1.. SPDX-License-Identifier: GPL-2.0
2
3====================================================
4pin_user_pages() and related calls
5====================================================
6
7.. contents:: :local:
8
9Overview
10========
11
12This document describes the following functions::
13
14 pin_user_pages()
15 pin_user_pages_fast()
16 pin_user_pages_remote()
17
18Basic description of FOLL_PIN
19=============================
20
21FOLL_PIN and FOLL_LONGTERM are flags that can be passed to the get_user_pages*()
22("gup") family of functions. FOLL_PIN has significant interactions and
23interdependencies with FOLL_LONGTERM, so both are covered here.
24
25FOLL_PIN is internal to gup, meaning that it should not appear at the gup call
26sites. This allows the associated wrapper functions  (pin_user_pages*() and
27others) to set the correct combination of these flags, and to check for problems
28as well.
29
30FOLL_LONGTERM, on the other hand, *is* allowed to be set at the gup call sites.
31This is in order to avoid creating a large number of wrapper functions to cover
32all combinations of get*(), pin*(), FOLL_LONGTERM, and more. Also, the
33pin_user_pages*() APIs are clearly distinct from the get_user_pages*() APIs, so
34that's a natural dividing line, and a good point to make separate wrapper calls.
35In other words, use pin_user_pages*() for DMA-pinned pages, and
36get_user_pages*() for other cases. There are five cases described later on in
37this document, to further clarify that concept.
38
39FOLL_PIN and FOLL_GET are mutually exclusive for a given gup call. However,
40multiple threads and call sites are free to pin the same struct pages, via both
41FOLL_PIN and FOLL_GET. It's just the call site that needs to choose one or the
42other, not the struct page(s).
43
44The FOLL_PIN implementation is nearly the same as FOLL_GET, except that FOLL_PIN
45uses a different reference counting technique.
46
47FOLL_PIN is a prerequisite to FOLL_LONGTERM. Another way of saying that is,
48FOLL_LONGTERM is a specific case, more restrictive case of FOLL_PIN.
49
50Which flags are set by each wrapper
51===================================
52
53For these pin_user_pages*() functions, FOLL_PIN is OR'd in with whatever gup
54flags the caller provides. The caller is required to pass in a non-null struct
55pages* array, and the function then pins pages by incrementing each by a special
56value: GUP_PIN_COUNTING_BIAS.
57
58For compound pages, the GUP_PIN_COUNTING_BIAS scheme is not used. Instead,
59an exact form of pin counting is achieved, by using the 2nd struct page
60in the compound page. A new struct page field, compound_pincount, has
61been added in order to support this.
62
63This approach for compound pages avoids the counting upper limit problems that
64are discussed below. Those limitations would have been aggravated severely by
65huge pages, because each tail page adds a refcount to the head page. And in
66fact, testing revealed that, without a separate compound_pincount field,
67page overflows were seen in some huge page stress tests.
68
69This also means that huge pages and compound pages do not suffer
70from the false positives problem that is mentioned below.::
71
72 Function
73 --------
74 pin_user_pages          FOLL_PIN is always set internally by this function.
75 pin_user_pages_fast     FOLL_PIN is always set internally by this function.
76 pin_user_pages_remote   FOLL_PIN is always set internally by this function.
77
78For these get_user_pages*() functions, FOLL_GET might not even be specified.
79Behavior is a little more complex than above. If FOLL_GET was *not* specified,
80but the caller passed in a non-null struct pages* array, then the function
81sets FOLL_GET for you, and proceeds to pin pages by incrementing the refcount
82of each page by +1.::
83
84 Function
85 --------
86 get_user_pages           FOLL_GET is sometimes set internally by this function.
87 get_user_pages_fast      FOLL_GET is sometimes set internally by this function.
88 get_user_pages_remote    FOLL_GET is sometimes set internally by this function.
89
90Tracking dma-pinned pages
91=========================
92
93Some of the key design constraints, and solutions, for tracking dma-pinned
94pages:
95
96* An actual reference count, per struct page, is required. This is because
97  multiple processes may pin and unpin a page.
