Re: [PATCH 21/41] CacheFiles: Permit the page lock state to be monitored [ver #48]

[David Howells]

Nick Piggin <nickpiggin@xxxxxxxxxxxx> wrote:

> I would really like to know exactly why it *has* to be asynchronous,
> and see what the diff looks like between a simple synchronous design
> and what you have here.

Okay.  See attached patch.

Three benchmarks, all with data preloaded into the cache, and all with all the
data in memory on the server:

 (1) dd of a 100MB file.

 (2) dd of a 200MB file.

 (3) Eight parallel tars of 340MB kernel trees.

	Benchmark	Without Patch	With Patch
	1		1.971s		3.371s
	2		3.750s		7.093s
	3		18 mins		22 mins

With this patch, we wait for the pages to be read from the backing fs and copy
them in fscache_read_or_alloc_pages().

I'm sure there are ways to optimise things, but whilst doing a synchronous
read at this in fscache_read_or_alloc_pages() might help an start-to-end read
as done by most programs, it's less likely to help random-access reads, such
as page-in for mmapped sections.

Note also: (1) the caller of the netfs's readpages() has already done
readahead speculation, and we should probably make best use of that, rather
than doing it again; and (2) all the metadata for where the data blocks are on
the backing fs has already been read by dint of calling bmap() to determine
whether the data blocks exist or not.

What might be cute is to have cachefiles_read_or_alloc_pages() dentry_open()
the backing file with O_DIRECT|O_NOHOLE, use ->read() to load the data into
the netfs pages directly, and then fput() the file once it has finished.

What would be even cuter is to do readv() for all the netfs pages
simultaneously, but that's impractical as each page has to be kmap()'d whilst
it is being read.

David
---
From: David Howells <dhowells@xxxxxxxxxx>
Subject: [PATCH] CacheFiles: Sync read demo for Nick Piggin


---

 fs/cachefiles/rdwr.c |  198 +++++++-------------------------------------------
 1 files changed, 28 insertions(+), 170 deletions(-)


diff --git a/fs/cachefiles/rdwr.c b/fs/cachefiles/rdwr.c
index a69787e..d99e43d 100644
--- a/fs/cachefiles/rdwr.c
+++ b/fs/cachefiles/rdwr.c
@@ -14,53 +14,6 @@
 #include "internal.h"
 
 /*
- * detect wake up events generated by the unlocking of pages in which we're
- * interested
- * - we use this to detect read completion of backing pages
- * - the caller holds the waitqueue lock
- */
-static int cachefiles_read_waiter(wait_queue_t *wait, unsigned mode,
-				  int sync, void *_key)
-{
-	struct cachefiles_one_read *monitor =
-		container_of(wait, struct cachefiles_one_read, monitor);
-	struct cachefiles_object *object;
-	struct wait_bit_key *key = _key;
-	struct page *page = wait->private;
-
-	ASSERT(key);
-
-	_enter("{%lu},%u,%d,{%p,%u}",
-	       monitor->netfs_page->index, mode, sync,
-	       key->flags, key->bit_nr);
-
-	if (key->flags != &page->flags ||
-	    key->bit_nr != PG_locked)
-		return 0;
-
-	_debug("--- monitor %p %lx ---", page, page->flags);
-
-	if (!PageUptodate(page) && !PageError(page))
-		dump_stack();
-
-	/* remove from the waitqueue */
-	list_del(&wait->task_list);
-
-	/* move onto the action list and queue for FS-Cache thread pool */
-	ASSERT(monitor->op);
-
-	object = container_of(monitor->op->op.object,
-			      struct cachefiles_object, fscache);
-
-	spin_lock(&object->work_lock);
-	list_add_tail(&monitor->op_link, &monitor->op->to_do);
-	spin_unlock(&object->work_lock);
-
-	fscache_enqueue_retrieval(monitor->op);
-	return 0;
-}
-
-/*
  * copy data from backing pages to netfs pages to complete a read operation
  * - driven by FS-Cache's thread pool
  */
@@ -139,7 +92,6 @@ static int cachefiles_read_backing_file_one(struct cachefiles_object *object,
 					    struct page *netpage,
 					    struct pagevec *pagevec)
 {
-	struct cachefiles_one_read *monitor;
 	struct address_space *bmapping;
 	struct page *newpage, *backpage;
 	int ret;
@@ -151,15 +103,6 @@ static int cachefiles_read_backing_file_one(struct cachefiles_object *object,
 	_debug("read back %p{%lu,%d}",
 	       netpage, netpage->index, page_count(netpage));
 
