linux-kernel - Re: [PATCH 0/3] Fadvise: Directory level page cache cleaning support

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Date:	Mon, 30 Dec 2013 12:40:05 -0700
From:	Andreas Dilger <adilger@...ger.ca>
To:	Dave Hansen <dave.hansen@...el.com>
Cc:	Li Wang <liwang@...ntukylin.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Cong Wang <xiyou.wangcong@...il.com>,
	Zefan Li <lizefan@...wei.com>, Matthew Wilcox <matthew@....cx>
Subject: Re: [PATCH 0/3] Fadvise: Directory level page cache cleaning support

On Dec 30, 2013, at 12:18, Dave Hansen <dave.hansen@...el.com> wrote:
> 
> Why is this necessary to do in the kernel?  Why not leave it to
> userspace to walk the filesystem(s)?

I would suspect that trying to do it in userspace would be quite bad. It would require traversing the whole directory tree to issue cache flushed for each subdirectory, but it doesn't know when to stop traversal. That would mean the "cache flush" would turn into "cache pollute" and cause a lot of disk IO for subdirectories not in cache to begin with. 

Cheers, Andreas--
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