[PATCH 0/2 RFC v3] Livelock avoidance for data integrity writeback

[PATCH 0/2 RFC v3] Livelock avoidance for data integrity writeback

[Jan Kara]

  Hi,

  I've revived my patches to implement livelock avoidance for data integrity
writes. Due to some concerns whether tagging of pages before writeout cannot
be too costly to use for WB_SYNC_NONE mode (where we stop after nr_to_write
pages) I've changed the patch to use page tagging only in WB_SYNC_ALL mode
where we are sure that we write out all the tagged pages. Later, we can think
about using tagging for livelock avoidance for WB_SYNC_NONE mode as well...
  As always comments are welcome.

                                                                        Honza

PS: I'm sorry for sending this twice. I've screwed up the list address in the
first posting.

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[PATCH 1/2] radix-tree: Implement function radix_tree_gang_tag_if_tagged

[Jan Kara]

Implement function for setting one tag if another tag is set
for each item in given range.

Signed-off-by: Jan Kara <jack@suse.cz>
---
 include/linux/radix-tree.h |    3 ++
 lib/radix-tree.c           |   82 ++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 85 insertions(+), 0 deletions(-)

diff --git a/include/linux/radix-tree.h b/include/linux/radix-tree.h
index 55ca73c..efdfb07 100644
--- a/include/linux/radix-tree.h
+++ b/include/linux/radix-tree.h
@@ -192,6 +192,9 @@ unsigned int
 radix_tree_gang_lookup_tag_slot(struct radix_tree_root *root, void ***results,
 		unsigned long first_index, unsigned int max_items,
 		unsigned int tag);
+unsigned long radix_tree_gang_tag_if_tagged(struct radix_tree_root *root,
+		unsigned long first_index, unsigned long last_index,
+		unsigned int fromtag, unsigned int totag);
 int radix_tree_tagged(struct radix_tree_root *root, unsigned int tag);
 
 static inline void radix_tree_preload_end(void)
diff --git a/lib/radix-tree.c b/lib/radix-tree.c
index 05da38b..c4595b2 100644
--- a/lib/radix-tree.c
+++ b/lib/radix-tree.c
@@ -609,6 +609,88 @@ int radix_tree_tag_get(struct radix_tree_root *root,
 EXPORT_SYMBOL(radix_tree_tag_get);
 
 /**
+ * radix_tree_gang_tag_if_tagged - for each item in given range set given
+ *				   tag if item has another tag set
+ * @root:		radix tree root
+ * @first_index:	starting index of a range to scan
+ * @last_index:		last index of a range to scan
+ * @iftag: 		tag index to test
+ * @settag:		tag index to set if tested tag is set
+ *
+ * This function scans range of radix tree from first_index to last_index.
+ * For each item in the range if iftag is set, the function sets also
+ * settag.
+ *
+ * The function returns number of leaves where the tag was set.
+ */
+unsigned long radix_tree_gang_tag_if_tagged(struct radix_tree_root *root,
+                unsigned long first_index, unsigned long last_index,
+                unsigned int iftag, unsigned int settag)
+{
+	unsigned int height = root->height, shift;
+	unsigned long tagged = 0, index = first_index;
+	struct radix_tree_node *open_slots[height], *slot;
+
+	last_index = min(last_index, radix_tree_maxindex(height));
+	if (first_index > last_index)
+		return 0;
+	if (!root_tag_get(root, iftag))
+		return 0;
+	if (height == 0) {
+		root_tag_set(root, settag);
+		return 1;
+	}
+
+	shift = (height - 1) * RADIX_TREE_MAP_SHIFT;
+	slot = radix_tree_indirect_to_ptr(root->rnode);
+
+	for (;;) {
+		int offset;
+
+		offset = (index >> shift) & RADIX_TREE_MAP_MASK;
+		if (!slot->slots[offset])
+			goto next;
+		if (!tag_get(slot, iftag, offset))
+			goto next;
+		tag_set(slot, settag, offset);
+		if (height == 1) {
+			tagged++;
+			goto next;
+		}
+		/* Go down one level */
+		height--;
+		shift -= RADIX_TREE_MAP_SHIFT;
+		open_slots[height] = slot;
+		slot = slot->slots[offset];
+		continue;
+next:
+		/* Go to next item at level determined by 'shift' */
+		index = ((index >> shift) + 1) << shift;
+		if (index > last_index)
+			break;
+		while (((index >> shift) & RADIX_TREE_MAP_MASK) == 0) {
+			/*
+			 * We've fully scanned this node. Go up. Because
+			 * last_index is guaranteed to be in the tree, what
+			 * we do below cannot wander astray.
+			 */
+			slot = open_slots[height];
+			height++;
+			shift += RADIX_TREE_MAP_SHIFT;
+		}
+	}
+	/*
+	 * The iftag must have been set somewhere because otherwise
+	 * we would return immediated at the beginning of the function
+	 */
+	root_tag_set(root, settag);
+
+	return tagged;
+}
+EXPORT_SYMBOL(radix_tree_gang_tag_if_tagged);
+
+
+/**
  *	radix_tree_next_hole    -    find the next hole (not-present entry)
  *	@root:		tree root
  *	@index:		index key
-- 
1.6.4.2

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[PATCH 2/2] mm: Implement writeback livelock avoidance using page tagging

[Jan Kara]

We try to avoid livelocks of writeback when some steadily creates
dirty pages in a mapping we are writing out. For memory-cleaning
writeback, using nr_to_write works reasonably well but we cannot
really use it for data integrity writeback. This patch tries to
solve the problem.

The idea is simple: Tag all pages that should be written back
with a special tag (TOWRITE) in the radix tree. This can be done
rather quickly and thus livelocks should not happen in practice.
Then we start doing the hard work of locking pages and sending
them to disk only for those pages that have TOWRITE tag set.

