1 .. SPDX-License-Identifier: GPL-2.0+
2
3 ===========
4 Folio Queue
5 ===========
6
7 :Author: David Howells <dhowells@redhat.com>
8
9 .. Contents:
10
11 * Overview
12 * Initialisation
13 * Adding and removing folios
14 * Querying information about a folio
15 * Querying information about a folio_queue
16 * Folio queue iteration
17 * Folio marks
18 * Lockless simultaneous production/consumption issues
19
20
21 Overview
22 ========
23
24 The folio_queue struct forms a single segment in a segmented list of folios
25 that can be used to form an I/O buffer. As such, the list can be iterated over
26 using the ITER_FOLIOQ iov_iter type.
27
28 The publicly accessible members of the structure are::
29
30 struct folio_queue {
31 struct folio_queue *next;
32 struct folio_queue *prev;
33 ...
34 };
35
36 A pair of pointers are provided, ``next`` and ``prev``, that point to the
37 segments on either side of the segment being accessed. Whilst this is a
38 doubly-linked list, it is intentionally not a circular list; the outward
39 sibling pointers in terminal segments should be NULL.
40
41 Each segment in the list also stores:
42
43 * an ordered sequence of folio pointers,
44 * the size of each folio and
45 * three 1-bit marks per folio,
46
47 but hese should not be accessed directly as the underlying data structure may
48 change, but rather the access functions outlined below should be used.
49
50 The facility can be made accessible by::
51
52 #include <linux/folio_queue.h>
53
54 and to use the iterator::
55
56 #include <linux/uio.h>
57
58
59 Initialisation
60 ==============
61
62 A segment should be initialised by calling::
63
64 void folioq_init(struct folio_queue *folioq);
65
66 with a pointer to the segment to be initialised. Note that this will not
67 necessarily initialise all the folio pointers, so care must be taken to check
68 the number of folios added.
69
70
71 Adding and removing folios
72 ==========================
73
74 Folios can be set in the next unused slot in a segment struct by calling one
75 of::
76
77 unsigned int folioq_append(struct folio_queue *folioq,
78 struct folio *folio);
79
80 unsigned int folioq_append_mark(struct folio_queue *folioq,
81 struct folio *folio);
82
83 Both functions update the stored folio count, store the folio and note its
84 size. The second function also sets the first mark for the folio added. Both
85 functions return the number of the slot used. [!] Note that no attempt is made
86 to check that the capacity wasn't overrun and the list will not be extended
87 automatically.
88
89 A folio can be excised by calling::
90
91 void folioq_clear(struct folio_queue *folioq, unsigned int slot);
92
93 This clears the slot in the array and also clears all the marks for that folio,
94 but doesn't change the folio count - so future accesses of that slot must check
95 if the slot is occupied.
96
97
98 Querying information about a folio
99 ==================================
100
101 Information about the folio in a particular slot may be queried by the
102 following function::
103
104 struct folio *folioq_folio(const struct folio_queue *folioq,
105 unsigned int slot);
106
107 If a folio has not yet been set in that slot, this may yield an undefined
108 pointer. The size of the folio in a slot may be queried with either of::
109
110 unsigned int folioq_folio_order(const struct folio_queue *folioq,
111 unsigned int slot);
112
113 size_t folioq_folio_size(const struct folio_queue *folioq,
114 unsigned int slot);
115
116 The first function returns the size as an order and the second as a number of
117 bytes.
118
119
120 Querying information about a folio_queue
121 ========================================
122
123 Information may be retrieved about a particular segment with the following
124 functions::
125
126 unsigned int folioq_nr_slots(const struct folio_queue *folioq);
127
128 unsigned int folioq_count(struct folio_queue *folioq);
129
130 bool folioq_full(struct folio_queue *folioq);
131
132 The first function returns the maximum capacity of a segment. It must not be
133 assumed that this won't vary between segments. The second returns the number
134 of folios added to a segments and the third is a shorthand to indicate if the
135 segment has been filled to capacity.
136
137 Not that the count and fullness are not affected by clearing folios from the
138 segment. These are more about indicating how many slots in the array have been
139 initialised, and it assumed that slots won't get reused, but rather the segment
140 will get discarded as the queue is consumed.
141
142
143 Folio marks
144 ===========
145
146 Folios within a queue can also have marks assigned to them. These marks can be
147 used to note information such as if a folio needs folio_put() calling upon it.
148 There are three marks available to be set for each folio.
149
150 The marks can be set by::
151
152 void folioq_mark(struct folio_queue *folioq, unsigned int slot);
153 void folioq_mark2(struct folio_queue *folioq, unsigned int slot);
154 void folioq_mark3(struct folio_queue *folioq, unsigned int slot);
155
156 Cleared by::
157
158 void folioq_unmark(struct folio_queue *folioq, unsigned int slot);
159 void folioq_unmark2(struct folio_queue *folioq, unsigned int slot);
160 void folioq_unmark3(struct folio_queue *folioq, unsigned int slot);
161
162 And the marks can be queried by::
163
164 bool folioq_is_marked(const struct folio_queue *folioq, unsigned int slot);
165 bool folioq_is_marked2(const struct folio_queue *folioq, unsigned int slot);
166 bool folioq_is_marked3(const struct folio_queue *folioq, unsigned int slot);
167
168 The marks can be used for any purpose and are not interpreted by this API.
169
170
171 Folio queue iteration
172 =====================
173
174 A list of segments may be iterated over using the I/O iterator facility using
175 an ``iov_iter`` iterator of ``ITER_FOLIOQ`` type. The iterator may be
176 initialised with::
177
178 void iov_iter_folio_queue(struct iov_iter *i, unsigned int direction,
179 const struct folio_queue *folioq,
180 unsigned int first_slot, unsigned int offset,
181 size_t count);
182
183 This may be told to start at a particular segment, slot and offset within a
184 queue. The iov iterator functions will follow the next pointers when advancing
185 and prev pointers when reverting when needed.
186
187
188 Lockless simultaneous production/consumption issues
189 ===================================================
190
191 If properly managed, the list can be extended by the producer at the head end
192 and shortened by the consumer at the tail end simultaneously without the need
193 to take locks. The ITER_FOLIOQ iterator inserts appropriate barriers to aid
194 with this.
195
196 Care must be taken when simultaneously producing and consuming a list. If the
197 last segment is reached and the folios it refers to are entirely consumed by
198 the IOV iterators, an iov_iter struct will be left pointing to the last segment
199 with a slot number equal to the capacity of that segment. The iterator will
200 try to continue on from this if there's another segment available when it is
201 used again, but care must be taken lest the segment got removed and freed by
202 the consumer before the iterator was advanced.
203
204 It is recommended that the queue always contain at least one segment, even if
205 that segment has never been filled or is entirely spent. This prevents the
206 head and tail pointers from collapsing.
207
208
209 API Function Reference
210 ======================
211
212 .. kernel-doc:: include/linux/folio_queue.h
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