kernel / fs / netfs / io.c

// SPDX-License-Identifier: GPL-2.0-or-later
/* Network filesystem high-level read support.
 *
 * Copyright (C) 2021 Red Hat, Inc. All Rights Reserved.
 * Written by David Howells ([e-mail])
 */

#include <linux/module.h>
#include <linux/export.h>
#include <linux/fs.h>
#include <linux/mm.h>
#include <linux/pagemap.h>
#include <linux/slab.h>
#include <linux/uio.h>
#include <linux/sched/mm.h>
#include <linux/task_io_accounting_ops.h>
#include "internal.h"

/*
 * Clear the unread part of an I/O request.
 */
static void netfs_clear_unread(struct netfs_io_subrequest *subreq)
{
	iov_iter_zero(iov_iter_count(&subreq->io_iter), &subreq->io_iter);
}

static void netfs_cache_read_terminated(void *priv, ssize_t transferred_or_error,
					bool was_async)
{
	struct netfs_io_subrequest *subreq = priv;

	netfs_subreq_terminated(subreq, transferred_or_error, was_async);
}

/*
 * Issue a read against the cache.
 * - Eats the caller's ref on subreq.
 */
static void netfs_read_from_cache(struct netfs_io_request *rreq,
				  struct netfs_io_subrequest *subreq,
				  enum netfs_read_from_hole read_hole)
{
	struct netfs_cache_resources *cres = &rreq->cache_resources;

	netfs_stat(&netfs_n_rh_read);
	cres->ops->read(cres, subreq->start, &subreq->io_iter, read_hole,
			netfs_cache_read_terminated, subreq);
}

/*
 * Fill a subrequest region with zeroes.
 */
static void netfs_fill_with_zeroes(struct netfs_io_request *rreq,
				   struct netfs_io_subrequest *subreq)
{
	netfs_stat(&netfs_n_rh_zero);
	__set_bit(NETFS_SREQ_CLEAR_TAIL, &subreq->flags);
	netfs_subreq_terminated(subreq, 0, false);
}

/*
 * Ask the netfs to issue a read request to the server for us.
 *
 * The netfs is expected to read from subreq->pos + subreq->transferred to
 * subreq->pos + subreq->len - 1.  It may not backtrack and write data into the
 * buffer prior to the transferred point as it might clobber dirty data
 * obtained from the cache.
 *
 * Alternatively, the netfs is allowed to indicate one of two things:
 *
 * - NETFS_SREQ_SHORT_READ: A short read - it will get called again to try and
 *   make progress.
 *
 * - NETFS_SREQ_CLEAR_TAIL: A short read - the rest of the buffer will be
 *   cleared.
 */
static void netfs_read_from_server(struct netfs_io_request *rreq,
				   struct netfs_io_subrequest *subreq)
{
	netfs_stat(&netfs_n_rh_download);

	if (rreq->origin != NETFS_DIO_READ &&
	    iov_iter_count(&subreq->io_iter) != subreq->len - subreq->transferred)
		pr_warn("R=%08x[%u] ITER PRE-MISMATCH %zx != %zx-%zx %lx\n",
			rreq->debug_id, subreq->debug_index,
			iov_iter_count(&subreq->io_iter), subreq->len,
			subreq->transferred, subreq->flags);
	rreq->netfs_ops->issue_read(subreq);
}

/*
 * Release those waiting.
 */
static void netfs_rreq_completed(struct netfs_io_request *rreq, bool was_async)
{
	trace_netfs_rreq(rreq, netfs_rreq_trace_done);
	netfs_clear_subrequests(rreq, was_async);
	netfs_put_request(rreq, was_async, netfs_rreq_trace_put_complete);
}

/*
 * [DEPRECATED] Deal with the completion of writing the data to the cache.  We
 * have to clear the PG_fscache bits on the folios involved and release the
 * caller's ref.
 *
 * May be called in softirq mode and we inherit a ref from the caller.
 */
static void netfs_rreq_unmark_after_write(struct netfs_io_request *rreq,
					  bool was_async)
{
	struct netfs_io_subrequest *subreq;
	struct folio *folio;
	pgoff_t unlocked = 0;
	bool have_unlocked = false;

	rcu_read_lock();

	list_for_each_entry(subreq, &rreq->subrequests, rreq_link) {
		XA_STATE(xas, &rreq->mapping->i_pages, subreq->start / PAGE_SIZE);

		xas_for_each(&xas, folio, (subreq->start + subreq->len - 1) / PAGE_SIZE) {
			if (xas_retry(&xas, folio))
				continue;

