On Tue, Dec 4, 2018 at 1:56 PM John Hubbard <jhubbard@xxxxxxxxxx> wrote: > > On 12/4/18 12:28 PM, Dan Williams wrote: > > On Mon, Dec 3, 2018 at 4:17 PM <john.hubbard@xxxxxxxxx> wrote: > >> > >> From: John Hubbard <jhubbard@xxxxxxxxxx> > >> > >> Introduces put_user_page(), which simply calls put_page(). > >> This provides a way to update all get_user_pages*() callers, > >> so that they call put_user_page(), instead of put_page(). > >> > >> Also introduces put_user_pages(), and a few dirty/locked variations, > >> as a replacement for release_pages(), and also as a replacement > >> for open-coded loops that release multiple pages. > >> These may be used for subsequent performance improvements, > >> via batching of pages to be released. > >> > >> This is the first step of fixing the problem described in [1]. The steps > >> are: > >> > >> 1) (This patch): 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 at Plumbers we talked about using a page bit to tag pages > > that have had their reference count elevated by get_user_pages()? That > > way there is no need to distinguish put_page() from put_user_page() it > > just happens internally to put_page(). At the conference Matthew was > > offering to free up a page bit for this purpose. > > > > ...but then, upon further discussion in that same session, we realized that > that doesn't help. You need a reference count. Otherwise a random put_page > could affect your dma-pinned pages, etc, etc. Ok, sorry, I mis-remembered. So, you're effectively trying to capture the end of the page pin event separate from the final 'put' of the page? Makes sense. > I was not able to actually find any place where a single additional page > bit would help our situation, which is why this still uses LRU fields for > both the two bits required (the RFC [1] still applies), and the dma_pinned_count. Except the LRU fields are already in use for ZONE_DEVICE pages... how does this proposal interact with those? > [1] https://lore.kernel.org/r/20181110085041.10071-7-jhubbard@xxxxxxxxxx > > >> [1] https://lwn.net/Articles/753027/ : "The Trouble with get_user_pages()" > >> > >> Reviewed-by: Jan Kara <jack@xxxxxxx> > > > > Wish, you could have been there Jan. I'm missing why it's safe to > > assume that a single put_user_page() is paired with a get_user_page()? > > > > A put_user_page() per page, or a put_user_pages() for an array of pages. See > patch 0002 for several examples. Yes, however I was more concerned about validation and trying to locate missed places where put_page() is used instead of put_user_page(). It would be interesting to see if we could have a debug mode where get_user_pages() returned dynamically allocated pages from a known address range and catch drivers that operate on a user-pinned page without using the proper helper to 'put' it. I think we might also need a ref_user_page() for drivers that may do their own get_page() and expect the dma_pinned_count to also increase.