Signed-off-by: Jan Kara <jack@suse.cz>
---
 include/linux/fs.h         |    1 +
 include/linux/radix-tree.h |    2 +-
 mm/page-writeback.c        |   44 ++++++++++++++++++++++++++++++++++++++++++--
 3 files changed, 44 insertions(+), 3 deletions(-)

diff --git a/include/linux/fs.h b/include/linux/fs.h
index 3428393..fe308f0 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -685,6 +685,7 @@ struct block_device {
  */
 #define PAGECACHE_TAG_DIRTY	0
 #define PAGECACHE_TAG_WRITEBACK	1
+#define PAGECACHE_TAG_TOWRITE	2
 
 int mapping_tagged(struct address_space *mapping, int tag);
 
diff --git a/include/linux/radix-tree.h b/include/linux/radix-tree.h
index efdfb07..f7ebff8 100644
--- a/include/linux/radix-tree.h
+++ b/include/linux/radix-tree.h
@@ -55,7 +55,7 @@ static inline int radix_tree_is_indirect_ptr(void *ptr)
 
 /*** radix-tree API starts here ***/
 
-#define RADIX_TREE_MAX_TAGS 2
+#define RADIX_TREE_MAX_TAGS 3
 
 /* root tags are stored in gfp_mask, shifted by __GFP_BITS_SHIFT */
 struct radix_tree_root {
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index b289310..f590a12 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -807,6 +807,30 @@ void __init page_writeback_init(void)
 }
 
 /**
+ * tag_pages_for_writeback - tag pages to be written by write_cache_pages
+ * @mapping: address space structure to write
+ * @start: starting page index
+ * @end: ending page index (inclusive)
+ *
+ * This function scans the page range from @start to @end and tags all pages
+ * that have DIRTY tag set with a special TOWRITE tag. The idea is that
+ * write_cache_pages (or whoever calls this function) will then use TOWRITE tag
+ * to identify pages eligible for writeback.  This mechanism is used to avoid
+ * livelocking of writeback by a process steadily creating new dirty pages in
+ * the file (thus it is important for this function to be damn quick so that it
+ * can tag pages faster than a dirtying process can create them).
+ */
+void tag_pages_for_writeback(struct address_space *mapping,
+			     pgoff_t start, pgoff_t end)
+{
+	spin_lock_irq(&mapping->tree_lock);
+	radix_tree_gang_tag_if_tagged(&mapping->page_tree, start, end,
+				PAGECACHE_TAG_DIRTY, PAGECACHE_TAG_TOWRITE);
+	spin_unlock_irq(&mapping->tree_lock);
+}
+EXPORT_SYMBOL(tag_pages_for_writeback);
+
+/**
  * write_cache_pages - walk the list of dirty pages of the given address space and write all of them.
  * @mapping: address space structure to write
  * @wbc: subtract the number of written pages from *@wbc->nr_to_write
@@ -820,6 +844,13 @@ void __init page_writeback_init(void)
  * the call was made get new I/O started against them.  If wbc->sync_mode is
  * WB_SYNC_ALL then we were called for data integrity and we must wait for
  * existing IO to complete.
+ *
+ * To avoid livelocks (when other process dirties new pages), we first tag
+ * pages which should be written back with TOWRITE tag and only then start
+ * writing them. For data-integrity sync we have to be careful so that we do
+ * not miss some pages (e.g., because some other process has cleared TOWRITE
+ * tag we set). The rule we follow is that TOWRITE tag can be cleared only
+ * by the process clearing the DIRTY tag (and submitting the page for IO).
  */
 int write_cache_pages(struct address_space *mapping,
 		      struct writeback_control *wbc, writepage_t writepage,
@@ -836,6 +867,7 @@ int write_cache_pages(struct address_space *mapping,
 	int cycled;
 	int range_whole = 0;
 	long nr_to_write = wbc->nr_to_write;
+	int tag;
 
 	pagevec_init(&pvec, 0);
 	if (wbc->range_cyclic) {
@@ -853,13 +885,18 @@ int write_cache_pages(struct address_space *mapping,
 			range_whole = 1;
 		cycled = 1; /* ignore range_cyclic tests */
 	}
+	if (wbc->sync_mode == WB_SYNC_ALL)
+		tag = PAGECACHE_TAG_TOWRITE;
+	else
+		tag = PAGECACHE_TAG_DIRTY;
 retry:
+	if (wbc->sync_mode == WB_SYNC_ALL)
+		tag_pages_for_writeback(mapping, index, end);
 	done_index = index;
 	while (!done && (index <= end)) {
 		int i;
 
-		nr_pages = pagevec_lookup_tag(&pvec, mapping, &index,
-			      PAGECACHE_TAG_DIRTY,
+		nr_pages = pagevec_lookup_tag(&pvec, mapping, &index, tag,
 			      min(end - index, (pgoff_t)PAGEVEC_SIZE-1) + 1);
 		if (nr_pages == 0)
 			break;
@@ -1319,6 +1356,9 @@ int test_set_page_writeback(struct page *page)
 			radix_tree_tag_clear(&mapping->page_tree,
 						page_index(page),
 						PAGECACHE_TAG_DIRTY);
+		radix_tree_tag_clear(&mapping->page_tree,
+				     page_index(page),
+				     PAGECACHE_TAG_TOWRITE);
 		spin_unlock_irqrestore(&mapping->tree_lock, flags);
 	} else {
 		ret = TestSetPageWriteback(page);
-- 
1.6.4.2

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Re: [PATCH 2/2] mm: Implement writeback livelock avoidance using page tagging

[Nick Piggin]

