On 12/4/18 4:40 PM, Dan Williams wrote: > On Tue, Dec 4, 2018 at 4:37 PM Jerome Glisse <jglisse@xxxxxxxxxx> wrote: >> >> On Tue, Dec 04, 2018 at 03:03:02PM -0800, Dan Williams wrote: >>> 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. Good idea about a new ref_user_page() call. It's going to hard to find those places at all of the call sites, btw. >> >> Total crazy idea for this, but this is the right time of day >> for this (for me at least it is beer time :)) What about mapping >> all struct page in two different range of kernel virtual address >> and when get user space is use it returns a pointer from the second >> range of kernel virtual address to the struct page. Then in put_page >> you know for sure if the code putting the page got it from GUP or >> from somewhere else. page_to_pfn() would need some trickery to >> handle that. > > Yes, exactly what I was thinking, if only as a debug mode since > instrumenting every pfn/page translation would be expensive. > That does sound viable as a debug mode. I'll try it out. A reliable way (in both directions) of sorting out put_page() vs. put_user_page() would be a huge improvement, even if just in debug mode. >> Dunno if we are running out of kernel virtual address (outside >> 32bits that i believe we are trying to shot down quietly behind >> the bar). > > There's room, KASAN is in a roughly similar place. > Looks like I'd better post a new version of the entire RFC, rather than just these two patches. It's still less fully-baked than I'd hoped. :) thanks, -- John Hubbard NVIDIA
