xref: /openbmc/linux/block/blk-flush.c (revision f7c35abe)
1 /*
2  * Functions to sequence FLUSH and FUA writes.
3  *
4  * Copyright (C) 2011		Max Planck Institute for Gravitational Physics
5  * Copyright (C) 2011		Tejun Heo <tj@kernel.org>
6  *
7  * This file is released under the GPLv2.
8  *
9  * REQ_{FLUSH|FUA} requests are decomposed to sequences consisted of three
10  * optional steps - PREFLUSH, DATA and POSTFLUSH - according to the request
11  * properties and hardware capability.
12  *
13  * If a request doesn't have data, only REQ_PREFLUSH makes sense, which
14  * indicates a simple flush request.  If there is data, REQ_PREFLUSH indicates
15  * that the device cache should be flushed before the data is executed, and
16  * REQ_FUA means that the data must be on non-volatile media on request
17  * completion.
18  *
19  * If the device doesn't have writeback cache, FLUSH and FUA don't make any
20  * difference.  The requests are either completed immediately if there's no
21  * data or executed as normal requests otherwise.
22  *
23  * If the device has writeback cache and supports FUA, REQ_PREFLUSH is
24  * translated to PREFLUSH but REQ_FUA is passed down directly with DATA.
25  *
26  * If the device has writeback cache and doesn't support FUA, REQ_PREFLUSH
27  * is translated to PREFLUSH and REQ_FUA to POSTFLUSH.
28  *
29  * The actual execution of flush is double buffered.  Whenever a request
30  * needs to execute PRE or POSTFLUSH, it queues at
31  * fq->flush_queue[fq->flush_pending_idx].  Once certain criteria are met, a
32  * REQ_OP_FLUSH is issued and the pending_idx is toggled.  When the flush
33  * completes, all the requests which were pending are proceeded to the next
34  * step.  This allows arbitrary merging of different types of FLUSH/FUA
35  * requests.
36  *
37  * Currently, the following conditions are used to determine when to issue
38  * flush.
39  *
40  * C1. At any given time, only one flush shall be in progress.  This makes
41  *     double buffering sufficient.
42  *
43  * C2. Flush is deferred if any request is executing DATA of its sequence.
44  *     This avoids issuing separate POSTFLUSHes for requests which shared
45  *     PREFLUSH.
46  *
47  * C3. The second condition is ignored if there is a request which has
48  *     waited longer than FLUSH_PENDING_TIMEOUT.  This is to avoid
49  *     starvation in the unlikely case where there are continuous stream of
50  *     FUA (without FLUSH) requests.
51  *
52  * For devices which support FUA, it isn't clear whether C2 (and thus C3)
53  * is beneficial.
54  *
55  * Note that a sequenced FLUSH/FUA request with DATA is completed twice.
56  * Once while executing DATA and again after the whole sequence is
57  * complete.  The first completion updates the contained bio but doesn't
58  * finish it so that the bio submitter is notified only after the whole
59  * sequence is complete.  This is implemented by testing RQF_FLUSH_SEQ in
60  * req_bio_endio().
61  *
62  * The above peculiarity requires that each FLUSH/FUA request has only one
63  * bio attached to it, which is guaranteed as they aren't allowed to be
64  * merged in the usual way.
65  */
66 
67 #include <linux/kernel.h>
68 #include <linux/module.h>
69 #include <linux/bio.h>
70 #include <linux/blkdev.h>
71 #include <linux/gfp.h>
72 #include <linux/blk-mq.h>
73 
74 #include "blk.h"
75 #include "blk-mq.h"
76 #include "blk-mq-tag.h"
77 #include "blk-mq-sched.h"
78 
79 /* FLUSH/FUA sequences */
80 enum {
81 	REQ_FSEQ_PREFLUSH	= (1 << 0), /* pre-flushing in progress */
82 	REQ_FSEQ_DATA		= (1 << 1), /* data write in progress */
83 	REQ_FSEQ_POSTFLUSH	= (1 << 2), /* post-flushing in progress */
84 	REQ_FSEQ_DONE		= (1 << 3),
85 
86 	REQ_FSEQ_ACTIONS	= REQ_FSEQ_PREFLUSH | REQ_FSEQ_DATA |
87 				  REQ_FSEQ_POSTFLUSH,
88 
89 	/*
90 	 * If flush has been pending longer than the following timeout,
91 	 * it's issued even if flush_data requests are still in flight.
