OK, as requested, I've split the tracking patch into 6 smaller patches, and it should be *much* easier to understand and review now.
============================================================ Changes since v1:
* Split the tracking patch into 6 smaller patches
* Rebased onto today's linux-next/akpm (there weren't any conflicts).
* Fixed an "unsigned int" vs. "int" problem in gup_benchmark, reported by Nathan Chancellor. (I don't see it in my local builds, probably because they use gcc, but an LLVM test found the mismatch.)
* Fixed a huge page pincount problem (add/subtract vs. increment/decrement), spotted by Jan Kara. ============================================================
There is a reasonable case to be made for merging two of the patches (patches 4 and 5), given that patch 4 provides tracking that has upper limits on the number of pins that can be done with huge pages. Let me know if anyone wants those merged, but unless there is some weird chance of someone grabbing patch 4 and not patch 5, I don't really see the need. Meanwhile, it's easier to review in this form.
Also, patch 3 has been revived. Earlier reviewers asked for it to be merged into the tracking patch (one cannot please everyone, heh), but now it's back out on it's own.
This activates tracking of FOLL_PIN pages. This is in support of fixing the get_user_pages()+DMA problem described in [1]-[4].
It is based on today's (Jan 28) linux-next (branch: akpm), commit 280e9cb00b41 ("drivers/media/platform/sti/delta/delta-ipc.c: fix read buffer overflow")
There is a git repo and branch, for convenience in reviewing:
git@github.com:johnhubbard/linux.git track_user_pages_v2_linux-next_akpm_28Jan2020
FOLL_PIN support is (so far) in mmotm and linux-next. However, the patch to use FOLL_PIN to track pages was *not* submitted, because Leon saw an RDMA test suite failure that involved (I think) page refcount overflows when huge pages were used.
This patch definitively solves that kind of overflow problem, by adding an exact pincount, for compound pages (of order > 1), in the 3rd struct page of a compound page. If available, that form of pincounting is used, instead of the GUP_PIN_COUNTING_BIAS approach. Thanks again to Jan Kara for that idea.
Here's the last reviewed version of the tracking patch (v11):
https://lore.kernel.org/r/20191216222537.491123-1-jhubbard@nvidia.com
Jan Kara had provided a reviewed-by tag for that, but I've had to remove it (again) here, due to having changed the patch "a little bit", in order to add the feature described above.
Other interesting changes:
* dump_page(): added one, or two new things to report for compound pages: head refcount (for all compound pages), and map_pincount (for compound pages of order > 1).
* Documentation/core-api/pin_user_pages.rst: removed the "TODO" for the huge page refcount upper limit problems, and added notes about how it works now. Also added a note about the dump_page() enhancements.
* Added some comments in gup.c and mm.h, to explain that there are two ways to count pinned pages: exact (for compound pages of order > 1) and fuzzy (GUP_PIN_COUNTING_BIAS: for all other pages).
============================================================ General notes about the tracking patch:
This is a prerequisite to solving the problem of proper interactions between file-backed pages, and [R]DMA activities, as discussed in [1], [2], [3], [4] and in a remarkable number of email threads since about 2017. :)
In contrast to earlier approaches, the page tracking can be incrementally applied to the kernel call sites that, until now, have been simply calling get_user_pages() ("gup"). In other words, opt-in by changing from this:
get_user_pages() (sets FOLL_GET) put_page()
to this: pin_user_pages() (sets FOLL_PIN) unpin_user_page()
============================================================ Next steps:
* Convert more subsystems from get_user_pages() to pin_user_pages(). * Work with Ira and others to connect this all up with file system leases.
[1] Some slow progress on get_user_pages() (Apr 2, 2019): https://lwn.net/Articles/784574/
[2] DMA and get_user_pages() (LPC: Dec 12, 2018): https://lwn.net/Articles/774411/
[3] The trouble with get_user_pages() (Apr 30, 2018): https://lwn.net/Articles/753027/
[4] LWN kernel index: get_user_pages() https://lwn.net/Kernel/Index/#Memory_management-get_user_pages
John Hubbard (8): mm: dump_page: print head page's refcount, for compound pages mm/gup: split get_user_pages_remote() into two routines mm/gup: pass a flags arg to __gup_device_* functions mm/gup: track FOLL_PIN pages mm/gup: page->hpage_pinned_refcount: exact pin counts for huge pages mm/gup: /proc/vmstat: pin_user_pages (FOLL_PIN) reporting mm/gup_benchmark: support pin_user_pages() and related calls selftests/vm: run_vmtests: invoke gup_benchmark with basic FOLL_PIN coverage
Documentation/core-api/pin_user_pages.rst | 47 +-- include/linux/mm.h | 109 ++++- include/linux/mm_types.h | 7 +- include/linux/mmzone.h | 2 + include/linux/page_ref.h | 10 + mm/debug.c | 22 +- mm/gup.c | 460 ++++++++++++++++----- mm/gup_benchmark.c | 71 +++- mm/huge_memory.c | 29 +- mm/hugetlb.c | 44 +- mm/page_alloc.c | 2 + mm/rmap.c | 6 + mm/vmstat.c | 2 + tools/testing/selftests/vm/gup_benchmark.c | 15 +- tools/testing/selftests/vm/run_vmtests | 22 + 15 files changed, 681 insertions(+), 167 deletions(-)