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SubjectRe: [PATCH 2/4] writeback: avoid duplicate balance_dirty_pages_ratelimited() calls
On Thu, Apr 14, 2011 at 05:53:07AM +0800, Jan Kara wrote:
> On Wed 13-04-11 16:59:39, Wu Fengguang wrote:
> > When dd in 512bytes, balance_dirty_pages_ratelimited() could be called 8
> > times for the same page, but obviously the page is only dirtied once.
> >
> > Fix it with a (slightly racy) PageDirty() test.
> >
> > Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
> > ---
> > mm/filemap.c | 5 ++++-
> > 1 file changed, 4 insertions(+), 1 deletion(-)
> >
> > --- linux-next.orig/mm/filemap.c 2011-04-13 16:46:01.000000000 +0800
> > +++ linux-next/mm/filemap.c 2011-04-13 16:47:26.000000000 +0800
> > @@ -2313,6 +2313,7 @@ static ssize_t generic_perform_write(str
> > long status = 0;
> > ssize_t written = 0;
> > unsigned int flags = 0;
> > + unsigned int dirty;
> >
> > /*
> > * Copies from kernel address space cannot fail (NFSD is a big user).
> > @@ -2361,6 +2362,7 @@ again:
> > pagefault_enable();
> > flush_dcache_page(page);
> >
> > + dirty = PageDirty(page);
> This isn't completely right as we sometimes dirty the page in
> ->write_begin() (see e.g. block_write_begin() when we allocate blocks under
> an already uptodate page) and in such cases we would not call
> balance_dirty_pages(). So I'm not sure we can really do this
> optimization (although it's sad)...

Good catch, thanks! I evaluated three possible options, the last one
looks most promising (however is a radical change).

- do radix_tree_tag_get() before calling ->write_begin()
simple but heavy weight

- add balance_dirty_pages_ratelimited() in __block_write_begin()
seems not easy, too

- accurately account the dirtied pages in account_page_dirtied() rather than
in balance_dirty_pages_ratelimited_nr(). This diff on top of my patchset
illustrates the idea, but will need to sort out cases like direct IO ...

--- linux-next.orig/mm/page-writeback.c 2011-04-14 07:50:09.000000000 +0800
+++ linux-next/mm/page-writeback.c 2011-04-14 07:52:35.000000000 +0800
@@ -1295,8 +1295,6 @@ void balance_dirty_pages_ratelimited_nr(
if (!bdi_cap_account_dirty(bdi))
return;

- current->nr_dirtied += nr_pages_dirtied;
-
if (dirty_exceeded_recently(bdi, MAX_PAUSE)) {
unsigned long max = current->nr_dirtied +
(128 >> (PAGE_SHIFT - 10));
@@ -1752,6 +1750,7 @@ void account_page_dirtied(struct page *p
__inc_bdi_stat(mapping->backing_dev_info, BDI_DIRTIED);
task_dirty_inc(current);
task_io_account_write(PAGE_CACHE_SIZE);
+ current->nr_dirtied++;
}
}
EXPORT_SYMBOL(account_page_dirtied);
> > mark_page_accessed(page);
> > status = a_ops->write_end(file, mapping, pos, bytes, copied,
> > page, fsdata);
> > @@ -2387,7 +2389,8 @@ again:
> > pos += copied;
> > written += copied;
> >
> > - balance_dirty_pages_ratelimited(mapping);
> > + if (!dirty)
> > + balance_dirty_pages_ratelimited(mapping);
> >
> > } while (iov_iter_count(i));
>
> Honza
> --
> Jan Kara <jack@suse.cz>
> SUSE Labs, CR


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