[Devel] Re: [PATCH v4 06/11] memcg: add dirty page accounting infrastructure

[KAMEZAWA Hiroyuki]

On Fri, 29 Oct 2010 19:13:00 +0800
Wu Fengguang <fengguang.wu at intel.com> wrote:

> On Fri, Oct 29, 2010 at 03:09:09PM +0800, Greg Thelen wrote:
> 
> > +
> > +	case MEMCG_NR_FILE_DIRTY:
> > +		/* Use Test{Set,Clear} to only un/charge the memcg once. */
> > +		if (val > 0) {
> > +			if (TestSetPageCgroupFileDirty(pc))
> > +				val = 0;
> > +		} else {
> > +			if (!TestClearPageCgroupFileDirty(pc))
> > +				val = 0;
> > +		}
> 
> I'm wondering why TestSet/TestClear and even the cgroup page flags for
> dirty/writeback/unstable pages are necessary at all (it helps to
> document in changelog if there are any). For example, VFS will call
> TestSetPageDirty() before calling
> mem_cgroup_inc_page_stat(MEMCG_NR_FILE_DIRTY), so there should be no
> chance of false double counting.
> 

1. flag is necessary for moving accounting information between cgroups
   when account_move() occurs.

2. TestSet... is required because there are always race with page_cgroup_lock()'s
   lock bit.

Thanks,
-Kame

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