			/* We might have multiple writes from the same huge
			 * folio, but we mustn't unlock a folio more than once.
			 */
			if (have_unlocked && folio->index <= unlocked)
				continue;
			unlocked = folio_next_index(folio) - 1;
			trace_netfs_folio(folio, netfs_folio_trace_end_copy);
			folio_end_private_2(folio);
			have_unlocked = true;
		}
	}

	rcu_read_unlock();
	netfs_rreq_completed(rreq, was_async);
}

static void netfs_rreq_copy_terminated(void *priv, ssize_t transferred_or_error,
				       bool was_async) /* [DEPRECATED] */
{
	struct netfs_io_subrequest *subreq = priv;
	struct netfs_io_request *rreq = subreq->rreq;

	if (IS_ERR_VALUE(transferred_or_error)) {
		netfs_stat(&netfs_n_rh_write_failed);
		trace_netfs_failure(rreq, subreq, transferred_or_error,
				    netfs_fail_copy_to_cache);
	} else {
		netfs_stat(&netfs_n_rh_write_done);
	}

	trace_netfs_sreq(subreq, netfs_sreq_trace_write_term);

	/* If we decrement nr_copy_ops to 0, the ref belongs to us. */
	if (atomic_dec_and_test(&rreq->nr_copy_ops))
		netfs_rreq_unmark_after_write(rreq, was_async);

	netfs_put_subrequest(subreq, was_async, netfs_sreq_trace_put_terminated);
}

/*
 * [DEPRECATED] Perform any outstanding writes to the cache.  We inherit a ref
 * from the caller.
 */
static void netfs_rreq_do_write_to_cache(struct netfs_io_request *rreq)
{
	struct netfs_cache_resources *cres = &rreq->cache_resources;
	struct netfs_io_subrequest *subreq, *next, *p;
	struct iov_iter iter;
	int ret;

	trace_netfs_rreq(rreq, netfs_rreq_trace_copy);

	/* We don't want terminating writes trying to wake us up whilst we're
	 * still going through the list.
	 */
	atomic_inc(&rreq->nr_copy_ops);

	list_for_each_entry_safe(subreq, p, &rreq->subrequests, rreq_link) {
		if (!test_bit(NETFS_SREQ_COPY_TO_CACHE, &subreq->flags)) {
			list_del_init(&subreq->rreq_link);
			netfs_put_subrequest(subreq, false,
					     netfs_sreq_trace_put_no_copy);
		}
	}

	list_for_each_entry(subreq, &rreq->subrequests, rreq_link) {
		/* Amalgamate adjacent writes */
		while (!list_is_last(&subreq->rreq_link, &rreq->subrequests)) {
			next = list_next_entry(subreq, rreq_link);
			if (next->start != subreq->start + subreq->len)
				break;
			subreq->len += next->len;
			list_del_init(&next->rreq_link);
			netfs_put_subrequest(next, false,
					     netfs_sreq_trace_put_merged);
		}

		ret = cres->ops->prepare_write(cres, &subreq->start, &subreq->len,
					       subreq->len, rreq->i_size, true);
		if (ret < 0) {
			trace_netfs_failure(rreq, subreq, ret, netfs_fail_prepare_write);
			trace_netfs_sreq(subreq, netfs_sreq_trace_write_skip);
			continue;
		}

		iov_iter_xarray(&iter, ITER_SOURCE, &rreq->mapping->i_pages,
				subreq->start, subreq->len);

		atomic_inc(&rreq->nr_copy_ops);
		netfs_stat(&netfs_n_rh_write);
		netfs_get_subrequest(subreq, netfs_sreq_trace_get_copy_to_cache);
		trace_netfs_sreq(subreq, netfs_sreq_trace_write);
		cres->ops->write(cres, subreq->start, &iter,
				 netfs_rreq_copy_terminated, subreq);
	}