On Fri, Jun 04, 2010 at 08:47:11PM +0200, Jan Kara wrote:
> We try to avoid livelocks of writeback when some steadily creates
> dirty pages in a mapping we are writing out. For memory-cleaning
> writeback, using nr_to_write works reasonably well but we cannot
> really use it for data integrity writeback. This patch tries to
> solve the problem.
> 
> The idea is simple: Tag all pages that should be written back
> with a special tag (TOWRITE) in the radix tree. This can be done
> rather quickly and thus livelocks should not happen in practice.
> Then we start doing the hard work of locking pages and sending
> them to disk only for those pages that have TOWRITE tag set.
> 
> Signed-off-by: Jan Kara <jack@suse.cz>
> ---
>  include/linux/fs.h         |    1 +
>  include/linux/radix-tree.h |    2 +-
>  mm/page-writeback.c        |   44 ++++++++++++++++++++++++++++++++++++++++++--
>  3 files changed, 44 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 3428393..fe308f0 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -685,6 +685,7 @@ struct block_device {
>   */
>  #define PAGECACHE_TAG_DIRTY	0
>  #define PAGECACHE_TAG_WRITEBACK	1
> +#define PAGECACHE_TAG_TOWRITE	2
>  
>  int mapping_tagged(struct address_space *mapping, int tag);
>  
> diff --git a/include/linux/radix-tree.h b/include/linux/radix-tree.h
> index efdfb07..f7ebff8 100644
> --- a/include/linux/radix-tree.h
> +++ b/include/linux/radix-tree.h
> @@ -55,7 +55,7 @@ static inline int radix_tree_is_indirect_ptr(void *ptr)
>  
>  /*** radix-tree API starts here ***/
>  
> -#define RADIX_TREE_MAX_TAGS 2
> +#define RADIX_TREE_MAX_TAGS 3
>  
>  /* root tags are stored in gfp_mask, shifted by __GFP_BITS_SHIFT */
>  struct radix_tree_root {
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index b289310..f590a12 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -807,6 +807,30 @@ void __init page_writeback_init(void)
>  }
>  
>  /**
> + * tag_pages_for_writeback - tag pages to be written by write_cache_pages
> + * @mapping: address space structure to write
> + * @start: starting page index
> + * @end: ending page index (inclusive)
> + *
> + * This function scans the page range from @start to @end and tags all pages
> + * that have DIRTY tag set with a special TOWRITE tag. The idea is that
> + * write_cache_pages (or whoever calls this function) will then use TOWRITE tag
> + * to identify pages eligible for writeback.  This mechanism is used to avoid
> + * livelocking of writeback by a process steadily creating new dirty pages in
> + * the file (thus it is important for this function to be damn quick so that it
> + * can tag pages faster than a dirtying process can create them).
> + */
> +void tag_pages_for_writeback(struct address_space *mapping,
> +			     pgoff_t start, pgoff_t end)
> +{
> +	spin_lock_irq(&mapping->tree_lock);
> +	radix_tree_gang_tag_if_tagged(&mapping->page_tree, start, end,
> +				PAGECACHE_TAG_DIRTY, PAGECACHE_TAG_TOWRITE);
> +	spin_unlock_irq(&mapping->tree_lock);
> +}
> +EXPORT_SYMBOL(tag_pages_for_writeback);
> +
> +/**
>   * write_cache_pages - walk the list of dirty pages of the given address space and write all of them.
>   * @mapping: address space structure to write
>   * @wbc: subtract the number of written pages from *@wbc->nr_to_write
> @@ -820,6 +844,13 @@ void __init page_writeback_init(void)
>   * the call was made get new I/O started against them.  If wbc->sync_mode is
>   * WB_SYNC_ALL then we were called for data integrity and we must wait for
>   * existing IO to complete.
> + *
> + * To avoid livelocks (when other process dirties new pages), we first tag
> + * pages which should be written back with TOWRITE tag and only then start
> + * writing them. For data-integrity sync we have to be careful so that we do
> + * not miss some pages (e.g., because some other process has cleared TOWRITE
> + * tag we set). The rule we follow is that TOWRITE tag can be cleared only
> + * by the process clearing the DIRTY tag (and submitting the page for IO).
>   */
>  int write_cache_pages(struct address_space *mapping,
>  		      struct writeback_control *wbc, writepage_t writepage,
> @@ -836,6 +867,7 @@ int write_cache_pages(struct address_space *mapping,
>  	int cycled;
>  	int range_whole = 0;
>  	long nr_to_write = wbc->nr_to_write;
> +	int tag;
>  
>  	pagevec_init(&pvec, 0);
>  	if (wbc->range_cyclic) {
> @@ -853,13 +885,18 @@ int write_cache_pages(struct address_space *mapping,
>  			range_whole = 1;
>  		cycled = 1; /* ignore range_cyclic tests */
>  	}
> +	if (wbc->sync_mode == WB_SYNC_ALL)
> +		tag = PAGECACHE_TAG_TOWRITE;
> +	else
> +		tag = PAGECACHE_TAG_DIRTY;
>  retry:
> +	if (wbc->sync_mode == WB_SYNC_ALL)
> +		tag_pages_for_writeback(mapping, index, end);

I wonder if this is too much spinlock latency in a huge dirty file?
Some kid of batching of the operation perhaps would be good?


>  	done_index = index;
>  	while (!done && (index <= end)) {
>  		int i;
>  
> -		nr_pages = pagevec_lookup_tag(&pvec, mapping, &index,
> -			      PAGECACHE_TAG_DIRTY,
> +		nr_pages = pagevec_lookup_tag(&pvec, mapping, &index, tag,
>  			      min(end - index, (pgoff_t)PAGEVEC_SIZE-1) + 1);
>  		if (nr_pages == 0)
>  			break;

Would it be neat to clear the tag even if we didn't set page to
writeback? It should be uncommon case.


> @@ -1319,6 +1356,9 @@ int test_set_page_writeback(struct page *page)
>  			radix_tree_tag_clear(&mapping->page_tree,
>  						page_index(page),
>  						PAGECACHE_TAG_DIRTY);
> +		radix_tree_tag_clear(&mapping->page_tree,
> +				     page_index(page),
> +				     PAGECACHE_TAG_TOWRITE);
>  		spin_unlock_irqrestore(&mapping->tree_lock, flags);
>  	} else {
>  		ret = TestSetPageWriteback(page);

It would be nice to have bitwise tag clearing so we can clear multiple
at once. Then

clear_tag = PAGECACHE_TAG_TOWRITE;
if (!PageDirty(page))
  clear_tag |= PAGECACHE_TAG_DIRTY;

That could reduce overhead a bit more.


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Re: [PATCH 2/2] mm: Implement writeback livelock avoidance using page tagging

[Jan Kara]

On Sat 05-06-10 11:38:02, Nick Piggin wrote:
> On Fri, Jun 04, 2010 at 08:47:11PM +0200, Jan Kara wrote:
> > We try to avoid livelocks of writeback when some steadily creates
> > dirty pages in a mapping we are writing out. For memory-cleaning
> > writeback, using nr_to_write works reasonably well but we cannot
> > really use it for data integrity writeback. This patch tries to
> > solve the problem.
> > 
> > The idea is simple: Tag all pages that should be written back
> > with a special tag (TOWRITE) in the radix tree. This can be done
> > rather quickly and thus livelocks should not happen in practice.
> > Then we start doing the hard work of locking pages and sending
> > them to disk only for those pages that have TOWRITE tag set.
> > 
> > Signed-off-by: Jan Kara <jack@suse.cz>
> > ---
> >  include/linux/fs.h         |    1 +
> >  include/linux/radix-tree.h |    2 +-
> >  mm/page-writeback.c        |   44 ++++++++++++++++++++++++++++++++++++++++++--
> >  3 files changed, 44 insertions(+), 3 deletions(-)
...
> > diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> > index b289310..f590a12 100644
> > --- a/mm/page-writeback.c
> > +++ b/mm/page-writeback.c
> > @@ -807,6 +807,30 @@ void __init page_writeback_init(void)
> >  }
> >  
> >  /**
> > + * tag_pages_for_writeback - tag pages to be written by write_cache_pages
> > + * @mapping: address space structure to write
> > + * @start: starting page index
> > + * @end: ending page index (inclusive)
> > + *
> > + * This function scans the page range from @start to @end and tags all pages
> > + * that have DIRTY tag set with a special TOWRITE tag. The idea is that
> > + * write_cache_pages (or whoever calls this function) will then use TOWRITE tag
> > + * to identify pages eligible for writeback.  This mechanism is used to avoid
> > + * livelocking of writeback by a process steadily creating new dirty pages in
> > + * the file (thus it is important for this function to be damn quick so that it
> > + * can tag pages faster than a dirtying process can create them).
> > + */
> > +void tag_pages_for_writeback(struct address_space *mapping,
> > +			     pgoff_t start, pgoff_t end)
> > +{
> > +	spin_lock_irq(&mapping->tree_lock);
> > +	radix_tree_gang_tag_if_tagged(&mapping->page_tree, start, end,
> > +				PAGECACHE_TAG_DIRTY, PAGECACHE_TAG_TOWRITE);
> > +	spin_unlock_irq(&mapping->tree_lock);
> > +}
> > +EXPORT_SYMBOL(tag_pages_for_writeback);
> > +
> > +/**
> >   * write_cache_pages - walk the list of dirty pages of the given address space and write all of them.
> >   * @mapping: address space structure to write
> >   * @wbc: subtract the number of written pages from *@wbc->nr_to_write
> > @@ -820,6 +844,13 @@ void __init page_writeback_init(void)
> >   * the call was made get new I/O started against them.  If wbc->sync_mode is
> >   * WB_SYNC_ALL then we were called for data integrity and we must wait for
> >   * existing IO to complete.
> > + *
> > + * To avoid livelocks (when other process dirties new pages), we first tag
> > + * pages which should be written back with TOWRITE tag and only then start
> > + * writing them. For data-integrity sync we have to be careful so that we do
> > + * not miss some pages (e.g., because some other process has cleared TOWRITE
> > + * tag we set). The rule we follow is that TOWRITE tag can be cleared only
> > + * by the process clearing the DIRTY tag (and submitting the page for IO).
> >   */
> >  int write_cache_pages(struct address_space *mapping,
> >  		      struct writeback_control *wbc, writepage_t writepage,
> > @@ -836,6 +867,7 @@ int write_cache_pages(struct address_space *mapping,
> >  	int cycled;
> >  	int range_whole = 0;
> >  	long nr_to_write = wbc->nr_to_write;
> > +	int tag;
> >  
> >  	pagevec_init(&pvec, 0);
> >  	if (wbc->range_cyclic) {
> > @@ -853,13 +885,18 @@ int write_cache_pages(struct address_space *mapping,
> >  			range_whole = 1;
> >  		cycled = 1; /* ignore range_cyclic tests */
> >  	}
> > +	if (wbc->sync_mode == WB_SYNC_ALL)
> > +		tag = PAGECACHE_TAG_TOWRITE;
> > +	else
> > +		tag = PAGECACHE_TAG_DIRTY;
> >  retry:
> > +	if (wbc->sync_mode == WB_SYNC_ALL)
> > +		tag_pages_for_writeback(mapping, index, end);
> 
> I wonder if this is too much spinlock latency in a huge dirty file?
> Some kid of batching of the operation perhaps would be good?
  You mean like copy tags for 4096 pages, then cond_resched the spin lock
and continue? That should be doable but it will give tasks that try to
livelock us more time (i.e. if there were 4096 tasks creating dirty pages
than probably they would be able to livelock us, won't they? Maybe we don't
care?).

> >  	done_index = index;
> >  	while (!done && (index <= end)) {
> >  		int i;
> >  
> > -		nr_pages = pagevec_lookup_tag(&pvec, mapping, &index,
> > -			      PAGECACHE_TAG_DIRTY,
> > +		nr_pages = pagevec_lookup_tag(&pvec, mapping, &index, tag,
> >  			      min(end - index, (pgoff_t)PAGEVEC_SIZE-1) + 1);
> >  		if (nr_pages == 0)
> >  			break;
> 
> Would it be neat to clear the tag even if we didn't set page to
> writeback? It should be uncommon case.
  Yeah, why not.

> > @@ -1319,6 +1356,9 @@ int test_set_page_writeback(struct page *page)
> >  			radix_tree_tag_clear(&mapping->page_tree,
> >  						page_index(page),
> >  						PAGECACHE_TAG_DIRTY);
> > +		radix_tree_tag_clear(&mapping->page_tree,
> > +				     page_index(page),
> > +				     PAGECACHE_TAG_TOWRITE);
> >  		spin_unlock_irqrestore(&mapping->tree_lock, flags);
> >  	} else {
> >  		ret = TestSetPageWriteback(page);
> 
> It would be nice to have bitwise tag clearing so we can clear multiple
> at once. Then
> 
> clear_tag = PAGECACHE_TAG_TOWRITE;
> if (!PageDirty(page))
>   clear_tag |= PAGECACHE_TAG_DIRTY;
> 
> That could reduce overhead a bit more.
  Good idea. Will do.

