On Wed, Oct 30, 2019 at 03:49:30PM -0700, John Hubbard wrote:
Document the new pin_user_pages() and related calls and behavior.
Thanks to Jan Kara and Vlastimil Babka for explaining the 4 cases in this documentation. (I've reworded it and expanded on it slightly.)
As I said before I think this may be better in a previous patch where you reference it.
Ira
Cc: Jonathan Corbet corbet@lwn.net Signed-off-by: John Hubbard jhubbard@nvidia.com
Documentation/vm/index.rst | 1 + Documentation/vm/pin_user_pages.rst | 213 ++++++++++++++++++++++++++++ 2 files changed, 214 insertions(+) create mode 100644 Documentation/vm/pin_user_pages.rst
diff --git a/Documentation/vm/index.rst b/Documentation/vm/index.rst index e8d943b21cf9..7194efa3554a 100644 --- a/Documentation/vm/index.rst +++ b/Documentation/vm/index.rst @@ -44,6 +44,7 @@ descriptions of data structures and algorithms. page_migration page_frags page_owner
- pin_user_pages remap_file_pages slub split_page_table_lock
diff --git a/Documentation/vm/pin_user_pages.rst b/Documentation/vm/pin_user_pages.rst new file mode 100644 index 000000000000..7110bca3f188 --- /dev/null +++ b/Documentation/vm/pin_user_pages.rst @@ -0,0 +1,213 @@ +.. SPDX-License-Identifier: GPL-2.0
+==================================================== +pin_user_pages() and related calls +====================================================
+.. contents:: :local:
+Overview +========
+This document describes the following functions: ::
- pin_user_pages
- pin_user_pages_fast
- pin_user_pages_remote
- pin_longterm_pages
- pin_longterm_pages_fast
- pin_longterm_pages_remote
+Basic description of FOLL_PIN +=============================
+A new flag for get_user_pages ("gup") has been added: FOLL_PIN. FOLL_PIN has +significant interactions and interdependencies with FOLL_LONGTERM, so both are +covered here.
+Both FOLL_PIN and FOLL_LONGTERM are "internal" to gup, meaning that neither +FOLL_PIN nor FOLL_LONGTERM should not appear at the gup call sites. This allows +the associated wrapper functions (pin_user_pages and others) to set the correct +combination of these flags, and to check for problems as well.
+FOLL_PIN and FOLL_GET are mutually exclusive for a given gup call. However, +multiple threads and call sites are free to pin the same struct pages, via both +FOLL_PIN and FOLL_GET. It's just the call site that needs to choose one or the +other, not the struct page(s).
+The FOLL_PIN implementation is nearly the same as FOLL_GET, except that FOLL_PIN +uses a different reference counting technique.
+FOLL_PIN is a prerequisite to FOLL_LONGTGERM. Another way of saying that is, +FOLL_LONGTERM is a specific case, more restrictive case of FOLL_PIN.
+Which flags are set by each wrapper +===================================
+Only FOLL_PIN and FOLL_LONGTERM are covered here. These flags are added to +whatever flags the caller provides::
- Function gup flags (FOLL_PIN or FOLL_LONGTERM only)
- pin_user_pages FOLL_PIN
- pin_user_pages_fast FOLL_PIN
- pin_user_pages_remote FOLL_PIN
- pin_longterm_pages FOLL_PIN | FOLL_LONGTERM
- pin_longterm_pages_fast FOLL_PIN | FOLL_LONGTERM
- pin_longterm_pages_remote FOLL_PIN | FOLL_LONGTERM
+Tracking dma-pinned pages +=========================
+Some of the key design constraints, and solutions, for tracking dma-pinned +pages:
+* An actual reference count, per struct page, is required. This is because
- multiple processes may pin and unpin a page.
+* False positives (reporting that a page is dma-pinned, when in fact it is not)
- are acceptable, but false negatives are not.
+* struct page may not be increased in size for this, and all fields are already
- used.
+* Given the above, we can overload the page->_refcount field by using, sort of,
- the upper bits in that field for a dma-pinned count. "Sort of", means that,
- rather than dividing page->_refcount into bit fields, we simple add a medium-
- large value (GUP_PIN_COUNTING_BIAS, initially chosen to be 1024: 10 bits) to
- page->_refcount. This provides fuzzy behavior: if a page has get_page() called
- on it 1024 times, then it will appear to have a single dma-pinned count.
- And again, that's acceptable.
+This also leads to limitations: there are only 32-10==22 bits available for a +counter that increments 10 bits at a time.
+TODO: for 1GB and larger huge pages, this is cutting it close. That's because +when pin_user_pages() follows such pages, it increments the head page by "1" +(where "1" used to mean "+1" for get_user_pages(), but now means "+1024" for +pin_user_pages()) for each tail page. So if you have a 1GB huge page:
+* There are 256K (18 bits) worth of 4 KB tail pages. +* There are 22 bits available to count up via GUP_PIN_COUNTING_BIAS (that is,
- 10 bits at a time)
+* There are 22 - 18 == 4 bits available to count. Except that there aren't,
- because you need to allow for a few normal get_page() calls on the head page,
- as well. Fortunately, the approach of using addition, rather than "hard"
- bitfields, within page->_refcount, allows for sharing these bits gracefully.
