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