92 	 */
93 	FLUSH_PENDING_TIMEOUT	= 5 * HZ,
94 };
95 
96 static bool blk_kick_flush(struct request_queue *q,
97 			   struct blk_flush_queue *fq);
98 
99 static unsigned int blk_flush_policy(unsigned long fflags, struct request *rq)
100 {
101 	unsigned int policy = 0;
102 
103 	if (blk_rq_sectors(rq))
104 		policy |= REQ_FSEQ_DATA;
105 
106 	if (fflags & (1UL << QUEUE_FLAG_WC)) {
107 		if (rq->cmd_flags & REQ_PREFLUSH)
108 			policy |= REQ_FSEQ_PREFLUSH;
109 		if (!(fflags & (1UL << QUEUE_FLAG_FUA)) &&
110 		    (rq->cmd_flags & REQ_FUA))
111 			policy |= REQ_FSEQ_POSTFLUSH;
112 	}
113 	return policy;
114 }
115 
116 static unsigned int blk_flush_cur_seq(struct request *rq)
117 {
118 	return 1 << ffz(rq->flush.seq);
119 }
120 
121 static void blk_flush_restore_request(struct request *rq)
122 {
123 	/*
124 	 * After flush data completion, @rq->bio is %NULL but we need to
125 	 * complete the bio again.  @rq->biotail is guaranteed to equal the
126 	 * original @rq->bio.  Restore it.
127 	 */
128 	rq->bio = rq->biotail;
129 
130 	/* make @rq a normal request */
131 	rq->rq_flags &= ~RQF_FLUSH_SEQ;
132 	rq->end_io = rq->flush.saved_end_io;
133 }
134 
135 static bool blk_flush_queue_rq(struct request *rq, bool add_front)
136 {
137 	if (rq->q->mq_ops) {
138 		blk_mq_add_to_requeue_list(rq, add_front, true);
139 		return false;
140 	} else {
141 		if (add_front)
142 			list_add(&rq->queuelist, &rq->q->queue_head);
143 		else
144 			list_add_tail(&rq->queuelist, &rq->q->queue_head);
145 		return true;
146 	}
147 }
148 
149 /**
150  * blk_flush_complete_seq - complete flush sequence
151  * @rq: FLUSH/FUA request being sequenced
152  * @fq: flush queue
153  * @seq: sequences to complete (mask of %REQ_FSEQ_*, can be zero)
154  * @error: whether an error occurred
155  *
156  * @rq just completed @seq part of its flush sequence, record the
157  * completion and trigger the next step.
158  *
159  * CONTEXT:
160  * spin_lock_irq(q->queue_lock or fq->mq_flush_lock)
161  *
162  * RETURNS:
163  * %true if requests were added to the dispatch queue, %false otherwise.
164  */
165 static bool blk_flush_complete_seq(struct request *rq,
166 				   struct blk_flush_queue *fq,
167 				   unsigned int seq, int error)
168 {
169 	struct request_queue *q = rq->q;
170 	struct list_head *pending = &fq->flush_queue[fq->flush_pending_idx];
171 	bool queued = false, kicked;
172 
173 	BUG_ON(rq->flush.seq & seq);
174 	rq->flush.seq |= seq;
175 
176 	if (likely(!error))
177 		seq = blk_flush_cur_seq(rq);
178 	else
179 		seq = REQ_FSEQ_DONE;
180 
181 	switch (seq) {
182 	case REQ_FSEQ_PREFLUSH:
183 	case REQ_FSEQ_POSTFLUSH:
184 		/* queue for flush */
185 		if (list_empty(pending))
186 			fq->flush_pending_since = jiffies;
187 		list_move_tail(&rq->flush.list, pending);
188 		break;
189 
190 	case REQ_FSEQ_DATA:
191 		list_move_tail(&rq->flush.list, &fq->flush_data_in_flight);
192 		queued = blk_flush_queue_rq(rq, true);
193 		break;
194 
195 	case REQ_FSEQ_DONE:
196 		/*
197 		 * @rq was previously adjusted by blk_flush_issue() for
198 		 * flush sequencing and may already have gone through the
199 		 * flush data request completion path.  Restore @rq for
200 		 * normal completion and end it.
