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mm/gup: introduce pin_user_pages*() and FOLL_PIN
Introduce pin_user_pages*() variations of get_user_pages*() calls, and
also pin_longterm_pages*() variations.
For now, these are placeholder calls, until the various call sites are
converted to use the correct get_user_pages*() or pin_user_pages*() API.
These variants will eventually all set FOLL_PIN, which is also
introduced, and thoroughly documented.
pin_user_pages()
pin_user_pages_remote()
pin_user_pages_fast()
All pages that are pinned via the above calls, must be unpinned via
put_user_page().
The underlying rules are:
* FOLL_PIN is a gup-internal flag, so the call sites should not directly
set it. That behavior is enforced with assertions.
* Call sites that want to indicate that they are going to do DirectIO
("DIO") or something with similar characteristics, should call a
get_user_pages()-like wrapper call that sets FOLL_PIN. These wrappers
will:
* Start with "pin_user_pages" instead of "get_user_pages". That
makes it easy to find and audit the call sites.
* Set FOLL_PIN
* For pages that are received via FOLL_PIN, those pages must be returned
via put_user_page().
Thanks to Jan Kara and Vlastimil Babka for explaining the 4 cases in
this documentation. (I've reworded it and expanded upon it.)
Link: http://lkml.kernel.org/r/20200107224558.2362728-12-jhubbard@nvidia.com
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Mike Rapoport <rppt@linux.ibm.com> [Documentation]
Reviewed-by: Jérôme Glisse <jglisse@redhat.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Ira Weiny <ira.weiny@intel.com>
Cc: Alex Williamson <alex.williamson@redhat.com>
Cc: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Cc: Björn Töpel <bjorn.topel@intel.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Cc: Jason Gunthorpe <jgg@mellanox.com>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Kirill A. Shutemov <kirill@shutemov.name>
Cc: Leon Romanovsky <leonro@mellanox.com>
Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>- Loading branch information
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,232 @@ | ||
| .. SPDX-License-Identifier: GPL-2.0 | ||
| ==================================================== | ||
| pin_user_pages() and related calls | ||
| ==================================================== | ||
|
|
||
| .. contents:: :local: | ||
|
|
||
| Overview | ||
| ======== | ||
|
|
||
| This document describes the following functions:: | ||
|
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||
| pin_user_pages() | ||
| pin_user_pages_fast() | ||
| pin_user_pages_remote() | ||
|
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| Basic description of FOLL_PIN | ||
| ============================= | ||
|
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||
| FOLL_PIN and FOLL_LONGTERM are flags that can be passed to the get_user_pages*() | ||
| ("gup") family of functions. FOLL_PIN has significant interactions and | ||
| interdependencies with FOLL_LONGTERM, so both are covered here. | ||
|
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||
| FOLL_PIN is internal to gup, meaning that it 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_LONGTERM, on the other hand, *is* allowed to be set at the gup call sites. | ||
| This is in order to avoid creating a large number of wrapper functions to cover | ||
| all combinations of get*(), pin*(), FOLL_LONGTERM, and more. Also, the | ||
| pin_user_pages*() APIs are clearly distinct from the get_user_pages*() APIs, so | ||
| that's a natural dividing line, and a good point to make separate wrapper calls. | ||
| In other words, use pin_user_pages*() for DMA-pinned pages, and | ||
| get_user_pages*() for other cases. There are four cases described later on in | ||
| this document, to further clarify that concept. | ||
|
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||
| 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). | ||
|
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||
| The FOLL_PIN implementation is nearly the same as FOLL_GET, except that FOLL_PIN | ||
| uses a different reference counting technique. | ||
|
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| FOLL_PIN is a prerequisite to FOLL_LONGTERM. Another way of saying that is, | ||
| FOLL_LONGTERM is a specific case, more restrictive case of FOLL_PIN. | ||
|
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| Which flags are set by each wrapper | ||
| =================================== | ||
|
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| For these pin_user_pages*() functions, FOLL_PIN is OR'd in with whatever gup | ||
| flags the caller provides. The caller is required to pass in a non-null struct | ||
| pages* array, and the function then pin pages by incrementing each by a special | ||
| value. For now, that value is +1, just like get_user_pages*().:: | ||
|
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||
| Function | ||
| -------- | ||
| pin_user_pages FOLL_PIN is always set internally by this function. | ||
| pin_user_pages_fast FOLL_PIN is always set internally by this function. | ||
| pin_user_pages_remote FOLL_PIN is always set internally by this function. | ||
|
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||
| For these get_user_pages*() functions, FOLL_GET might not even be specified. | ||
| Behavior is a little more complex than above. If FOLL_GET was *not* specified, | ||
| but the caller passed in a non-null struct pages* array, then the function | ||
| sets FOLL_GET for you, and proceeds to pin pages by incrementing the refcount | ||
| of each page by +1.:: | ||
|
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||
| Function | ||
| -------- | ||
| get_user_pages FOLL_GET is sometimes set internally by this function. | ||
