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

[Andrew Morton <akpm@linux-foundation.org>]

On Fri,  4 Jun 2010 20:40:54 +0200
Jan Kara <jack@suse.cz> 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

I'd add "inclusive" here as well.  Add it everywhere, very carefully
(or "exclusive").  it really is a minefield and we've had off-by-ones
from this before


> 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);

Problem.  For how long can this disable interrupts?  Maybe 1TB of dirty
pagecache before the NMI watchdog starts getting involved?  Could be a
problem in some situations.

Easy enough to fix - just walk the range in 1000(?) page hunks,
dropping the lock and doing cond_resched() each time. 
radix_tree_gang_tag_if_tagged() will need to return next_index to make
that efficient with sparse files.

> +/**
>   * 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).
>   */

Seems simple enough and I can't think of any bugs which the obvious
races will cause.

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