201 		 */
202 		BUG_ON(!list_empty(&rq->queuelist));
203 		list_del_init(&rq->flush.list);
204 		blk_flush_restore_request(rq);
205 		if (q->mq_ops)
206 			blk_mq_end_request(rq, error);
207 		else
208 			__blk_end_request_all(rq, error);
209 		break;
210 
211 	default:
212 		BUG();
213 	}
214 
215 	kicked = blk_kick_flush(q, fq);
216 	return kicked | queued;
217 }
218 
219 static void flush_end_io(struct request *flush_rq, int error)
220 {
221 	struct request_queue *q = flush_rq->q;
222 	struct list_head *running;
223 	bool queued = false;
224 	struct request *rq, *n;
225 	unsigned long flags = 0;
226 	struct blk_flush_queue *fq = blk_get_flush_queue(q, flush_rq->mq_ctx);
227 
228 	if (q->mq_ops) {
229 		struct blk_mq_hw_ctx *hctx;
230 
231 		/* release the tag's ownership to the req cloned from */
232 		spin_lock_irqsave(&fq->mq_flush_lock, flags);
233 		hctx = blk_mq_map_queue(q, flush_rq->mq_ctx->cpu);
234 		blk_mq_tag_set_rq(hctx, flush_rq->tag, fq->orig_rq);
235 		flush_rq->tag = -1;
236 	}
237 
238 	running = &fq->flush_queue[fq->flush_running_idx];
239 	BUG_ON(fq->flush_pending_idx == fq->flush_running_idx);
240 
241 	/* account completion of the flush request */
242 	fq->flush_running_idx ^= 1;
243 
244 	if (!q->mq_ops)
245 		elv_completed_request(q, flush_rq);
246 
247 	/* and push the waiting requests to the next stage */
248 	list_for_each_entry_safe(rq, n, running, flush.list) {
249 		unsigned int seq = blk_flush_cur_seq(rq);
250 
251 		BUG_ON(seq != REQ_FSEQ_PREFLUSH && seq != REQ_FSEQ_POSTFLUSH);
252 		queued |= blk_flush_complete_seq(rq, fq, seq, error);
253 	}
254 
255 	/*
256 	 * Kick the queue to avoid stall for two cases:
257 	 * 1. Moving a request silently to empty queue_head may stall the
258 	 * queue.
259 	 * 2. When flush request is running in non-queueable queue, the
260 	 * queue is hold. Restart the queue after flush request is finished
261 	 * to avoid stall.
262 	 * This function is called from request completion path and calling
263 	 * directly into request_fn may confuse the driver.  Always use
264 	 * kblockd.
265 	 */
266 	if (queued || fq->flush_queue_delayed) {
267 		WARN_ON(q->mq_ops);
268 		blk_run_queue_async(q);
269 	}
270 	fq->flush_queue_delayed = 0;
271 	if (q->mq_ops)
272 		spin_unlock_irqrestore(&fq->mq_flush_lock, flags);
273 }
274 
275 /**
276  * blk_kick_flush - consider issuing flush request
277  * @q: request_queue being kicked
278  * @fq: flush queue
279  *
280  * Flush related states of @q have changed, consider issuing flush request.
281  * Please read the comment at the top of this file for more info.
282  *
283  * CONTEXT:
284  * spin_lock_irq(q->queue_lock or fq->mq_flush_lock)
285  *
286  * RETURNS:
287  * %true if flush was issued, %false otherwise.
