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 large folios, the GUP_PIN_COUNTING_BIAS scheme is not used. Instead,
59the extra space available in the struct folio is used to store the
60pincount directly.
61
62This approach for large folios avoids the counting upper limit problems
63that are discussed below. Those limitations would have been aggravated
64severely by huge pages, because each tail page adds a refcount to the
65head page. And in fact, testing revealed that, without a separate pincount
66field, refcount overflows were seen in some huge page stress tests.
67
68This also means that huge pages and large folios do not suffer
69from the false positives problem that is mentioned below.::
70
71 Function
72 --------
73 pin_user_pages          FOLL_PIN is always set internally by this function.
74 pin_user_pages_fast     FOLL_PIN is always set internally by this function.
75 pin_user_pages_remote   FOLL_PIN is always set internally by this function.
76
77For these get_user_pages*() functions, FOLL_GET might not even be specified.
78Behavior is a little more complex than above. If FOLL_GET was *not* specified,
79but the caller passed in a non-null struct pages* array, then the function
80sets FOLL_GET for you, and proceeds to pin pages by incrementing the refcount
81of each page by +1.::
82
83 Function
84 --------
85 get_user_pages           FOLL_GET is sometimes set internally by this function.
86 get_user_pages_fast      FOLL_GET is sometimes set internally by this function.
87 get_user_pages_remote    FOLL_GET is sometimes set internally by this function.
88
89Tracking dma-pinned pages
90=========================
91
92Some of the key design constraints, and solutions, for tracking dma-pinned
93pages:
94
95* An actual reference count, per struct page, is required. This is because
96  multiple processes may pin and unpin a page.
97
98* False positives (reporting that a page is dma-pinned, when in fact it is not)
99  are acceptable, but false negatives are not.
100
101* struct page may not be increased in size for this, and all fields are already
102  used.
103
104* Given the above, we can overload the page->_refcount field by using, sort of,
105  the upper bits in that field for a dma-pinned count. "Sort of", means that,
106  rather than dividing page->_refcount into bit fields, we simple add a medium-
107  large value (GUP_PIN_COUNTING_BIAS, initially chosen to be 1024: 10 bits) to
108  page->_refcount. This provides fuzzy behavior: if a page has get_page() called
109  on it 1024 times, then it will appear to have a single dma-pinned count.
110  And again, that's acceptable.
111
112This also leads to limitations: there are only 31-10==21 bits available for a
113counter that increments 10 bits at a time.
114
115* Because of that limitation, special handling is applied to the zero pages
116  when using FOLL_PIN.  We only pretend to pin a zero page - we don't alter its
117  refcount or pincount at all (it is permanent, so there's no need).  The
118  unpinning functions also don't do anything to a zero page.  This is
119  transparent to the caller.
120
121* Callers must specifically request "dma-pinned tracking of pages". In other
122  words, just calling get_user_pages() will not suffice; a new set of functions,
123  pin_user_page() and related, must be used.
124
125FOLL_PIN, FOLL_GET, FOLL_LONGTERM: when to use which flags
126==========================================================
127
128Thanks to Jan Kara, Vlastimil Babka and several other -mm people, for describing
129these categories:
130
131CASE 1: Direct IO (DIO)
132-----------------------
133There are GUP references to pages that are serving
134as DIO buffers. These buffers are needed for a relatively short time (so they
135are not "long term"). No special synchronization with page_mkclean() or
136munmap() is provided. Therefore, flags to set at the call site are: ::
137
138    FOLL_PIN
139
140...but rather than setting FOLL_PIN directly, call sites should use one of
141the pin_user_pages*() routines that set FOLL_PIN.
142
143CASE 2: RDMA
144------------
145There are GUP references to pages that are serving as DMA
146buffers. These buffers are needed for a long time ("long term"). No special
147synchronization with page_mkclean() or munmap() is provided. Therefore, flags
148to set at the call site are: ::
149
150    FOLL_PIN | FOLL_LONGTERM
151
152NOTE: Some pages, such as DAX pages, cannot be pinned with longterm pins. That's
153because DAX pages do not have a separate page cache, and so "pinning" implies
154locking down file system blocks, which is not (yet) supported in that way.
155
156CASE 3: MMU notifier registration, with or without page faulting hardware
157-------------------------------------------------------------------------
158Device drivers can pin pages via get_user_pages*(), and register for mmu
159notifier callbacks for the memory range. Then, upon receiving a notifier
160"invalidate range" callback , stop the device from using the range, and unpin
161the pages. There may be other possible schemes, such as for example explicitly
162synchronizing against pending IO, that accomplish approximately the same thing.
163
164Or, if the hardware supports replayable page faults, then the device driver can
165avoid pinning entirely (this is ideal), as follows: register for mmu notifier
166callbacks as above, but instead of stopping the device and unpinning in the
167callback, simply remove the range from the device's page tables.