98
99* False positives (reporting that a page is dma-pinned, when in fact it is not)
100  are acceptable, but false negatives are not.
101
102* struct page may not be increased in size for this, and all fields are already
103  used.
104
105* Given the above, we can overload the page->_refcount field by using, sort of,
106  the upper bits in that field for a dma-pinned count. "Sort of", means that,
107  rather than dividing page->_refcount into bit fields, we simple add a medium-
108  large value (GUP_PIN_COUNTING_BIAS, initially chosen to be 1024: 10 bits) to
109  page->_refcount. This provides fuzzy behavior: if a page has get_page() called
110  on it 1024 times, then it will appear to have a single dma-pinned count.
111  And again, that's acceptable.
112
113This also leads to limitations: there are only 31-10==21 bits available for a
114counter that increments 10 bits at a time.
115
116* Callers must specifically request "dma-pinned tracking of pages". In other
117  words, just calling get_user_pages() will not suffice; a new set of functions,
118  pin_user_page() and related, must be used.
119
120FOLL_PIN, FOLL_GET, FOLL_LONGTERM: when to use which flags
121==========================================================
122
123Thanks to Jan Kara, Vlastimil Babka and several other -mm people, for describing
124these categories:
125
126CASE 1: Direct IO (DIO)
127-----------------------
128There are GUP references to pages that are serving
129as DIO buffers. These buffers are needed for a relatively short time (so they
130are not "long term"). No special synchronization with page_mkclean() or
131munmap() is provided. Therefore, flags to set at the call site are: ::
132
133    FOLL_PIN
134
135...but rather than setting FOLL_PIN directly, call sites should use one of
136the pin_user_pages*() routines that set FOLL_PIN.
137
138CASE 2: RDMA
139------------
140There are GUP references to pages that are serving as DMA
141buffers. These buffers are needed for a long time ("long term"). No special
142synchronization with page_mkclean() or munmap() is provided. Therefore, flags
143to set at the call site are: ::
144
145    FOLL_PIN | FOLL_LONGTERM
146
147NOTE: Some pages, such as DAX pages, cannot be pinned with longterm pins. That's
148because DAX pages do not have a separate page cache, and so "pinning" implies
149locking down file system blocks, which is not (yet) supported in that way.
150
151CASE 3: MMU notifier registration, with or without page faulting hardware
152-------------------------------------------------------------------------
153Device drivers can pin pages via get_user_pages*(), and register for mmu
154notifier callbacks for the memory range. Then, upon receiving a notifier
155"invalidate range" callback , stop the device from using the range, and unpin
156the pages. There may be other possible schemes, such as for example explicitly
157synchronizing against pending IO, that accomplish approximately the same thing.
158
159Or, if the hardware supports replayable page faults, then the device driver can
160avoid pinning entirely (this is ideal), as follows: register for mmu notifier
161callbacks as above, but instead of stopping the device and unpinning in the
162callback, simply remove the range from the device's page tables.
163
164Either way, as long as the driver unpins the pages upon mmu notifier callback,
165then there is proper synchronization with both filesystem and mm
166(page_mkclean(), munmap(), etc). Therefore, neither flag needs to be set.
167
168CASE 4: Pinning for struct page manipulation only
169-------------------------------------------------
170If only struct page data (as opposed to the actual memory contents that a page
171is tracking) is affected, then normal GUP calls are sufficient, and neither flag
172needs to be set.
173
174CASE 5: Pinning in order to write to the data within the page
175-------------------------------------------------------------
176Even though neither DMA nor Direct IO is involved, just a simple case of "pin,
177write to a page's data, unpin" can cause a problem. Case 5 may be considered a
178superset of Case 1, plus Case 2, plus anything that invokes that pattern. In
179other words, if the code is neither Case 1 nor Case 2, it may still require
180FOLL_PIN, for patterns like this:
181
182Correct (uses FOLL_PIN calls):
183    pin_user_pages()
184    write to the data within the pages
185    unpin_user_pages()
186
187INCORRECT (uses FOLL_GET calls):
188    get_user_pages()
189    write to the data within the pages
190    put_page()
191
192page_maybe_dma_pinned(): the whole point of pinning
193===================================================
194
195The whole point of marking pages as "DMA-pinned" or "gup-pinned" is to be able
196to query, "is this page DMA-pinned?" That allows code such as page_mkclean()
197(and file system writeback code in general) to make informed decisions about
198what to do when a page cannot be unmapped due to such pins.