288  */
289 static bool blk_kick_flush(struct request_queue *q, struct blk_flush_queue *fq)
290 {
291 	struct list_head *pending = &fq->flush_queue[fq->flush_pending_idx];
292 	struct request *first_rq =
293 		list_first_entry(pending, struct request, flush.list);
294 	struct request *flush_rq = fq->flush_rq;
295 
296 	/* C1 described at the top of this file */
297 	if (fq->flush_pending_idx != fq->flush_running_idx || list_empty(pending))
298 		return false;
299 
300 	/* C2 and C3
301 	 *
302 	 * For blk-mq + scheduling, we can risk having all driver tags
303 	 * assigned to empty flushes, and we deadlock if we are expecting
304 	 * other requests to make progress. Don't defer for that case.
305 	 */
306 	if (!list_empty(&fq->flush_data_in_flight) &&
307 	    !(q->mq_ops && q->elevator) &&
308 	    time_before(jiffies,
309 			fq->flush_pending_since + FLUSH_PENDING_TIMEOUT))
310 		return false;
311 
312 	/*
313 	 * Issue flush and toggle pending_idx.  This makes pending_idx
314 	 * different from running_idx, which means flush is in flight.
315 	 */
316 	fq->flush_pending_idx ^= 1;
317 
318 	blk_rq_init(q, flush_rq);
319 
320 	/*
321 	 * Borrow tag from the first request since they can't
322 	 * be in flight at the same time. And acquire the tag's
323 	 * ownership for flush req.
324 	 */
325 	if (q->mq_ops) {
326 		struct blk_mq_hw_ctx *hctx;
327 
328 		flush_rq->mq_ctx = first_rq->mq_ctx;
329 		flush_rq->tag = first_rq->tag;
330 		fq->orig_rq = first_rq;
331 
332 		hctx = blk_mq_map_queue(q, first_rq->mq_ctx->cpu);
333 		blk_mq_tag_set_rq(hctx, first_rq->tag, flush_rq);
334 	}
335 
336 	flush_rq->cmd_flags = REQ_OP_FLUSH | REQ_PREFLUSH;
337 	flush_rq->rq_flags |= RQF_FLUSH_SEQ;
338 	flush_rq->rq_disk = first_rq->rq_disk;
339 	flush_rq->end_io = flush_end_io;
340 
341 	return blk_flush_queue_rq(flush_rq, false);
342 }
343 
344 static void flush_data_end_io(struct request *rq, int error)
345 {
346 	struct request_queue *q = rq->q;
347 	struct blk_flush_queue *fq = blk_get_flush_queue(q, NULL);
348 
349 	/*
350 	 * Updating q->in_flight[] here for making this tag usable
351 	 * early. Because in blk_queue_start_tag(),
352 	 * q->in_flight[BLK_RW_ASYNC] is used to limit async I/O and
353 	 * reserve tags for sync I/O.
354 	 *
355 	 * More importantly this way can avoid the following I/O
356 	 * deadlock:
357 	 *
358 	 * - suppose there are 40 fua requests comming to flush queue
359 	 *   and queue depth is 31
360 	 * - 30 rqs are scheduled then blk_queue_start_tag() can't alloc
361 	 *   tag for async I/O any more
362 	 * - all the 30 rqs are completed before FLUSH_PENDING_TIMEOUT
363 	 *   and flush_data_end_io() is called
364 	 * - the other rqs still can't go ahead if not updating
365 	 *   q->in_flight[BLK_RW_ASYNC] here, meantime these rqs
366 	 *   are held in flush data queue and make no progress of
367 	 *   handling post flush rq
368 	 * - only after the post flush rq is handled, all these rqs
369 	 *   can be completed
370 	 */
371 
372 	elv_completed_request(q, rq);
373 
374 	/* for avoiding double accounting */
375 	rq->rq_flags &= ~RQF_STARTED;
376 
377 	/*
378 	 * After populating an empty queue, kick it to avoid stall.  Read
379 	 * the comment in flush_end_io().