-	monitor = kzalloc(sizeof(*monitor), GFP_KERNEL);
-	if (!monitor)
-		goto nomem;
-
-	monitor->netfs_page = netpage;
-	monitor->op = fscache_get_retrieval(op);
-
-	init_waitqueue_func_entry(&monitor->monitor, cachefiles_read_waiter);
-
 	/* attempt to get hold of the backing page */
 	bmapping = object->backer->d_inode->i_mapping;
 	newpage = NULL;
@@ -172,7 +115,7 @@ static int cachefiles_read_backing_file_one(struct cachefiles_object *object,
 		if (!newpage) {
 			newpage = page_cache_alloc_cold(bmapping);
 			if (!newpage)
-				goto nomem_monitor;
+				goto nomem;
 		}
 
 		ret = add_to_page_cache(newpage, bmapping,
@@ -200,29 +143,12 @@ read_backing_page:
 	if (ret < 0)
 		goto read_error;
 
-	/* set the monitor to transfer the data across */
-monitor_backing_page:
-	_debug("- monitor add");
-
-	/* install the monitor */
-	page_cache_get(monitor->netfs_page);
-	page_cache_get(backpage);
-	monitor->back_page = backpage;
-	monitor->monitor.private = backpage;
-	add_page_wait_queue(backpage, &monitor->monitor);
-	monitor = NULL;
-
-	/* but the page may have been read before the monitor was installed, so
-	 * the monitor may miss the event - so we have to ensure that we do get
-	 * one in such a case */
-	if (trylock_page(backpage)) {
-		_debug("jumpstart %p {%lx}", backpage, backpage->flags);
-		unlock_page(backpage);
-	}
-	goto success;
+	/* wait for the page read to be complete */
+	wait_on_page_locked(backpage);
+	goto use_backing_page;
 
-	/* if the backing page is already present, it can be in one of
-	 * three states: read in progress, read failed or read okay */
+	/* if the backing page is already present, it can be in one of three
+	 * states: read in progress, read failed or read okay */
 backing_page_already_present:
 	_debug("- present");
 
@@ -231,14 +157,19 @@ backing_page_already_present:
 		newpage = NULL;
 	}
 
+use_backing_page:
 	if (PageError(backpage))
 		goto io_error;
 
 	if (PageUptodate(backpage))
 		goto backing_page_already_uptodate;
 
-	if (!trylock_page(backpage))
-		goto monitor_backing_page;
+	lock_page(backpage);
+	if (PageError(backpage) || PageUptodate(backpage)) {
+		unlock_page(backpage);
+		goto use_backing_page;
+	}
+
 	_debug("read %p {%lx}", backpage, backpage->flags);
 	goto read_backing_page;
 
@@ -252,18 +183,12 @@ backing_page_already_uptodate:
 
 	copy_highpage(netpage, backpage);
 	fscache_end_io(op, netpage, 0);
-
-success:
 	_debug("success");
 	ret = 0;
 
 out:
 	if (backpage)
 		page_cache_release(backpage);
-	if (monitor) {
-		fscache_put_retrieval(monitor->op);
-		kfree(monitor);
-	}
 	_leave(" = %d", ret);
 	return ret;
 
@@ -278,9 +203,6 @@ io_error:
 
 nomem_page:
 	page_cache_release(newpage);
-nomem_monitor:
-	fscache_put_retrieval(monitor->op);
-	kfree(monitor);
 nomem:
 	_leave(" = -ENOMEM");
 	return -ENOMEM;
@@ -379,7 +301,6 @@ static int cachefiles_read_backing_file(struct cachefiles_object *object,
 					struct list_head *list,
 					struct pagevec *mark_pvec)
 {
-	struct cachefiles_one_read *monitor = NULL;
 	struct address_space *bmapping = object->backer->d_inode->i_mapping;
 	struct pagevec lru_pvec;
 	struct page *newpage = NULL, *netpage, *_n, *backpage = NULL;
@@ -395,16 +316,6 @@ static int cachefiles_read_backing_file(struct cachefiles_object *object,
 		_debug("read back %p{%lu,%d}",
 		       netpage, netpage->index, page_count(netpage));
 
-		if (!monitor) {
-			monitor = kzalloc(sizeof(*monitor), GFP_KERNEL);
-			if (!monitor)
-				goto nomem;
-
-			monitor->op = fscache_get_retrieval(op);
-			init_waitqueue_func_entry(&monitor->monitor,
-						  cachefiles_read_waiter);
-		}
-
 		for (;;) {
 			backpage = find_get_page(bmapping, netpage->index);
 			if (backpage)
@@ -441,92 +352,44 @@ static int cachefiles_read_backing_file(struct cachefiles_object *object,
 		if (ret < 0)
 			goto read_error;
 