- But we're still looking at about 16 references.
+This, however, is a missing feature more than anything else, because it's easily +solved by addressing an obvious inefficiency in the original get_user_pages() +approach of retrieving pages: stop treating all the pages as if they were +PAGE_SIZE. Retrieve huge pages as huge pages. The callers need to be aware of +this, so some work is required. Once that's in place, this limitation mostly +disappears from view, because there will be ample refcounting range available.
+* Callers must specifically request "dma-pinned tracking of pages". In other
- words, just calling get_user_pages() will not suffice; a new set of functions,
- pin_user_page() and related, must be used.
+FOLL_PIN, FOLL_GET, FOLL_LONGTERM: when to use which flags +==========================================================
+Thanks to Jan Kara, Vlastimil Babka and several other -mm people, for describing +these categories:
+CASE 1: Direct IO (DIO) +----------------------- +There are GUP references to pages that are serving +as DIO buffers. These buffers are needed for a relatively short time (so they +are not "long term"). No special synchronization with page_mkclean() or +munmap() is provided. Therefore, flags to set at the call site are: ::
- FOLL_PIN
+...but rather than setting FOLL_PIN directly, call sites should use one of +the pin_user_pages*() routines that set FOLL_PIN.
+CASE 2: RDMA +------------ +There are GUP references to pages that are serving as DMA +buffers. These buffers are needed for a long time ("long term"). No special +synchronization with page_mkclean() or munmap() is provided. Therefore, flags +to set at the call site are: ::
- FOLL_PIN | FOLL_LONGTERM
+TODO: There is also a special case when the pages are DAX pages: in addition to +the above flags, the caller needs something like a layout lease on the +associated file. This is yet to be implemented. When it is implemented, it's +expected that the lease will be a prerequisite to setting FOLL_LONGTERM.
For now we probably want to leave this note out until we figure out how this is going to work. Best to say something like:
Some pages, such as DAX pages, can't be pinned with longterm pins and will fail.
Ira
+CASE 3: ODP +----------- +(Mellanox/Infiniband On Demand Paging: the hardware supports +replayable page faulting). There are GUP references to pages serving as DMA +buffers. For ODP, MMU notifiers are used to synchronize with page_mkclean() +and munmap(). Therefore, normal GUP calls are sufficient, so neither flag +needs to be set.
+CASE 4: Pinning for struct page manipulation only +------------------------------------------------- +Here, normal GUP calls are sufficient, so neither flag needs to be set.
+page_dma_pinned(): the whole point of pinning +=============================================
+The whole point of marking pages as "DMA-pinned" or "gup-pinned" is to be able +to query, "is this page DMA-pinned?" That allows code such as page_mkclean() +(and file system writeback code in general) to make informed decisions about +what to do when a page cannot be unmapped due to such pins.
+What to do in those cases is the subject of a years-long series of discussions +and debates (see the References at the end of this document). It's a TODO item +here: fill in the details once that's worked out. Meanwhile, it's safe to say +that having this available: ::
static inline bool page_dma_pinned(struct page *page)+...is a prerequisite to solving the long-running gup+DMA problem.
+Another way of thinking about FOLL_GET, FOLL_PIN, and FOLL_LONGTERM +===================================================================
+Another way of thinking about these flags is as a progression of restrictions: +FOLL_GET is for struct page manipulation, without affecting the data that the +struct page refers to. FOLL_PIN is a *replacement* for FOLL_GET, and is for +short term pins on pages whose data *will* get accessed. As such, FOLL_PIN is +a "more severe" form of pinning. And finally, FOLL_LONGTERM is an even more +restrictive case that has FOLL_PIN as a prerequisite: this is for pages that +will be pinned longterm, and whose data will be accessed.
+Unit testing +============ +This file::
- tools/testing/selftests/vm/gup_benchmark.c
+has the following new calls to exercise the new pin*() wrapper functions:
+* PIN_FAST_BENCHMARK (./gup_benchmark -a) +* PIN_LONGTERM_BENCHMARK (./gup_benchmark -a) +* PIN_BENCHMARK (./gup_benchmark -a)
+You can monitor how many total dma-pinned pages have been acquired and released +since the system was booted, via two new /proc/vmstat entries: ::
- /proc/vmstat/nr_foll_pin_requested
- /proc/vmstat/nr_foll_pin_requested
+Those are both going to show zero, unless CONFIG_DEBUG_VM is set. This is +because there is a noticeable performance drop in put_user_page(), when they +are activated.
+References +==========
+* `Some slow progress on get_user_pages() (Apr 2, 2019) https://lwn.net/Articles/784574/`_ +* `DMA and get_user_pages() (LPC: Dec 12, 2018) https://lwn.net/Articles/774411/`_ +* `The trouble with get_user_pages() (Apr 30, 2018) https://lwn.net/Articles/753027/`_
+John Hubbard, October, 2019
2.23.0