| get_user_pages_fast FOLL_GET is sometimes set internally by this function. | ||
| get_user_pages_remote FOLL_GET is sometimes set internally by this function. | ||
|
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||
| Tracking dma-pinned pages | ||
| ========================= | ||
|
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||
| Some of the key design constraints, and solutions, for tracking dma-pinned | ||
| pages: | ||
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| * An actual reference count, per struct page, is required. This is because | ||
| multiple processes may pin and unpin a page. | ||
|
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| * False positives (reporting that a page is dma-pinned, when in fact it is not) | ||
| are acceptable, but false negatives are not. | ||
|
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| * struct page may not be increased in size for this, and all fields are already | ||
| used. | ||
|
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| * 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. | ||
|
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||
| This also leads to limitations: there are only 31-10==21 bits available for a | ||
| counter that increments 10 bits at a time. | ||
|
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| 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: | ||
|
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||
| * There are 256K (18 bits) worth of 4 KB tail pages. | ||
| * There are 21 bits available to count up via GUP_PIN_COUNTING_BIAS (that is, | ||
| 10 bits at a time) | ||
| * There are 21 - 18 == 3 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 8 references. | ||
|
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||
| 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. | ||
|
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||
| * 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. | ||
|
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||
| 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: | ||
|
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||
| 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: :: | ||
|
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| FOLL_PIN | ||
|
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||
| ...but rather than setting FOLL_PIN directly, call sites should use one of | ||
| the pin_user_pages*() routines that set FOLL_PIN. | ||
|
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||
| 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: :: | ||
|
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||
| FOLL_PIN | FOLL_LONGTERM | ||
|
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||
| NOTE: Some pages, such as DAX pages, cannot be pinned with longterm pins. That's | ||
| because DAX pages do not have a separate page cache, and so "pinning" implies | ||
| locking down file system blocks, which is not (yet) supported in that way. | ||
|
|
||
| CASE 3: Hardware with page faulting support | ||
| ------------------------------------------- | ||
| Here, a well-written driver doesn't normally need to pin pages at all. However, | ||
| if the driver does choose to do so, it can register MMU notifiers for the range, | ||
| and will be called back upon invalidation. Either way (avoiding page pinning, or | ||
| using MMU notifiers to unpin upon request), there is proper synchronization with | ||
| both filesystem and mm (page_mkclean(), munmap(), etc). | ||
|
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||
| Therefore, neither flag needs to be set. | ||
|
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||
| In this case, ideally, neither get_user_pages() nor pin_user_pages() should be | ||
| called. Instead, the software should be written so that it does not pin pages. | ||
| This allows mm and filesystems to operate more efficiently and reliably. | ||
|
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||
| CASE 4: Pinning for struct page manipulation only | ||
| ------------------------------------------------- | ||
| Here, normal GUP calls are sufficient, so neither flag needs to be set. | ||
|
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||
| page_dma_pinned(): the whole point of pinning | ||
| ============================================= | ||
|
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||
| 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: :: | ||
|
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||
| static inline bool page_dma_pinned(struct page *page) | ||
|
|
||
| ...is a prerequisite to solving the long-running gup+DMA problem. | ||
|
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||
| 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. | ||
|
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| Unit testing | ||
| ============ | ||
| This file:: | ||
|
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||
| tools/testing/selftests/vm/gup_benchmark.c | ||
|
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| has the following new calls to exercise the new pin*() wrapper functions: | ||
|
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| * PIN_FAST_BENCHMARK (./gup_benchmark -a) | ||
| * PIN_BENCHMARK (./gup_benchmark -b) | ||
|
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| 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: :: | ||
|
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||
| /proc/vmstat/nr_foll_pin_requested | ||
| /proc/vmstat/nr_foll_pin_requested | ||
|
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| 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. | ||
|
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||
| 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 |
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