								Honza

-- 
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SUSE Labs, CR

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Re: [PATCH 2/2] mm: Implement writeback livelock avoidance using page tagging

[Nick Piggin]

On Mon, Jun 07, 2010 at 06:09:03PM +0200, Jan Kara wrote:
> On Sat 05-06-10 11:38:02, Nick Piggin wrote:
> > On Fri, Jun 04, 2010 at 08:47:11PM +0200, Jan Kara wrote:
> > > +	if (wbc->sync_mode == WB_SYNC_ALL)
> > > +		tag_pages_for_writeback(mapping, index, end);
> > 
> > I wonder if this is too much spinlock latency in a huge dirty file?
> > Some kid of batching of the operation perhaps would be good?
>   You mean like copy tags for 4096 pages, then cond_resched the spin lock
> and continue? That should be doable but it will give tasks that try to
> livelock us more time (i.e. if there were 4096 tasks creating dirty pages
> than probably they would be able to livelock us, won't they? Maybe we don't
> care?).

Not 100% sure. I think that if we've got the inode in I_SYNC state, it
should stop cleaning and dirtiers will get throttled.

Even if writeback was able to continue on that inode, it would be a big
achievement to dirty then clean pages as fast as we are able to tag them
in batches of 4096 :)

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Re: [PATCH 2/2] mm: Implement writeback livelock avoidance using page tagging

[Jan Kara]

On Tue 08-06-10 15:29:37, Nick Piggin wrote:
> On Mon, Jun 07, 2010 at 06:09:03PM +0200, Jan Kara wrote:
> > On Sat 05-06-10 11:38:02, Nick Piggin wrote:
> > > On Fri, Jun 04, 2010 at 08:47:11PM +0200, Jan Kara wrote:
> > > > +	if (wbc->sync_mode == WB_SYNC_ALL)
> > > > +		tag_pages_for_writeback(mapping, index, end);
> > > 
> > > I wonder if this is too much spinlock latency in a huge dirty file?
> > > Some kid of batching of the operation perhaps would be good?
> >   You mean like copy tags for 4096 pages, then cond_resched the spin lock
> > and continue? That should be doable but it will give tasks that try to
> > livelock us more time (i.e. if there were 4096 tasks creating dirty pages
> > than probably they would be able to livelock us, won't they? Maybe we don't
> > care?).
> 
> Not 100% sure. I think that if we've got the inode in I_SYNC state, it
> should stop cleaning and dirtiers will get throttled.
> 
> Even if writeback was able to continue on that inode, it would be a big
> achievement to dirty then clean pages as fast as we are able to tag them
> in batches of 4096 :)
  In practice, you are probably right that the writers will eventually get
throttled if they were aggressive enough to dirty lots of pages while
we cond_resched the lock...

									Honza
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SUSE Labs, CR

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Re: [PATCH 2/2] mm: Implement writeback livelock avoidance using page tagging

[Jan Kara]

On Mon 07-06-10 18:09:03, Jan Kara wrote:
> On Sat 05-06-10 11:38:02, Nick Piggin wrote:
> > On Fri, Jun 04, 2010 at 08:47:11PM +0200, Jan Kara wrote:
> > >  	done_index = index;
> > >  	while (!done && (index <= end)) {
> > >  		int i;
> > >  
> > > -		nr_pages = pagevec_lookup_tag(&pvec, mapping, &index,
> > > -			      PAGECACHE_TAG_DIRTY,
> > > +		nr_pages = pagevec_lookup_tag(&pvec, mapping, &index, tag,
> > >  			      min(end - index, (pgoff_t)PAGEVEC_SIZE-1) + 1);
> > >  		if (nr_pages == 0)
> > >  			break;
> > 
> > Would it be neat to clear the tag even if we didn't set page to
> > writeback? It should be uncommon case.
>   Yeah, why not.
  Looking at this more, we shouldn't leave any TOWRITE tags dangling in
WB_SYNC_ALL mode - as soon as someone writes the page, he does
set_page_writeback() which clears the tag. Similarly if the page is removed
from the mapping, the tag is cleared. Or am I missing something?

> > > @@ -1319,6 +1356,9 @@ int test_set_page_writeback(struct page *page)
> > >  			radix_tree_tag_clear(&mapping->page_tree,
> > >  						page_index(page),
> > >  						PAGECACHE_TAG_DIRTY);
> > > +		radix_tree_tag_clear(&mapping->page_tree,
> > > +				     page_index(page),
> > > +				     PAGECACHE_TAG_TOWRITE);
> > >  		spin_unlock_irqrestore(&mapping->tree_lock, flags);
> > >  	} else {
> > >  		ret = TestSetPageWriteback(page);
> > 
> > It would be nice to have bitwise tag clearing so we can clear multiple
> > at once. Then
> > 
> > clear_tag = PAGECACHE_TAG_TOWRITE;
> > if (!PageDirty(page))
> >   clear_tag |= PAGECACHE_TAG_DIRTY;
> > 
> > That could reduce overhead a bit more.
>   Good idea. Will do.
  On a second thought, will it bring us enough to justify a new interface
(which will be inconsistent with all the other radix tree interfaces
because they use tag numbers and not bitmaps)? Because looking at the code,
all we could save is the transformation of page index into a radix tree
path.  We would have to do all the other work for each tag separately
anyway and it won't probably have any great cache locality either because
radix trees for different tags are separate.