-		/* add the netfs page to the pagecache and LRU, and set the
-		 * monitor to transfer the data across */
-	monitor_backing_page:
-		_debug("- monitor add");
-
-		ret = add_to_page_cache(netpage, op->mapping, netpage->index,
-					GFP_KERNEL);
-		if (ret < 0) {
-			if (ret == -EEXIST) {
-				page_cache_release(netpage);
-				continue;
-			}
-			goto nomem;
-		}
-
-		page_cache_get(netpage);
-		if (!pagevec_add(&lru_pvec, netpage))
-			__pagevec_lru_add_file(&lru_pvec);
-
-		/* install a monitor */
-		page_cache_get(netpage);
-		monitor->netfs_page = netpage;
-
-		page_cache_get(backpage);
-		monitor->back_page = backpage;
-		monitor->monitor.private = backpage;
-		add_page_wait_queue(backpage, &monitor->monitor);
-		monitor = NULL;
-
-		/* but the page may have been read before the monitor was
-		 * installed, so the monitor may miss the event - so we have to
-		 * ensure that we do get one in such a case */
-		if (trylock_page(backpage)) {
-			_debug("2unlock %p {%lx}", backpage, backpage->flags);
-			unlock_page(backpage);
-		}
-
-		page_cache_release(backpage);
-		backpage = NULL;
-
-		page_cache_release(netpage);
-		netpage = NULL;
-		continue;
+		wait_on_page_locked(backpage);
 
 		/* if the backing page is already present, it can be in one of
 		 * three states: read in progress, read failed or read okay */
 	backing_page_already_present:
 		_debug("- present %p", backpage);
 
+	check_backing_page:
 		if (PageError(backpage))
 			goto io_error;
 
 		if (PageUptodate(backpage))
-			goto backing_page_already_uptodate;
+			goto copy_backing_page;
 
 		_debug("- not ready %p{%lx}", backpage, backpage->flags);
 
-		if (!trylock_page(backpage))
-			goto monitor_backing_page;
-
-		if (PageError(backpage)) {
-			_debug("error %lx", backpage->flags);
+		lock_page(backpage);
+		if (PageError(backpage) || PageUptodate(backpage)) {
 			unlock_page(backpage);
-			goto io_error;
+			goto check_backing_page;
 		}
 
-		if (PageUptodate(backpage))
-			goto backing_page_already_uptodate_unlock;
-
 		/* we've locked a page that's neither up to date nor erroneous,
 		 * so we need to attempt to read it again */
 		goto reread_backing_page;
 
-		/* the backing page is already up to date, attach the netfs
-		 * page to the pagecache and LRU and copy the data across */
-	backing_page_already_uptodate_unlock:
-		_debug("uptodate %lx", backpage->flags);
-		unlock_page(backpage);
-	backing_page_already_uptodate:
-		_debug("- uptodate");
+	copy_backing_page:
+		_debug("- copy page");
 
+		/* add the netfs page to the pagecache and LRU */
 		ret = add_to_page_cache(netpage, op->mapping, netpage->index,
 					GFP_KERNEL);
 		if (ret < 0) {
 			if (ret == -EEXIST) {
 				page_cache_release(netpage);
+				page_cache_release(backpage);
+				netpage = NULL;
+				backpage = NULL;
 				continue;
 			}
 			goto nomem;
@@ -537,17 +400,16 @@ static int cachefiles_read_backing_file(struct cachefiles_object *object,
 		page_cache_release(backpage);
 		backpage = NULL;
 
-		if (!pagevec_add(mark_pvec, netpage))
-			fscache_mark_pages_cached(op, mark_pvec);
-
 		page_cache_get(netpage);
 		if (!pagevec_add(&lru_pvec, netpage))
 			__pagevec_lru_add_file(&lru_pvec);
 
+		if (!pagevec_add(mark_pvec, netpage))
+			fscache_mark_pages_cached(op, mark_pvec);
+
 		fscache_end_io(op, netpage, 0);
 		page_cache_release(netpage);
 		netpage = NULL;
-		continue;
 	}
 
 	netpage = NULL;
@@ -564,10 +426,6 @@ out:
 		page_cache_release(netpage);
 	if (backpage)
 		page_cache_release(backpage);
-	if (monitor) {
-		fscache_put_retrieval(op);
-		kfree(monitor);
-	}
 
 	list_for_each_entry_safe(netpage, _n, list, lru) {
 		list_del(&netpage->lru);

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