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

[Wu Fengguang]

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.

Thanks,
Fengguang
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