Re: [LSF/MM TOPIC] get_user_pages() pins in file mappings

[Jan Kara <jack@suse.cz>]

Hi John,

On Mon 04-02-19 15:46:10, John Hubbard wrote:
> On 1/24/19 1:04 AM, Jan Kara wrote:
> 
> > In particular we hope to have reasonably robust mechanism of identifying
> > pages pinned by GUP (patches will be posted soon) - I'd like to run that by
> > MM folks (unless discussion happens on mailing lists before LSF/MM). We
> > also have ideas how filesystems should react to pinned page in their
> > writepages methods - there will be some changes needed in some filesystems
> > to bounce the page if they need stable page contents. So I'd like to
> > explain why we chose to do bouncing to fs people (i.e., why we cannot just
> > wait, skip the page, do something else etc.) to save us from the same
> > discussion with each fs separately and also hash out what the API for
> > filesystems to do this should look like. Finally we plan to keep pinned
> > page permanently dirty - again something I'd like to explain why we do this
> > and gather input from other people.
> 
> Hi Jan,
> 
> Say, I was just talking through this point with someone on our driver team, 
> and suddenly realized that I'm now slightly confused on one point. If we end
> up keeping the gup-pinned pages effectively permanently dirty while pinned,
> then maybe the call sites no longer need to specify "dirty" (or not) when
> they call put_user_page*()?
> 
> In other words, the RFC [1] has this API:
> 
>     void put_user_page(struct page *page);
>     void put_user_pages_dirty(struct page **pages, unsigned long npages);
>     void put_user_pages_dirty_lock(struct page **pages, unsigned long npages);
>     void put_user_pages(struct page **pages, unsigned long npages);
> 
> But maybe we only really need this:
> 
>     void put_user_page(struct page *page);
>     void put_user_pages(struct page **pages, unsigned long npages);
> 
> ?
> 
> [1] https://lkml.kernel.org/r/20190204052135.25784-1-jhubbard@nvidia.com

So you are right that if we keep gup-pinned pages dirty, drivers could get
away without marking them as such. However I view "keep pages dirty" as an
implementation detail, rather than a promise of the API. So I'd like to
leave us the flexibility of choosing a different implementation in the
future. And as such I'd just leave the put_user_pages_dirty() variants in
place.

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR