Messages in this thread | | | Date | Wed, 29 Apr 2009 16:59:40 -0700 | From | Andrew Morton <> | Subject | Re: [PATCH] Fix dirty page accounting in redirty_page_for_writepage() |
| |
On Wed, 29 Apr 2009 19:25:46 -0400 Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca> wrote:
> Basically, the following execution : > > dd if=/dev/zero of=/tmp/testfile > > will slowly fill _all_ ram available without taking into account memory > pressure. > > This is because the dirty page accounting is incorrect in > redirty_page_for_writepage. > > This patch adds missing dirty page accounting in redirty_page_for_writepage().
The patch changes __set_page_dirty_nobuffers(), not redirty_page_for_writepage().
__set_page_dirty_nobuffers() has a huge number of callers.
> --- linux-2.6-lttng.orig/mm/page-writeback.c 2009-04-29 18:14:48.000000000 -0400 > +++ linux-2.6-lttng/mm/page-writeback.c 2009-04-29 18:23:59.000000000 -0400 > @@ -1237,6 +1237,12 @@ int __set_page_dirty_nobuffers(struct pa > if (!mapping) > return 1; > > + /* > + * Take care of setting back page accounting correctly. > + */ > + inc_zone_page_state(page, NR_FILE_DIRTY); > + inc_bdi_stat(mapping->backing_dev_info, BDI_RECLAIMABLE); > + > spin_lock_irq(&mapping->tree_lock); > mapping2 = page_mapping(page); > if (mapping2) { /* Race with truncate? */ >
But __set_page_dirty_nobuffers() calls account_page_dirtied(), which already does the above two operations. afacit we're now double-accounting.
Now, it's possible that the accounting goes wrong very occasionally in the "/* Race with truncate? */" case. If the truncate path clears the page's dirty bit then it will decrement the dirty-page accounting, but this code path will fail to perform the increment of the dirty-page accounting. IOW, once this function has set PG_Dirty, it is committed to altering some or all of the page-dirty accounting.
But afacit your test case will not trigger the race-with-truncate anyway?
Can you determine at approximately what frequency (pages-per-second) this accounting leak is occurring in your test?
| |