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SubjectRe: [PATCH 3/7] writeback: fix dirtied pages accounting on sub-page writes
On Wed 07-12-11 20:08:18, Wu Fengguang wrote:
> On Wed, Dec 07, 2011 at 06:53:40PM +0800, Jan Kara wrote:
> > On Mon 28-11-11 21:53:41, Wu Fengguang wrote:
> > > When dd in 512bytes, generic_perform_write() calls
> > > balance_dirty_pages_ratelimited() 8 times for the same page, but
> > > obviously the page is only dirtied once.
> > Actually, for ppc where pages can be 64 KB the problem is even worse.
>
> Ah yes.
>
> > > Fix it by accounting tsk->nr_dirtied and bdp_ratelimits at page dirty time.
> > I was wondering about one more thing - couldn't we rather check in
> > generic_perform_write() whether the page was dirty before calling
> > ->write_end and call balance_dirty_pages_ratelimited() only if it wasn't?
>
> Cough.. the very original version does that exactly, then you raised
> some concern here:
>
> https://lkml.org/lkml/2011/4/13/554
>
> The discussion goes on and eventually I get to the current version
> that looks most acceptable in the three options.
Good point. I should have researched web (or my memory) more closely ;)
Thanks for the pointer - it has reminded me why using PageDirty isn't quite
perfect.

> > For generic_perform_write() it doesn't really matter that much since we
> > do things page-by-page anyway but other callers could be more efficient...
>
> That's right.
You can add:
Acked-by: Jan Kara <jack@suse.cz>

Honza

> > > Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
> > > Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
> > > ---
> > > mm/page-writeback.c | 13 +++++--------
> > > 1 file changed, 5 insertions(+), 8 deletions(-)
> > >
> > > --- linux-next.orig/mm/page-writeback.c 2011-11-28 21:23:20.000000000 +0800
> > > +++ linux-next/mm/page-writeback.c 2011-11-28 21:23:23.000000000 +0800
> > > @@ -1239,8 +1239,6 @@ void balance_dirty_pages_ratelimited_nr(
> > > if (bdi->dirty_exceeded)
> > > ratelimit = min(ratelimit, 32 >> (PAGE_SHIFT - 10));
> > >
> > > - current->nr_dirtied += nr_pages_dirtied;
> > > -
> > > preempt_disable();
> > > /*
> > > * This prevents one CPU to accumulate too many dirtied pages without
> > > @@ -1251,12 +1249,9 @@ void balance_dirty_pages_ratelimited_nr(
> > > p = &__get_cpu_var(bdp_ratelimits);
> > > if (unlikely(current->nr_dirtied >= ratelimit))
> > > *p = 0;
> > > - else {
> > > - *p += nr_pages_dirtied;
> > > - if (unlikely(*p >= ratelimit_pages)) {
> > > - *p = 0;
> > > - ratelimit = 0;
> > > - }
> > > + else if (unlikely(*p >= ratelimit_pages)) {
> > > + *p = 0;
> > > + ratelimit = 0;
> > > }
> > > /*
> > > * Pick up the dirtied pages by the exited tasks. This avoids lots of
> > > @@ -1749,6 +1744,8 @@ void account_page_dirtied(struct page *p
> > > __inc_bdi_stat(mapping->backing_dev_info, BDI_RECLAIMABLE);
> > > __inc_bdi_stat(mapping->backing_dev_info, BDI_DIRTIED);
> > > task_io_account_write(PAGE_CACHE_SIZE);
> > > + current->nr_dirtied++;
> > > + this_cpu_inc(bdp_ratelimits);
> > > }
> > > }
> > > EXPORT_SYMBOL(account_page_dirtied);
> > >
> > >
> > --
> > Jan Kara <jack@suse.cz>
> > SUSE Labs, CR
--
Jan Kara <jack@suse.cz>
SUSE Labs, CR


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