380 	 */
381 	if (blk_flush_complete_seq(rq, fq, REQ_FSEQ_DATA, error))
382 		blk_run_queue_async(q);
383 }
384 
385 static void mq_flush_data_end_io(struct request *rq, int error)
386 {
387 	struct request_queue *q = rq->q;
388 	struct blk_mq_hw_ctx *hctx;
389 	struct blk_mq_ctx *ctx = rq->mq_ctx;
390 	unsigned long flags;
391 	struct blk_flush_queue *fq = blk_get_flush_queue(q, ctx);
392 
393 	hctx = blk_mq_map_queue(q, ctx->cpu);
394 
395 	/*
396 	 * After populating an empty queue, kick it to avoid stall.  Read
397 	 * the comment in flush_end_io().
398 	 */
399 	spin_lock_irqsave(&fq->mq_flush_lock, flags);
400 	blk_flush_complete_seq(rq, fq, REQ_FSEQ_DATA, error);
401 	spin_unlock_irqrestore(&fq->mq_flush_lock, flags);
402 
403 	blk_mq_run_hw_queue(hctx, true);
404 }
405 
406 /**
407  * blk_insert_flush - insert a new FLUSH/FUA request
408  * @rq: request to insert
409  *
410  * To be called from __elv_add_request() for %ELEVATOR_INSERT_FLUSH insertions.
411  * or __blk_mq_run_hw_queue() to dispatch request.
412  * @rq is being submitted.  Analyze what needs to be done and put it on the
413  * right queue.
414  *
415  * CONTEXT:
416  * spin_lock_irq(q->queue_lock) in !mq case
417  */
418 void blk_insert_flush(struct request *rq)
419 {
420 	struct request_queue *q = rq->q;
421 	unsigned long fflags = q->queue_flags;	/* may change, cache */
422 	unsigned int policy = blk_flush_policy(fflags, rq);
423 	struct blk_flush_queue *fq = blk_get_flush_queue(q, rq->mq_ctx);
424 
425 	/*
426 	 * @policy now records what operations need to be done.  Adjust
427 	 * REQ_PREFLUSH and FUA for the driver.
428 	 */
429 	rq->cmd_flags &= ~REQ_PREFLUSH;
430 	if (!(fflags & (1UL << QUEUE_FLAG_FUA)))
431 		rq->cmd_flags &= ~REQ_FUA;
432 
433 	/*
434 	 * REQ_PREFLUSH|REQ_FUA implies REQ_SYNC, so if we clear any
435 	 * of those flags, we have to set REQ_SYNC to avoid skewing
436 	 * the request accounting.
437 	 */
438 	rq->cmd_flags |= REQ_SYNC;
439 
440 	/*
441 	 * An empty flush handed down from a stacking driver may
442 	 * translate into nothing if the underlying device does not
443 	 * advertise a write-back cache.  In this case, simply
444 	 * complete the request.
445 	 */
446 	if (!policy) {
447 		if (q->mq_ops)
448 			blk_mq_end_request(rq, 0);
449 		else
450 			__blk_end_bidi_request(rq, 0, 0, 0);
451 		return;
452 	}
453 
454 	BUG_ON(rq->bio != rq->biotail); /*assumes zero or single bio rq */
455 
456 	/*
457 	 * If there's data but flush is not necessary, the request can be
458 	 * processed directly without going through flush machinery.  Queue
459 	 * for normal execution.
460 	 */
461 	if ((policy & REQ_FSEQ_DATA) &&
462 	    !(policy & (REQ_FSEQ_PREFLUSH | REQ_FSEQ_POSTFLUSH))) {
463 		if (q->mq_ops)
464 			blk_mq_sched_insert_request(rq, false, true, false, false);
465 		else
466 			list_add_tail(&rq->queuelist, &q->queue_head);
467 		return;
468 	}
469 
470 	/*
471 	 * @rq should go through flush machinery.  Mark it part of flush
472 	 * sequence and submit for further processing.
