>    the existing struct page refcounting.
> 
> 4) Use the tracking and identification of these pages, to implement
>    special handling (especially in writeback paths) when the pages are
>    backed by a filesystem. Again, [1] provides details as to why that is
>    desirable.

I thought we agreed at LSFMM that the future is a new get_user_bvec()
/ put_user_bvec().  This is largely going to touch the same places as
step 2 in your list above.  Is it worth doing step 2?

One of the advantages of put_user_bvec() is that it would be quite easy
to miss a conversion from put_page() to put_user_page(), but it'll be
a type error to miss a conversion from put_page() to put_user_bvec().

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