									Honza
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Jan Kara <jack@suse.cz>
SUSE Labs, CR

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Re: [PATCH 2/2] mm: Implement writeback livelock avoidance using page tagging

[Christoph Hellwig]

I have an oops with current Linus' tree in xfstests 217 that looks
like it was caused by this patch:

217 149s ...[ 5105.342605] XFS mounting filesystem vdb6
[ 5105.373481] Ending clean XFS mount for filesystem: vdb6
[ 5115.405061] XFS mounting filesystem loop0
[ 5115.548654] Ending clean XFS mount for filesystem: loop0
[ 5115.588067] BUG: unable to handle kernel paging request at f7f14000
[ 5115.588067] IP: [<c07224fd>]
radix_tree_range_tag_if_tagged+0x15d/0x1c0
[ 5115.588067] *pde = 00007067 *pte = 00000000 
[ 5115.588067] Oops: 0000 [#1] SMP 
[ 5115.588067] last sysfs file:
/sys/devices/virtual/block/loop0/removable

Entering kdb (current=0xf7868100, pid 15675) on processor 0 Oops: (null) due to oops @ 0xc07224fd
<d>Modules linked in:
<c>
<d>Pid: 15675, comm: mkfs.xfs Not tainted 2.6.35+ #305 /Bochs
<d>EIP: 0060:[<c07224fd>] EFLAGS: 00010002 CPU: 0
EIP is at radix_tree_range_tag_if_tagged+0x15d/0x1c0
<d>EAX: f7f14000 EBX: 00000000 ECX: 482bb4f8 EDX: 0c0748d4
<d>ESI: 2031756d EDI: 00000000 EBP: c7d41d10 ESP: c7d41cb0
<d> DS: 007b ES: 007b FS: 00d8 GS: 0033 SS: 0068
<0>Process mkfs.xfs (pid: 15675, ti=c7d40000 task=f7868100
task.ti=c7d40000)
<0>Stack: ffffffff f774a1d8 f774a598 f774aa98 cfd9c598 e7c89458 00000001 00000000
<0> c01e0caf c01e0caf 00000046 00000010 00000000 e7c89568 c7d41d28 c355ad50
<0> ffffffff 00000208 00000007 c7d41cb0 00000003 ffffffff c355ad5c ffffffff
<0>Call Trace:
<0> [<c01e0caf>] ? tag_pages_for_writeback+0x1f/0xc0
<0> [<c01e0caf>] ? tag_pages_for_writeback+0x1f/0xc0
<0> [<c01e0cd3>] ? tag_pages_for_writeback+0x43/0xc0
<0> [<c01e172b>] ? write_cache_pages+0x23b/0x370
<0> [<c01e0980>] ? __writepage+0x0/0x30
<0> [<c0528979>] ? xfs_vm_writepages+0x29/0x50
[0]more> 
Only 'q' or 'Q' are processed at more prompt, input ignored
<0> [<c0528979>] ? xfs_vm_writepages+0x29/0x50
<0> [<c05296e0>] ? xfs_vm_writepage+0x0/0x630
<0> [<c01e187f>] ? generic_writepages+0x1f/0x30
<0> [<c0528992>] ? xfs_vm_writepages+0x42/0x50
<0> [<c01e18a7>] ? do_writepages+0x17/0x30
<0> [<c01da57c>] ? __filemap_fdatawrite_range+0x5c/0x70
<0> [<c01da8a6>] ? filemap_fdatawrite+0x26/0x30
<0> [<c01da8dd>] ? filemap_write_and_wait+0x2d/0x50
<0> [<c052e812>] ? xfs_flushinval_pages+0x72/0xe0
<0> [<c0506c9c>] ? xfs_ilock+0x7c/0xd0
<0> [<c052d7d7>] ? xfs_file_aio_read+0x307/0x340
<0> [<c020e69c>] ? do_sync_read+0x9c/0xd0
<0> [<c0189636>] ? up_read+0x16/0x30
<0> [<c020e957>] ? vfs_read+0x97/0x140
<0> [<c020e600>] ? do_sync_read+0x0/0xd0
<0> [<c0181c18>] ? __task_pid_nr_ns+0x88/0xd0
<0> [<c020f1c3>] ? sys_pread64+0x63/0x80
<0> [<c09cb7ed>] ? syscall_call+0x7/0xb
<0>Code: f0 83 c3 01 d3 e3 39 5d e0 72 51 8b 45 08 39 45 e4 73 49 89 d8
d3 e8 a8 3f 0f 85 47 ff ff ff 8b 75 ec 8d 04 96 90 83 c1 06 89 df <8b>
30 d3 ef 83 c2 01 83 c0 04 83 e7 3f 74 ec e9 27 ff ff ff 8b 

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Re: [PATCH 2/2] mm: Implement writeback livelock avoidance using page tagging

[Jan Kara]

[-- Attachment #1: Type: text/plain, Size: 2400 bytes --]

On Thu 12-08-10 14:35:47, Christoph Hellwig wrote:
> I have an oops with current Linus' tree in xfstests 217 that looks
> like it was caused by this patch:
  Thanks for report!

> 217 149s ...[ 5105.342605] XFS mounting filesystem vdb6
> [ 5105.373481] Ending clean XFS mount for filesystem: vdb6
> [ 5115.405061] XFS mounting filesystem loop0
> [ 5115.548654] Ending clean XFS mount for filesystem: loop0
> [ 5115.588067] BUG: unable to handle kernel paging request at f7f14000
> [ 5115.588067] IP: [<c07224fd>] radix_tree_range_tag_if_tagged+0x15d/0x1c0
> [ 5115.588067] *pde = 00007067 *pte = 00000000 
> [ 5115.588067] Oops: 0000 [#1] SMP 
> [ 5115.588067] last sysfs file:
> /sys/devices/virtual/block/loop0/removable
  We seem to oops at:
                while (((index >> shift) & RADIX_TREE_MAP_MASK) == 0) {
                        /*
                         * We've fully scanned this node. Go up. Because
                         * last_index is guaranteed to be in the tree, what
                         * we do below cannot wander astray.
                         */
>>>>>                   slot = open_slots[height];
                        height++;
                        shift += RADIX_TREE_MAP_SHIFT;
                }

> Entering kdb (current=0xf7868100, pid 15675) on processor 0 Oops: (null) due to oops @ 0xc07224fd
> <d>Modules linked in:
> <c>
> <d>Pid: 15675, comm: mkfs.xfs Not tainted 2.6.35+ #305 /Bochs
> <d>EIP: 0060:[<c07224fd>] EFLAGS: 00010002 CPU: 0
> EIP is at radix_tree_range_tag_if_tagged+0x15d/0x1c0
> <d>EAX: f7f14000 EBX: 00000000 ECX: 482bb4f8 EDX: 0c0748d4
> <d>ESI: 2031756d EDI: 00000000 EBP: c7d41d10 ESP: c7d41cb0
  And from the values in registers the loop seems to have went astray
because "index" was zero at the point we entered the loop... looking
around...  Ah, I see, you create files with 16TB size which creates
radix tree of such height that radix_tree_maxindex(height) == ~0UL and
if write_cache_pages() passes in ~0UL as end, we can overflow the index.
Hmm, I haven't realized that is possible.
  OK, attached is a patch that should fix the issue. There is just still an
issue that *first_indexp will overflow in this case as well and thus we
could in theory loop indefinitely. I'll have to think how to best handle
this overflow - checking in caller is kind of prone to errors...

