Re: [patch 4/8] mm: allow not updating BDI stats in end_page_writeback()

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

On Mon, 17 Mar 2008 20:19:12 +0100 Miklos Szeredi <miklos@szeredi.hu> wrote:

> From: Miklos Szeredi <mszeredi@suse.cz>
> 
> Fuse's writepage will need to clear page writeback separately from
> updating the per BDI counters.
> 
> This patch renames end_page_writeback() to __end_page_writeback() and
> adds a boolean parameter to indicate if the per BDI stats need to be
> updated.
> 
> Regular callers get an inline end_page_writeback() without the boolean
> parameter.
> 
> ...
> 
> Index: linux/include/linux/page-flags.h
> ===================================================================
> --- linux.orig/include/linux/page-flags.h	2008-03-17 18:24:13.000000000 +0100
> +++ linux/include/linux/page-flags.h	2008-03-17 18:25:53.000000000 +0100
> @@ -300,7 +300,7 @@ struct page;	/* forward declaration */
>  
>  extern void cancel_dirty_page(struct page *page, unsigned int account_size);
>  
> -int test_clear_page_writeback(struct page *page);
> +int test_clear_page_writeback(struct page *page, bool bdi_stats);
>  int test_set_page_writeback(struct page *page);
>  
>  static inline void set_page_writeback(struct page *page)
> Index: linux/include/linux/pagemap.h
> ===================================================================
> --- linux.orig/include/linux/pagemap.h	2008-03-17 18:24:13.000000000 +0100
> +++ linux/include/linux/pagemap.h	2008-03-17 18:25:53.000000000 +0100
> @@ -223,7 +223,12 @@ static inline void wait_on_page_writebac
>  		wait_on_page_bit(page, PG_writeback);
>  }
>  
> -extern void end_page_writeback(struct page *page);
> +extern void __end_page_writeback(struct page *page, bool bdi_stats);
> +
> +static inline void end_page_writeback(struct page *page)
> +{
> +	__end_page_writeback(page, true);
> +}
>  
>  /*
>   * Fault a userspace page into pagetables.  Return non-zero on a fault.
> Index: linux/mm/filemap.c
> ===================================================================
> --- linux.orig/mm/filemap.c	2008-03-17 18:25:38.000000000 +0100
> +++ linux/mm/filemap.c	2008-03-17 18:25:53.000000000 +0100
> @@ -574,19 +574,20 @@ EXPORT_SYMBOL(unlock_page);
>  /**
>   * end_page_writeback - end writeback against a page
>   * @page: the page
> + * @bdi_stats: update the per-bdi writeback counter
>   */
> -void end_page_writeback(struct page *page)
> +void __end_page_writeback(struct page *page, bool bdi_stats)
>  {
>  	if (TestClearPageReclaim(page))
>  		rotate_reclaimable_page(page);
>  
> -	if (!test_clear_page_writeback(page))
> +	if (!test_clear_page_writeback(page, bdi_stats))
>  		BUG();
>  
>  	smp_mb__after_clear_bit();
>  	wake_up_page(page, PG_writeback);
>  }
> -EXPORT_SYMBOL(end_page_writeback);
> +EXPORT_SYMBOL(__end_page_writeback);
>  
>  /**
>   * __lock_page - get a lock on the page, assuming we need to sleep to get it
> Index: linux/mm/page-writeback.c
> ===================================================================
> --- linux.orig/mm/page-writeback.c	2008-03-17 18:25:17.000000000 +0100
> +++ linux/mm/page-writeback.c	2008-03-17 18:25:53.000000000 +0100
> @@ -1242,7 +1242,7 @@ int clear_page_dirty_for_io(struct page 
>  }
>  EXPORT_SYMBOL(clear_page_dirty_for_io);
>  
> -int test_clear_page_writeback(struct page *page)
> +int test_clear_page_writeback(struct page *page, bool bdi_stats)
>  {
>  	struct address_space *mapping = page_mapping(page);
>  	int ret;
> @@ -1257,7 +1257,7 @@ int test_clear_page_writeback(struct pag
>  			radix_tree_tag_clear(&mapping->page_tree,
>  						page_index(page),
>  						PAGECACHE_TAG_WRITEBACK);
> -			if (bdi_cap_writeback_dirty(bdi)) {
> +			if (bdi_stats && bdi_cap_writeback_dirty(bdi)) {
>  				__dec_bdi_stat(bdi, BDI_WRITEBACK);
>  				__bdi_writeout_inc(bdi);
>  			}

Adding `mode' flags to a core function is generally considered poor form. 
And it adds additional overhead and possibly stack utilisation for all
callers.

We generally prefer that a new function be created.  After all, that's what
you've done here, only the code has gone and wedged two different functions
into one.


Another approach might be to add a new bdi_cap_foo() flag.  We could then do

	if (bdi_cap_writeback_dirty(bdi) && bdi_cap_mumble(bdi)) {

here.  But even better would be to create a new BDI capability which
indicates that this address_space doesn't want this treatment in
test_clear_page_writeback(), then go fix up all the
!bdi_cap_writeback_dirty() address_spaces to set that flag.

So then the code becomes

	if (!bdi_cap_account_writeback_in_test_clear_page_writeback(bdi)) {

(good luck thinking up a better name ;))

Reason: bdi_cap_writeback_dirty() is kinda weirdly intrepreted to mean
various different things in different places and we really should separate
its multiple interpretations into separate flags.

Note that this becomes a standalone VFS cleanup patch, and the fuse code
can then just use it later on.  

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