473 	 */
474 	memset(&rq->flush, 0, sizeof(rq->flush));
475 	INIT_LIST_HEAD(&rq->flush.list);
476 	rq->rq_flags |= RQF_FLUSH_SEQ;
477 	rq->flush.saved_end_io = rq->end_io; /* Usually NULL */
478 	if (q->mq_ops) {
479 		rq->end_io = mq_flush_data_end_io;
480 
481 		spin_lock_irq(&fq->mq_flush_lock);
482 		blk_flush_complete_seq(rq, fq, REQ_FSEQ_ACTIONS & ~policy, 0);
483 		spin_unlock_irq(&fq->mq_flush_lock);
484 		return;
485 	}
486 	rq->end_io = flush_data_end_io;
487 
488 	blk_flush_complete_seq(rq, fq, REQ_FSEQ_ACTIONS & ~policy, 0);
489 }
490 
491 /**
492  * blkdev_issue_flush - queue a flush
493  * @bdev:	blockdev to issue flush for
494  * @gfp_mask:	memory allocation flags (for bio_alloc)
495  * @error_sector:	error sector
496  *
497  * Description:
498  *    Issue a flush for the block device in question. Caller can supply
499  *    room for storing the error offset in case of a flush error, if they
500  *    wish to. If WAIT flag is not passed then caller may check only what
501  *    request was pushed in some internal queue for later handling.
502  */
503 int blkdev_issue_flush(struct block_device *bdev, gfp_t gfp_mask,
504 		sector_t *error_sector)
505 {
506 	struct request_queue *q;
507 	struct bio *bio;
508 	int ret = 0;
509 
510 	if (bdev->bd_disk == NULL)
511 		return -ENXIO;
512 
513 	q = bdev_get_queue(bdev);
514 	if (!q)
515 		return -ENXIO;
516 
517 	/*
518 	 * some block devices may not have their queue correctly set up here
519 	 * (e.g. loop device without a backing file) and so issuing a flush
520 	 * here will panic. Ensure there is a request function before issuing
521 	 * the flush.
522 	 */
523 	if (!q->make_request_fn)
524 		return -ENXIO;
525 
526 	bio = bio_alloc(gfp_mask, 0);
527 	bio->bi_bdev = bdev;
528 	bio->bi_opf = REQ_OP_WRITE | REQ_PREFLUSH;
529 
530 	ret = submit_bio_wait(bio);
531 
532 	/*
533 	 * The driver must store the error location in ->bi_sector, if
534 	 * it supports it. For non-stacked drivers, this should be
535 	 * copied from blk_rq_pos(rq).
536 	 */
537 	if (error_sector)
538 		*error_sector = bio->bi_iter.bi_sector;
539 
540 	bio_put(bio);
541 	return ret;
542 }
543 EXPORT_SYMBOL(blkdev_issue_flush);
544 
545 struct blk_flush_queue *blk_alloc_flush_queue(struct request_queue *q,
546 		int node, int cmd_size)
547 {
548 	struct blk_flush_queue *fq;
549 	int rq_sz = sizeof(struct request);
550 
551 	fq = kzalloc_node(sizeof(*fq), GFP_KERNEL, node);
552 	if (!fq)
553 		goto fail;
554 
555 	if (q->mq_ops)
556 		spin_lock_init(&fq->mq_flush_lock);
557 
558 	rq_sz = round_up(rq_sz + cmd_size, cache_line_size());
559 	fq->flush_rq = kzalloc_node(rq_sz, GFP_KERNEL, node);
560 	if (!fq->flush_rq)
561 		goto fail_rq;
562 
563 	INIT_LIST_HEAD(&fq->flush_queue[0]);
564 	INIT_LIST_HEAD(&fq->flush_queue[1]);
565 	INIT_LIST_HEAD(&fq->flush_data_in_flight);
566 
567 	return fq;
568 
569  fail_rq:
570 	kfree(fq);
571  fail:
572 	return NULL;
573 }
574 
575 void blk_free_flush_queue(struct blk_flush_queue *fq)
576 {
577 	/* bio based request queue hasn't flush queue */
578 	if (!fq)
579 		return;
580 
581 	kfree(fq->flush_rq);
582 	kfree(fq);
583 }
584