								Honza
-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR

[-- Attachment #2: 0001-mm-Fix-overflow-in-radix_tree_range_tag_if_tagged.patch --]
[-- Type: text/x-patch, Size: 0 bytes --]

Re: [PATCH 2/2] mm: Implement writeback livelock avoidance using page tagging

[Christoph Hellwig]

On Fri, Aug 13, 2010 at 12:28:58AM +0200, Jan Kara wrote:
>   And from the values in registers the loop seems to have went astray
> because "index" was zero at the point we entered the loop... looking
> around...  Ah, I see, you create files with 16TB size which creates
> radix tree of such height that radix_tree_maxindex(height) == ~0UL and
> if write_cache_pages() passes in ~0UL as end, we can overflow the index.
> Hmm, I haven't realized that is possible.
>   OK, attached is a patch that should fix the issue.

This seems to fix the case for me.

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Re: [PATCH 0/2 RFC v3] Livelock avoidance for data integrity writeback

[Nick Piggin]

On Fri, Jun 04, 2010 at 08:47:09PM +0200, Jan Kara wrote:
> 
>   Hi,
> 
>   I've revived my patches to implement livelock avoidance for data integrity
> writes. Due to some concerns whether tagging of pages before writeout cannot
> be too costly to use for WB_SYNC_NONE mode (where we stop after nr_to_write
> pages) I've changed the patch to use page tagging only in WB_SYNC_ALL mode
> where we are sure that we write out all the tagged pages. Later, we can think
> about using tagging for livelock avoidance for WB_SYNC_NONE mode as well...

Hmm what concerns? Do you have any numbers?

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Re: [PATCH 0/2 RFC v3] Livelock avoidance for data integrity writeback

[Wu Fengguang]

On Sat, Jun 05, 2010 at 11:14:47AM +1000, Nick Piggin wrote:
> On Fri, Jun 04, 2010 at 08:47:09PM +0200, Jan Kara wrote:
> > 
> >   Hi,
> > 
> >   I've revived my patches to implement livelock avoidance for data integrity
> > writes. Due to some concerns whether tagging of pages before writeout cannot
> > be too costly to use for WB_SYNC_NONE mode (where we stop after nr_to_write
> > pages) I've changed the patch to use page tagging only in WB_SYNC_ALL mode
> > where we are sure that we write out all the tagged pages. Later, we can think
> > about using tagging for livelock avoidance for WB_SYNC_NONE mode as well...
> 
> Hmm what concerns? Do you have any numbers?

sync() is performed in two stages: the WB_SYNC_NONE run and the
WB_SYNC_ALL run. The WB_SYNC_NONE stage can still be livelocked.

We may switch to wbc.for_sync (instead of testing WB_SYNC_ALL only)
provided by the following patch. Jan, would you add it to your series
(with necessary improvements)?

Thanks,
Fengguang
---
introduce wbc.for_sync to cover the two sync() stages

The sync() is performed in two stages: the WB_SYNC_NONE sync and
the WB_SYNC_ALL sync. It is necessary to tag both stages with
wbc.for_sync, so as to prevent either of them being livelocked.

The next patch will utilize this flag to do the livelock prevention.

btw, the nr_pages param in bdi_start_writeback() is dropped:

1) better livelock prevention will be used. Limiting nr_to_write was
   inferior in that it may miss some pages to sync. And it does not
   really avoid livelock for single huge file: write_cache_pages()
   ignores nr_to_write for WB_SYNC_NONE at all.

2) It is very unlikely to impact some not-for-sync existing users.
   For example the ubifs call writeback_inodes_sb() to writeback some
   pages. And the ext4 also intents to write somehow enough pages with
   writeback_inodes_sb_if_idle().  However they seem not really care to
   write more pages.

CC: Jan Kara <jack@suse.cz>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
---
 fs/fs-writeback.c           |   28 ++++++++--------------------
 include/linux/backing-dev.h |    2 +-
 include/linux/writeback.h   |    8 ++++++++
 mm/page-writeback.c         |    2 +-
 4 files changed, 18 insertions(+), 22 deletions(-)

--- linux.orig/fs/fs-writeback.c	2010-06-06 11:16:42.000000000 +0800
+++ linux/fs/fs-writeback.c	2010-06-06 11:21:13.000000000 +0800
@@ -42,6 +42,7 @@ struct wb_writeback_args {
 	long nr_pages;
 	struct super_block *sb;
 	enum writeback_sync_modes sync_mode;
+	int for_sync:1;
 	unsigned int for_kupdate:1;
 	unsigned int range_cyclic:1;
 	unsigned int for_background:1;
@@ -228,6 +229,7 @@ static void bdi_sync_writeback(struct ba
 	struct wb_writeback_args args = {
 		.sb		= sb,
 		.sync_mode	= WB_SYNC_ALL,
+		.for_sync	= 1,
 		.nr_pages	= LONG_MAX,
 		.range_cyclic	= 0,
 	};
@@ -244,7 +246,6 @@ static void bdi_sync_writeback(struct ba
  * bdi_start_writeback - start writeback
  * @bdi: the backing device to write from
  * @sb: write inodes from this super_block
- * @nr_pages: the number of pages to write
  *
  * Description:
  *   This does WB_SYNC_NONE opportunistic writeback. The IO is only
@@ -253,24 +254,17 @@ static void bdi_sync_writeback(struct ba
  *
  */
 void bdi_start_writeback(struct backing_dev_info *bdi, struct super_block *sb,
-			 long nr_pages)
+			 long mission)
 {
 	struct wb_writeback_args args = {
 		.sb		= sb,
 		.sync_mode	= WB_SYNC_NONE,
-		.nr_pages	= nr_pages,
+		.nr_pages	= LONG_MAX,
+		.for_background	= mission == WB_FOR_BACKGROUND,
+		.for_sync	= mission == WB_FOR_SYNC,
 		.range_cyclic	= 1,
 	};
 
