linux-kernel - Re: [PATCH v5 07/11] mm, dev_pagemap: introduce CONFIG_DEV_PAGEMAP

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Date:   Mon, 12 Mar 2018 11:17:25 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Jerome Glisse <jglisse@...hat.com>
Cc:     linux-nvdimm <linux-nvdimm@...ts.01.org>,
        Michal Hocko <mhocko@...e.com>, david <david@...morbit.com>,
        Christoph Hellwig <hch@....de>,
        linux-xfs <linux-xfs@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Jan Kara <jack@...e.cz>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5 07/11] mm, dev_pagemap: introduce CONFIG_DEV_PAGEMAP_OPS

On Mon, Mar 12, 2018 at 7:17 AM, Jerome Glisse <jglisse@...hat.com> wrote:
> On Fri, Mar 09, 2018 at 10:55:26PM -0800, Dan Williams wrote:
>> The HMM sub-system extended dev_pagemap to arrange a callback when a
>> dev_pagemap managed page is freed. Since a dev_pagemap page is free /
>> idle when its reference count is 1 it requires an additional branch to
>> check the page-type at put_page() time. Given put_page() is a hot-path
>> we do not want to incur that check if HMM is not in use, so a static
>> branch is used to avoid that overhead when not necessary.
>>
>> Now, the FS_DAX implementation wants to reuse this mechanism for
>> receiving dev_pagemap ->page_free() callbacks. Rework the HMM-specific
>> static-key into a generic mechanism that either HMM or FS_DAX code paths
>> can enable.
>
> Why EXPORT_SYMBOL_GPL and not EXPORT_SYMBOL like it was prior to this
> patch ? Not i care that much about that, just wondering. Maybe keep it
> EXPORT_SYMBOL() ?

HMM has significant integrations and useful hacks within the mm
sub-system, I think we should go further than just these symbols and
also include devm_memremap_pages() as EXPORT_SYMBOL_GPL. It was simply
an oversight that these were EXPORT_SYMBOL previously.

> In any case
> Reviewed-by: "Jérôme Glisse" <jglisse@...hat.com>

Thanks!