	/* If we decrement nr_copy_ops to 0, the usage ref belongs to us. */
	if (atomic_dec_and_test(&rreq->nr_copy_ops))
		netfs_rreq_unmark_after_write(rreq, false);
}

static void netfs_rreq_write_to_cache_work(struct work_struct *work) /* [DEPRECATED] */
{
	struct netfs_io_request *rreq =
		container_of(work, struct netfs_io_request, work);

	netfs_rreq_do_write_to_cache(rreq);
}

static void netfs_rreq_write_to_cache(struct netfs_io_request *rreq) /* [DEPRECATED] */
{
	rreq->work.func = netfs_rreq_write_to_cache_work;
	if (!queue_work(system_unbound_wq, &rreq->work))
		BUG();
}

/*
 * Handle a short read.
 */
static void netfs_rreq_short_read(struct netfs_io_request *rreq,
				  struct netfs_io_subrequest *subreq)
{
	__clear_bit(NETFS_SREQ_SHORT_IO, &subreq->flags);
	__set_bit(NETFS_SREQ_SEEK_DATA_READ, &subreq->flags);

	netfs_stat(&netfs_n_rh_short_read);
	trace_netfs_sreq(subreq, netfs_sreq_trace_resubmit_short);

	netfs_get_subrequest(subreq, netfs_sreq_trace_get_short_read);
	atomic_inc(&rreq->nr_outstanding);
	if (subreq->source == NETFS_READ_FROM_CACHE)
		netfs_read_from_cache(rreq, subreq, NETFS_READ_HOLE_CLEAR);
	else
		netfs_read_from_server(rreq, subreq);
}

/*
 * Reset the subrequest iterator prior to resubmission.
 */
static void netfs_reset_subreq_iter(struct netfs_io_request *rreq,
				    struct netfs_io_subrequest *subreq)
{
	size_t remaining = subreq->len - subreq->transferred;
	size_t count = iov_iter_count(&subreq->io_iter);

	if (count == remaining)
		return;

	_debug("R=%08x[%u] ITER RESUB-MISMATCH %zx != %zx-%zx-%llx %x",
	       rreq->debug_id, subreq->debug_index,
	       iov_iter_count(&subreq->io_iter), subreq->transferred,
	       subreq->len, rreq->i_size,
	       subreq->io_iter.iter_type);

	if (count < remaining)
		iov_iter_revert(&subreq->io_iter, remaining - count);
	else
		iov_iter_advance(&subreq->io_iter, count - remaining);
}

/*
 * Resubmit any short or failed operations.  Returns true if we got the rreq
 * ref back.
 */
static bool netfs_rreq_perform_resubmissions(struct netfs_io_request *rreq)
{
	struct netfs_io_subrequest *subreq;

	WARN_ON(in_interrupt());

	trace_netfs_rreq(rreq, netfs_rreq_trace_resubmit);

	/* We don't want terminating submissions trying to wake us up whilst
	 * we're still going through the list.
	 */
	atomic_inc(&rreq->nr_outstanding);

	__clear_bit(NETFS_RREQ_INCOMPLETE_IO, &rreq->flags);
	list_for_each_entry(subreq, &rreq->subrequests, rreq_link) {
		if (subreq->error) {
			if (subreq->source != NETFS_READ_FROM_CACHE)
				break;
			subreq->source = NETFS_DOWNLOAD_FROM_SERVER;
			subreq->error = 0;
			__set_bit(NETFS_SREQ_RETRYING, &subreq->flags);
			netfs_stat(&netfs_n_rh_download_instead);
			trace_netfs_sreq(subreq, netfs_sreq_trace_download_instead);
			netfs_get_subrequest(subreq, netfs_sreq_trace_get_resubmit);
			atomic_inc(&rreq->nr_outstanding);
			netfs_reset_subreq_iter(rreq, subreq);
			netfs_read_from_server(rreq, subreq);
		} else if (test_bit(NETFS_SREQ_SHORT_IO, &subreq->flags)) {
			__set_bit(NETFS_SREQ_RETRYING, &subreq->flags);
			netfs_reset_subreq_iter(rreq, subreq);
			netfs_rreq_short_read(rreq, subreq);
		}
	}

