Unverified Commit 796a4049 authored by David Howells's avatar David Howells Committed by Christian Brauner
Browse files

netfs: In readahead, put the folio refs as soon extracted 



netfslib currently defers dropping the ref on the folios it obtains during
readahead to after it has started I/O on the basis that we can do it whilst
we wait for the I/O to complete, but this runs the risk of the I/O
collection racing with this in future.

Furthermore, Matthew Wilcox strongly suggests that the refs should be
dropped immediately, as readahead_folio() does (netfslib is using
__readahead_batch() which doesn't drop the refs).

Fixes: ee4cdf7b ("netfs: Speed up buffered reading")
Suggested-by: default avatarMatthew Wilcox <willy@infradead.org>
Signed-off-by: default avatarDavid Howells <dhowells@redhat.com>
Link: https://lore.kernel.org/r/3771538.1728052438@warthog.procyon.org.uk


cc: Jeff Layton <jlayton@kernel.org>
cc: netfs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.org
Signed-off-by: default avatarChristian Brauner <brauner@kernel.org>
parent 8cf0b939

fs/netfs/buffered_read.c

+14 −33
Original line number Diff line number Diff line
@@ -67,7 +67,8 @@ static int netfs_begin_cache_read(struct netfs_io_request *rreq, struct netfs_in 
 * Decant the list of folios to read into a rolling buffer.

 */

static size_t netfs_load_buffer_from_ra(struct netfs_io_request *rreq,

					struct folio_queue *folioq)

					struct folio_queue *folioq,

					struct folio_batch *put_batch)

{

	unsigned int order, nr;

	size_t size = 0;
@@ -82,6 +83,9 @@ static size_t netfs_load_buffer_from_ra(struct netfs_io_request *rreq, 
		order = folio_order(folio);

		folioq->orders[i] = order;

		size += PAGE_SIZE << order;



		if (!folio_batch_add(put_batch, folio))

			folio_batch_release(put_batch);

	}



	for (int i = nr; i < folioq_nr_slots(folioq); i++)
@@ -120,6 +124,9 @@ static ssize_t netfs_prepare_read_iterator(struct netfs_io_subrequest *subreq) 
		 * that we will need to release later - but we don't want to do

		 * that until after we've started the I/O.

		 */

		struct folio_batch put_batch;



		folio_batch_init(&put_batch);

		while (rreq->submitted < subreq->start + rsize) {

			struct folio_queue *tail = rreq->buffer_tail, *new;

			size_t added;
@@ -132,10 +139,11 @@ static ssize_t netfs_prepare_read_iterator(struct netfs_io_subrequest *subreq) 
			new->prev = tail;

			tail->next = new;

			rreq->buffer_tail = new;

			added = netfs_load_buffer_from_ra(rreq, new);

			added = netfs_load_buffer_from_ra(rreq, new, &put_batch);

			rreq->iter.count += added;

			rreq->submitted += added;

		}

		folio_batch_release(&put_batch);

	}



	subreq->len = rsize;
@@ -348,6 +356,7 @@ static int netfs_wait_for_read(struct netfs_io_request *rreq) 
static int netfs_prime_buffer(struct netfs_io_request *rreq)

{

	struct folio_queue *folioq;

	struct folio_batch put_batch;

	size_t added;



	folioq = kmalloc(sizeof(*folioq), GFP_KERNEL);
@@ -360,39 +369,14 @@ static int netfs_prime_buffer(struct netfs_io_request *rreq) 
	rreq->submitted = rreq->start;

	iov_iter_folio_queue(&rreq->iter, ITER_DEST, folioq, 0, 0, 0);



	added = netfs_load_buffer_from_ra(rreq, folioq);

	folio_batch_init(&put_batch);

	added = netfs_load_buffer_from_ra(rreq, folioq, &put_batch);

	folio_batch_release(&put_batch);

	rreq->iter.count += added;

	rreq->submitted += added;

	return 0;

}



/*

 * Drop the ref on each folio that we inherited from the VM readahead code.  We

 * still have the folio locks to pin the page until we complete the I/O.

 *

 * Note that we can't just release the batch in each queue struct as we use the

 * occupancy count in other places.

 */

static void netfs_put_ra_refs(struct folio_queue *folioq)

{

	struct folio_batch fbatch;



	folio_batch_init(&fbatch);

	while (folioq) {

		for (unsigned int slot = 0; slot < folioq_count(folioq); slot++) {

			struct folio *folio = folioq_folio(folioq, slot);

			if (!folio)

				continue;

			trace_netfs_folio(folio, netfs_folio_trace_read_put);

			if (!folio_batch_add(&fbatch, folio))

				folio_batch_release(&fbatch);

		}

		folioq = folioq->next;

	}



	folio_batch_release(&fbatch);

}



/**

 * netfs_readahead - Helper to manage a read request

 * @ractl: The description of the readahead request
@@ -436,9 +420,6 @@ void netfs_readahead(struct readahead_control *ractl) 
		goto cleanup_free;

	netfs_read_to_pagecache(rreq);



	/* Release the folio refs whilst we're waiting for the I/O. */

	netfs_put_ra_refs(rreq->buffer);



	netfs_put_request(rreq, true, netfs_rreq_trace_put_return);

	return;

fs/netfs/read_collect.c

+2 −0
Original line number Diff line number Diff line
@@ -77,6 +77,8 @@ static void netfs_unlock_read_folio(struct netfs_io_subrequest *subreq, 
			folio_unlock(folio);

		}

	}



	folioq_clear(folioq, slot);

}



/*

include/trace/events/netfs.h

+0 −1
Original line number Diff line number Diff line
@@ -172,7 +172,6 @@ 
	EM(netfs_folio_trace_read,		"read")		\

	EM(netfs_folio_trace_read_done,		"read-done")	\

	EM(netfs_folio_trace_read_gaps,		"read-gaps")	\

	EM(netfs_folio_trace_read_put,		"read-put")	\

	EM(netfs_folio_trace_read_unlock,	"read-unlock")	\

	EM(netfs_folio_trace_redirtied,		"redirtied")	\

	EM(netfs_folio_trace_store,		"store")	\