168
169Either way, as long as the driver unpins the pages upon mmu notifier callback,
170then there is proper synchronization with both filesystem and mm
171(page_mkclean(), munmap(), etc). Therefore, neither flag needs to be set.
172
173CASE 4: Pinning for struct page manipulation only
174-------------------------------------------------
175If only struct page data (as opposed to the actual memory contents that a page
176is tracking) is affected, then normal GUP calls are sufficient, and neither flag
177needs to be set.
178
179CASE 5: Pinning in order to write to the data within the page
180-------------------------------------------------------------
181Even though neither DMA nor Direct IO is involved, just a simple case of "pin,
182write to a page's data, unpin" can cause a problem. Case 5 may be considered a
183superset of Case 1, plus Case 2, plus anything that invokes that pattern. In
184other words, if the code is neither Case 1 nor Case 2, it may still require
185FOLL_PIN, for patterns like this:
186
187Correct (uses FOLL_PIN calls):
188    pin_user_pages()
189    write to the data within the pages
190    unpin_user_pages()
191
192INCORRECT (uses FOLL_GET calls):
193    get_user_pages()
194    write to the data within the pages
195    put_page()
196
197page_maybe_dma_pinned(): the whole point of pinning
198===================================================
199
200The whole point of marking pages as "DMA-pinned" or "gup-pinned" is to be able
201to query, "is this page DMA-pinned?" That allows code such as page_mkclean()
202(and file system writeback code in general) to make informed decisions about
203what to do when a page cannot be unmapped due to such pins.
204
205What to do in those cases is the subject of a years-long series of discussions
206and debates (see the References at the end of this document). It's a TODO item
207here: fill in the details once that's worked out. Meanwhile, it's safe to say
208that having this available: ::
209
210        static inline bool page_maybe_dma_pinned(struct page *page)
211
212...is a prerequisite to solving the long-running gup+DMA problem.
213
214Another way of thinking about FOLL_GET, FOLL_PIN, and FOLL_LONGTERM
215===================================================================
216
217Another way of thinking about these flags is as a progression of restrictions:
218FOLL_GET is for struct page manipulation, without affecting the data that the
219struct page refers to. FOLL_PIN is a *replacement* for FOLL_GET, and is for
220short term pins on pages whose data *will* get accessed. As such, FOLL_PIN is
221a "more severe" form of pinning. And finally, FOLL_LONGTERM is an even more
222restrictive case that has FOLL_PIN as a prerequisite: this is for pages that
223will be pinned longterm, and whose data will be accessed.
224
225Unit testing
226============
227This file::
228
229 tools/testing/selftests/mm/gup_test.c
230
231has the following new calls to exercise the new pin*() wrapper functions:
232
233* PIN_FAST_BENCHMARK (./gup_test -a)
234* PIN_BASIC_TEST (./gup_test -b)
235
236You can monitor how many total dma-pinned pages have been acquired and released
237since the system was booted, via two new /proc/vmstat entries: ::
238
239    /proc/vmstat/nr_foll_pin_acquired
240    /proc/vmstat/nr_foll_pin_released
241
242Under normal conditions, these two values will be equal unless there are any
243long-term [R]DMA pins in place, or during pin/unpin transitions.
244
245* nr_foll_pin_acquired: This is the number of logical pins that have been
246  acquired since the system was powered on. For huge pages, the head page is
247  pinned once for each page (head page and each tail page) within the huge page.
248  This follows the same sort of behavior that get_user_pages() uses for huge
249  pages: the head page is refcounted once for each tail or head page in the huge
250  page, when get_user_pages() is applied to a huge page.
251
252* nr_foll_pin_released: The number of logical pins that have been released since
253  the system was powered on. Note that pages are released (unpinned) on a
254  PAGE_SIZE granularity, even if the original pin was applied to a huge page.
255  Becaused of the pin count behavior described above in "nr_foll_pin_acquired",
256  the accounting balances out, so that after doing this::
257
258    pin_user_pages(huge_page);
259    for (each page in huge_page)
260        unpin_user_page(page);
261
262...the following is expected::
263
264    nr_foll_pin_released == nr_foll_pin_acquired
265
266(...unless it was already out of balance due to a long-term RDMA pin being in
267place.)
268
269Other diagnostics
270=================
271
272dump_page() has been enhanced slightly to handle these new counting
273fields, and to better report on large folios in general.  Specifically,
274for large folios, the exact pincount is reported.
275
276References
277==========
278
279* `Some slow progress on get_user_pages() (Apr 2, 2019) <https://lwn.net/Articles/784574/>`_
280* `DMA and get_user_pages() (LPC: Dec 12, 2018) <https://lwn.net/Articles/774411/>`_
281* `The trouble with get_user_pages() (Apr 30, 2018) <https://lwn.net/Articles/753027/>`_
282* `LWN kernel index: get_user_pages() <https://lwn.net/Kernel/Index/#Memory_management-get_user_pages>`_
283
284John Hubbard, October, 2019
285