-	/*
-	 * We treat @nr_pages=0 as the special case to do background writeback,
-	 * ie. to sync pages until the background dirty threshold is reached.
-	 */
-	if (!nr_pages) {
-		args.nr_pages = LONG_MAX;
-		args.for_background = 1;
-	}
-
 	bdi_alloc_queue_work(bdi, &args);
 }
 
@@ -757,6 +751,7 @@ static long wb_writeback(struct bdi_writ
 		.older_than_this	= NULL,
 		.for_kupdate		= args->for_kupdate,
 		.for_background		= args->for_background,
+		.for_sync		= args->for_sync,
 		.range_cyclic		= args->range_cyclic,
 	};
 	unsigned long oldest_jif;
@@ -1216,14 +1211,7 @@ static void wait_sb_inodes(struct super_
  */
 void writeback_inodes_sb(struct super_block *sb)
 {
-	unsigned long nr_dirty = global_page_state(NR_FILE_DIRTY);
-	unsigned long nr_unstable = global_page_state(NR_UNSTABLE_NFS);
-	long nr_to_write;
-
-	nr_to_write = nr_dirty + nr_unstable +
-			(inodes_stat.nr_inodes - inodes_stat.nr_unused);
-
-	bdi_start_writeback(sb->s_bdi, sb, nr_to_write);
+	bdi_start_writeback(sb->s_bdi, sb, WB_FOR_SYNC);
 }
 EXPORT_SYMBOL(writeback_inodes_sb);
 
--- linux.orig/include/linux/backing-dev.h	2010-06-06 11:16:42.000000000 +0800
+++ linux/include/linux/backing-dev.h	2010-06-06 11:21:13.000000000 +0800
@@ -106,7 +106,7 @@ int bdi_register_dev(struct backing_dev_
 void bdi_unregister(struct backing_dev_info *bdi);
 int bdi_setup_and_register(struct backing_dev_info *, char *, unsigned int);
 void bdi_start_writeback(struct backing_dev_info *bdi, struct super_block *sb,
-				long nr_pages);
+				long mission);
 int bdi_writeback_task(struct bdi_writeback *wb);
 int bdi_has_dirty_io(struct backing_dev_info *bdi);
 void bdi_arm_supers_timer(void);
--- linux.orig/include/linux/writeback.h	2010-06-06 11:16:42.000000000 +0800
+++ linux/include/linux/writeback.h	2010-06-06 11:58:58.000000000 +0800
@@ -21,6 +21,13 @@ enum writeback_sync_modes {
 	WB_SYNC_ALL,	/* Wait on every mapping */
 };
 
+enum writeback_mission {
+	WB_FOR_BACKGROUND,	/* stop on hitting background threshold */
+	WB_FOR_SYNC,		/* write all now-dirty inodes/pages,
+				 * but take care not to live lock
+				 */
+};
+
 /*
  * A control structure which tells the writeback code what to do.  These are
  * always on the stack, and hence need no locking.  They are always initialised
@@ -53,6 +60,7 @@ struct writeback_control {
 	unsigned encountered_congestion:1; /* An output: a queue is full */
 	unsigned for_kupdate:1;		/* A kupdate writeback */
 	unsigned for_background:1;	/* A background writeback */
+	unsigned for_sync:1;		/* A writeback for sync */
 	unsigned for_reclaim:1;		/* Invoked from the page allocator */
 	unsigned range_cyclic:1;	/* range_start is cyclic */
 	unsigned more_io:1;		/* more io to be dispatched */
--- linux.orig/mm/page-writeback.c	2010-06-06 11:16:42.000000000 +0800
+++ linux/mm/page-writeback.c	2010-06-06 11:21:13.000000000 +0800
@@ -597,7 +597,7 @@ static void balance_dirty_pages(struct a
 	    (!laptop_mode && ((global_page_state(NR_FILE_DIRTY)
 			       + global_page_state(NR_UNSTABLE_NFS))
 					  > background_thresh)))
-		bdi_start_writeback(bdi, NULL, 0);
+		bdi_start_writeback(bdi, NULL, WB_FOR_BACKGROUND);
 }
 
 void set_page_dirty_balance(struct page *page, int page_mkwrite)

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Re: [PATCH 0/2 RFC v3] Livelock avoidance for data integrity writeback

[Nick Piggin]

On Sun, Jun 06, 2010 at 12:08:19PM +0800, Wu Fengguang wrote:
> On Sat, Jun 05, 2010 at 11:14:47AM +1000, Nick Piggin wrote:
> > On Fri, Jun 04, 2010 at 08:47:09PM +0200, Jan Kara wrote:
> > > 
> > >   Hi,
> > > 
> > >   I've revived my patches to implement livelock avoidance for data integrity
> > > writes. Due to some concerns whether tagging of pages before writeout cannot
> > > be too costly to use for WB_SYNC_NONE mode (where we stop after nr_to_write
> > > pages) I've changed the patch to use page tagging only in WB_SYNC_ALL mode
> > > where we are sure that we write out all the tagged pages. Later, we can think
> > > about using tagging for livelock avoidance for WB_SYNC_NONE mode as well...
> > 
> > Hmm what concerns? Do you have any numbers?
> 
> sync() is performed in two stages: the WB_SYNC_NONE run and the
> WB_SYNC_ALL run. The WB_SYNC_NONE stage can still be livelocked.

By concerns, I mean Jan's _performance_ concerns. I would prefer to
minimise them, and then try to get an idea of the performance impact
of doing tagging unconditionally.

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[PATCH 0/2 RFC v3] Livelock avoidance for data integrity writeback

[Jan Kara]

  Hi,

  I've revived my patches to implement livelock avoidance for data integrity
writes. Due to some concerns whether tagging of pages before writeout cannot
be too costly to use for WB_SYNC_NONE mode (where we stop after nr_to_write
pages) I've changed the patch to use page tagging only in WB_SYNC_ALL mode
where we are sure that we write out all the tagged pages. Later, we can think
about using tagging for livelock avoidance for WB_SYNC_NONE mode as well...
  As always comments are welcome.

									Honza

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