	/* If we decrement nr_outstanding to 0, the usage ref belongs to us. */
	if (atomic_dec_and_test(&rreq->nr_outstanding))
		return true;

	wake_up_var(&rreq->nr_outstanding);
	return false;
}

/*
 * Check to see if the data read is still valid.
 */
static void netfs_rreq_is_still_valid(struct netfs_io_request *rreq)
{
	struct netfs_io_subrequest *subreq;

	if (!rreq->netfs_ops->is_still_valid ||
	    rreq->netfs_ops->is_still_valid(rreq))
		return;

	list_for_each_entry(subreq, &rreq->subrequests, rreq_link) {
		if (subreq->source == NETFS_READ_FROM_CACHE) {
			subreq->error = -ESTALE;
			__set_bit(NETFS_RREQ_INCOMPLETE_IO, &rreq->flags);
		}
	}
}

/*
 * Determine how much we can admit to having read from a DIO read.
 */
static void netfs_rreq_assess_dio(struct netfs_io_request *rreq)
{
	struct netfs_io_subrequest *subreq;
	unsigned int i;
	size_t transferred = 0;

	for (i = 0; i < rreq->direct_bv_count; i++) {
		flush_dcache_page(rreq->direct_bv[i].bv_page);
		// TODO: cifs marks pages in the destination buffer
		// dirty under some circumstances after a read.  Do we
		// need to do that too?
		set_page_dirty(rreq->direct_bv[i].bv_page);
	}

	list_for_each_entry(subreq, &rreq->subrequests, rreq_link) {
		if (subreq->error || subreq->transferred == 0)
			break;
		transferred += subreq->transferred;
		if (subreq->transferred < subreq->len ||
		    test_bit(NETFS_SREQ_HIT_EOF, &subreq->flags))
			break;
	}

	for (i = 0; i < rreq->direct_bv_count; i++)
		flush_dcache_page(rreq->direct_bv[i].bv_page);

	rreq->transferred = transferred;
	task_io_account_read(transferred);

	if (rreq->iocb) {
		rreq->iocb->ki_pos += transferred;
		if (rreq->iocb->ki_complete)
			rreq->iocb->ki_complete(
				rreq->iocb, rreq->error ? rreq->error : transferred);
	}
	if (rreq->netfs_ops->done)
		rreq->netfs_ops->done(rreq);
	inode_dio_end(rreq->inode);
}

/*
 * Assess the state of a read request and decide what to do next.
 *
 * Note that we could be in an ordinary kernel thread, on a workqueue or in
 * softirq context at this point.  We inherit a ref from the caller.
 */
static void netfs_rreq_assess(struct netfs_io_request *rreq, bool was_async)
{
	trace_netfs_rreq(rreq, netfs_rreq_trace_assess);

again:
	netfs_rreq_is_still_valid(rreq);

	if (!test_bit(NETFS_RREQ_FAILED, &rreq->flags) &&
	    test_bit(NETFS_RREQ_INCOMPLETE_IO, &rreq->flags)) {
		if (netfs_rreq_perform_resubmissions(rreq))
			goto again;
		return;
	}

	if (rreq->origin != NETFS_DIO_READ)
		netfs_rreq_unlock_folios(rreq);
	else
		netfs_rreq_assess_dio(rreq);

	trace_netfs_rreq(rreq, netfs_rreq_trace_wake_ip);
	clear_bit_unlock(NETFS_RREQ_IN_PROGRESS, &rreq->flags);
	wake_up_bit(&rreq->flags, NETFS_RREQ_IN_PROGRESS);

	if (test_bit(NETFS_RREQ_COPY_TO_CACHE, &rreq->flags) &&
	    test_bit(NETFS_RREQ_USE_PGPRIV2, &rreq->flags))
		return netfs_rreq_write_to_cache(rreq);

	netfs_rreq_completed(rreq, was_async);
}

static void netfs_rreq_work(struct work_struct *work)
{
	struct netfs_io_request *rreq =
		container_of(work, struct netfs_io_request, work);
	netfs_rreq_assess(rreq, false);
}

/*
 * Handle the completion of all outstanding I/O operations on a read request.
 * We inherit a ref from the caller.
 */
static void netfs_rreq_terminated(struct netfs_io_request *rreq,
				  bool was_async)
{
	if (test_bit(NETFS_RREQ_INCOMPLETE_IO, &rreq->flags) &&
	    was_async) {
		if (!queue_work(system_unbound_wq, &rreq->work))
			BUG();
	} else {
		netfs_rreq_assess(rreq, was_async);
	}
}