199
200What to do in those cases is the subject of a years-long series of discussions
201and debates (see the References at the end of this document). It's a TODO item
202here: fill in the details once that's worked out. Meanwhile, it's safe to say
203that having this available: ::
204
205        static inline bool page_maybe_dma_pinned(struct page *page)
206
207...is a prerequisite to solving the long-running gup+DMA problem.
208
209Another way of thinking about FOLL_GET, FOLL_PIN, and FOLL_LONGTERM
210===================================================================
211
212Another way of thinking about these flags is as a progression of restrictions:
213FOLL_GET is for struct page manipulation, without affecting the data that the
214struct page refers to. FOLL_PIN is a *replacement* for FOLL_GET, and is for
215short term pins on pages whose data *will* get accessed. As such, FOLL_PIN is
216a "more severe" form of pinning. And finally, FOLL_LONGTERM is an even more
217restrictive case that has FOLL_PIN as a prerequisite: this is for pages that
218will be pinned longterm, and whose data will be accessed.
219
220Unit testing
221============
222This file::
223
224 tools/testing/selftests/vm/gup_test.c
225
226has the following new calls to exercise the new pin*() wrapper functions:
227
228* PIN_FAST_BENCHMARK (./gup_test -a)
229* PIN_BASIC_TEST (./gup_test -b)
230
231You can monitor how many total dma-pinned pages have been acquired and released
232since the system was booted, via two new /proc/vmstat entries: ::
233
234    /proc/vmstat/nr_foll_pin_acquired
235    /proc/vmstat/nr_foll_pin_released
236
237Under normal conditions, these two values will be equal unless there are any
238long-term [R]DMA pins in place, or during pin/unpin transitions.
239
240* nr_foll_pin_acquired: This is the number of logical pins that have been
241  acquired since the system was powered on. For huge pages, the head page is
242  pinned once for each page (head page and each tail page) within the huge page.
243  This follows the same sort of behavior that get_user_pages() uses for huge
244  pages: the head page is refcounted once for each tail or head page in the huge
245  page, when get_user_pages() is applied to a huge page.
246
247* nr_foll_pin_released: The number of logical pins that have been released since
248  the system was powered on. Note that pages are released (unpinned) on a
249  PAGE_SIZE granularity, even if the original pin was applied to a huge page.
250  Becaused of the pin count behavior described above in "nr_foll_pin_acquired",
251  the accounting balances out, so that after doing this::
252
253    pin_user_pages(huge_page);
254    for (each page in huge_page)
255        unpin_user_page(page);
256
257...the following is expected::
258
259    nr_foll_pin_released == nr_foll_pin_acquired
260
261(...unless it was already out of balance due to a long-term RDMA pin being in
262place.)
263
264Other diagnostics
265=================
266
267dump_page() has been enhanced slightly, to handle these new counting
268fields, and to better report on compound pages in general. Specifically,
269for compound pages, the exact (compound_pincount) pincount is reported.
270
271References
272==========
273
274* `Some slow progress on get_user_pages() (Apr 2, 2019) <https://lwn.net/Articles/784574/>`_
275* `DMA and get_user_pages() (LPC: Dec 12, 2018) <https://lwn.net/Articles/774411/>`_
276* `The trouble with get_user_pages() (Apr 30, 2018) <https://lwn.net/Articles/753027/>`_
277* `LWN kernel index: get_user_pages() <https://lwn.net/Kernel/Index/#Memory_management-get_user_pages>`_
278
279John Hubbard, October, 2019
280