/**
 * netfs_subreq_terminated - Note the termination of an I/O operation.
 * @subreq: The I/O request that has terminated.
 * @transferred_or_error: The amount of data transferred or an error code.
 * @was_async: The termination was asynchronous
 *
 * This tells the read helper that a contributory I/O operation has terminated,
 * one way or another, and that it should integrate the results.
 *
 * The caller indicates in @transferred_or_error the outcome of the operation,
 * supplying a positive value to indicate the number of bytes transferred, 0 to
 * indicate a failure to transfer anything that should be retried or a negative
 * error code.  The helper will look after reissuing I/O operations as
 * appropriate and writing downloaded data to the cache.
 *
 * If @was_async is true, the caller might be running in softirq or interrupt
 * context and we can't sleep.
 */
void netfs_subreq_terminated(struct netfs_io_subrequest *subreq,
			     ssize_t transferred_or_error,
			     bool was_async)
{
	struct netfs_io_request *rreq = subreq->rreq;
	int u;

	_enter("R=%x[%x]{%llx,%lx},%zd",
	       rreq->debug_id, subreq->debug_index,
	       subreq->start, subreq->flags, transferred_or_error);

	switch (subreq->source) {
	case NETFS_READ_FROM_CACHE:
		netfs_stat(&netfs_n_rh_read_done);
		break;
	case NETFS_DOWNLOAD_FROM_SERVER:
		netfs_stat(&netfs_n_rh_download_done);
		break;
	default:
		break;
	}

	if (IS_ERR_VALUE(transferred_or_error)) {
		subreq->error = transferred_or_error;
		trace_netfs_failure(rreq, subreq, transferred_or_error,
				    netfs_fail_read);
		goto failed;
	}

	if (WARN(transferred_or_error > subreq->len - subreq->transferred,
		 "Subreq overread: R%x[%x] %zd > %zu - %zu",
		 rreq->debug_id, subreq->debug_index,
		 transferred_or_error, subreq->len, subreq->transferred))
		transferred_or_error = subreq->len - subreq->transferred;

	subreq->error = 0;
	subreq->transferred += transferred_or_error;
	if (subreq->transferred < subreq->len &&
	    !test_bit(NETFS_SREQ_HIT_EOF, &subreq->flags))
		goto incomplete;

complete:
	__clear_bit(NETFS_SREQ_NO_PROGRESS, &subreq->flags);
	if (test_bit(NETFS_SREQ_COPY_TO_CACHE, &subreq->flags))
		set_bit(NETFS_RREQ_COPY_TO_CACHE, &rreq->flags);

out:
	trace_netfs_sreq(subreq, netfs_sreq_trace_terminated);

	/* If we decrement nr_outstanding to 0, the ref belongs to us. */
	u = atomic_dec_return(&rreq->nr_outstanding);
	if (u == 0)
		netfs_rreq_terminated(rreq, was_async);
	else if (u == 1)
		wake_up_var(&rreq->nr_outstanding);

	netfs_put_subrequest(subreq, was_async, netfs_sreq_trace_put_terminated);
	return;

incomplete:
	if (test_bit(NETFS_SREQ_CLEAR_TAIL, &subreq->flags)) {
		netfs_clear_unread(subreq);
		subreq->transferred = subreq->len;
		goto complete;
	}

	if (transferred_or_error == 0) {
		if (__test_and_set_bit(NETFS_SREQ_NO_PROGRESS, &subreq->flags)) {
			if (rreq->origin != NETFS_DIO_READ)
				subreq->error = -ENODATA;
			goto failed;
		}
	} else {
		__clear_bit(NETFS_SREQ_NO_PROGRESS, &subreq->flags);
	}

	__set_bit(NETFS_SREQ_SHORT_IO, &subreq->flags);
	set_bit(NETFS_RREQ_INCOMPLETE_IO, &rreq->flags);
	goto out;

failed:
	if (subreq->source == NETFS_READ_FROM_CACHE) {
		netfs_stat(&netfs_n_rh_read_failed);
		set_bit(NETFS_RREQ_INCOMPLETE_IO, &rreq->flags);
	} else {
		netfs_stat(&netfs_n_rh_download_failed);
		set_bit(NETFS_RREQ_FAILED, &rreq->flags);
		rreq->error = subreq->error;
	}
	goto out;
}
EXPORT_SYMBOL(netfs_subreq_terminated);

static enum netfs_io_source netfs_cache_prepare_read(struct netfs_io_subrequest *subreq,
						       loff_t i_size)
{
	struct netfs_io_request *rreq = subreq->rreq;
	struct netfs_cache_resources *cres = &rreq->cache_resources;

	if (cres->ops)
		return cres->ops->prepare_read(subreq, i_size);
	if (subreq->start >= rreq->i_size)
		return NETFS_FILL_WITH_ZEROES;
	return NETFS_DOWNLOAD_FROM_SERVER;
}

/*
 * Work out what sort of subrequest the next one will be.
 */
static enum netfs_io_source
netfs_rreq_prepare_read(struct netfs_io_request *rreq,
			struct netfs_io_subrequest *subreq,
			struct iov_iter *io_iter)
{
	enum netfs_io_source source = NETFS_DOWNLOAD_FROM_SERVER;
	struct netfs_inode *ictx = netfs_inode(rreq->inode);
	size_t lsize;

	_enter("%llx-%llx,%llx", subreq->start, subreq->start + subreq->len, rreq->i_size);

	if (rreq->origin != NETFS_DIO_READ) {
		source = netfs_cache_prepare_read(subreq, rreq->i_size);
		if (source == NETFS_INVALID_READ)
			goto out;
	}

	if (source == NETFS_DOWNLOAD_FROM_SERVER) {
		/* Call out to the netfs to let it shrink the request to fit
		 * its own I/O sizes and boundaries.  If it shinks it here, it
		 * will be called again to make simultaneous calls; if it wants
		 * to make serial calls, it can indicate a short read and then
		 * we will call it again.
		 */
		if (rreq->origin != NETFS_DIO_READ) {
			if (subreq->start >= ictx->zero_point) {
				source = NETFS_FILL_WITH_ZEROES;
				goto set;
			}
			if (subreq->len > ictx->zero_point - subreq->start)
				subreq->len = ictx->zero_point - subreq->start;

			/* We limit buffered reads to the EOF, but let the
			 * server deal with larger-than-EOF DIO/unbuffered
			 * reads.
			 */
			if (subreq->len > rreq->i_size - subreq->start)
				subreq->len = rreq->i_size - subreq->start;
		}
		if (rreq->rsize && subreq->len > rreq->rsize)
			subreq->len = rreq->rsize;

		if (rreq->netfs_ops->clamp_length &&
		    !rreq->netfs_ops->clamp_length(subreq)) {
			source = NETFS_INVALID_READ;
			goto out;
		}

		if (subreq->max_nr_segs) {
			lsize = netfs_limit_iter(io_iter, 0, subreq->len,
						 subreq->max_nr_segs);
			if (subreq->len > lsize) {
				subreq->len = lsize;
				trace_netfs_sreq(subreq, netfs_sreq_trace_limited);
			}
		}
	}

set:
	if (subreq->len > rreq->len)
		pr_warn("R=%08x[%u] SREQ>RREQ %zx > %llx\n",
			rreq->debug_id, subreq->debug_index,
			subreq->len, rreq->len);

	if (WARN_ON(subreq->len == 0)) {
		source = NETFS_INVALID_READ;
		goto out;
	}

	subreq->source = source;
	trace_netfs_sreq(subreq, netfs_sreq_trace_prepare);

	subreq->io_iter = *io_iter;
	iov_iter_truncate(&subreq->io_iter, subreq->len);
	iov_iter_advance(io_iter, subreq->len);
out:
	subreq->source = source;
	trace_netfs_sreq(subreq, netfs_sreq_trace_prepare);
	return source;
}

/*
 * Slice off a piece of a read request and submit an I/O request for it.
 */
static bool netfs_rreq_submit_slice(struct netfs_io_request *rreq,
				    struct iov_iter *io_iter)
{
	struct netfs_io_subrequest *subreq;
	enum netfs_io_source source;

	subreq = netfs_alloc_subrequest(rreq);
	if (!subreq)
		return false;

	subreq->start		= rreq->start + rreq->submitted;
	subreq->len		= io_iter->count;

	_debug("slice %llx,%zx,%llx", subreq->start, subreq->len, rreq->submitted);
	list_add_tail(&subreq->rreq_link, &rreq->subrequests);

	/* Call out to the cache to find out what it can do with the remaining
	 * subset.  It tells us in subreq->flags what it decided should be done
	 * and adjusts subreq->len down if the subset crosses a cache boundary.
	 *
	 * Then when we hand the subset, it can choose to take a subset of that
	 * (the starts must coincide), in which case, we go around the loop
	 * again and ask it to download the next piece.
	 */
	source = netfs_rreq_prepare_read(rreq, subreq, io_iter);
	if (source == NETFS_INVALID_READ)
		goto subreq_failed;

	atomic_inc(&rreq->nr_outstanding);

	rreq->submitted += subreq->len;

	trace_netfs_sreq(subreq, netfs_sreq_trace_submit);
	switch (source) {
	case NETFS_FILL_WITH_ZEROES:
		netfs_fill_with_zeroes(rreq, subreq);
		break;
	case NETFS_DOWNLOAD_FROM_SERVER:
		netfs_read_from_server(rreq, subreq);
		break;
	case NETFS_READ_FROM_CACHE:
		netfs_read_from_cache(rreq, subreq, NETFS_READ_HOLE_IGNORE);
		break;
	default:
		BUG();
	}

	return true;

subreq_failed:
	rreq->error = subreq->error;
	netfs_put_subrequest(subreq, false, netfs_sreq_trace_put_failed);
	return false;
}

/*
 * Begin the process of reading in a chunk of data, where that data may be
 * stitched together from multiple sources, including multiple servers and the
 * local cache.
 */
int netfs_begin_read(struct netfs_io_request *rreq, bool sync)
{
	struct iov_iter io_iter;
	int ret;

	_enter("R=%x %llx-%llx",
	       rreq->debug_id, rreq->start, rreq->start + rreq->len - 1);

	if (rreq->len == 0) {
		pr_err("Zero-sized read [R=%x]\n", rreq->debug_id);
		return -EIO;
	}

	if (rreq->origin == NETFS_DIO_READ)
		inode_dio_begin(rreq->inode);

	// TODO: Use bounce buffer if requested
	rreq->io_iter = rreq->iter;

	INIT_WORK(&rreq->work, netfs_rreq_work);

	/* Chop the read into slices according to what the cache and the netfs
	 * want and submit each one.
	 */
	netfs_get_request(rreq, netfs_rreq_trace_get_for_outstanding);
	atomic_set(&rreq->nr_outstanding, 1);
	io_iter = rreq->io_iter;
	do {
		_debug("submit %llx + %llx >= %llx",
		       rreq->start, rreq->submitted, rreq->i_size);
		if (!netfs_rreq_submit_slice(rreq, &io_iter))
			break;
		if (test_bit(NETFS_SREQ_NO_PROGRESS, &rreq->flags))
			break;
		if (test_bit(NETFS_RREQ_BLOCKED, &rreq->flags) &&
		    test_bit(NETFS_RREQ_NONBLOCK, &rreq->flags))
			break;

	} while (rreq->submitted < rreq->len);

	if (!rreq->submitted) {
		netfs_put_request(rreq, false, netfs_rreq_trace_put_no_submit);
		if (rreq->origin == NETFS_DIO_READ)
			inode_dio_end(rreq->inode);
		ret = 0;
		goto out;
	}

	if (sync) {
		/* Keep nr_outstanding incremented so that the ref always
		 * belongs to us, and the service code isn't punted off to a
		 * random thread pool to process.  Note that this might start
		 * further work, such as writing to the cache.
		 */
		wait_var_event(&rreq->nr_outstanding,
			       atomic_read(&rreq->nr_outstanding) == 1);
		if (atomic_dec_and_test(&rreq->nr_outstanding))
			netfs_rreq_assess(rreq, false);

		trace_netfs_rreq(rreq, netfs_rreq_trace_wait_ip);
		wait_on_bit(&rreq->flags, NETFS_RREQ_IN_PROGRESS,
			    TASK_UNINTERRUPTIBLE);

		ret = rreq->error;
		if (ret == 0) {
			if (rreq->origin == NETFS_DIO_READ) {
				ret = rreq->transferred;
			} else if (rreq->submitted < rreq->len) {
				trace_netfs_failure(rreq, NULL, ret, netfs_fail_short_read);
				ret = -EIO;
			}
		}
	} else {
		/* If we decrement nr_outstanding to 0, the ref belongs to us. */
		if (atomic_dec_and_test(&rreq->nr_outstanding))
			netfs_rreq_assess(rreq, false);
		ret = -EIOCBQUEUED;
	}

out:
	return ret;
}