[Ocfs2-devel] [PATCH 3/4] ocfs2: Add readhead during CoW.
Tao Ma
tao.ma at oracle.com
Tue Jun 29 18:46:30 PDT 2010
Hi Joel,
On 06/30/2010 07:23 AM, Joel Becker wrote:
> On Tue, Jun 29, 2010 at 04:35:40PM +0800, Tao Ma wrote:
>> @@ -2953,6 +2957,14 @@ static int ocfs2_duplicate_clusters_by_page(handle_t *handle,
>> if (PAGE_CACHE_SIZE<= OCFS2_SB(sb)->s_clustersize)
>> BUG_ON(PageDirty(page));
>>
>> + if (PageReadahead(page)&& context->file) {
>> + page_cache_async_readahead(mapping,
>> + &context->file->f_ra,
>> + context->file,
>> + page, page_index,
>> + readahead_pages);
>> + }
>
> This is merely re-sending the same pages that were already sent,
> right? In the previous patch, you asked the readahead code to try all
> pages in the hunk. Now you've discovered a page that isn't yet up to
> date, and you send it (and the 1M next to it) back to readahead.
> This is, I assume, because the readahead code doesn't actually
> read your entire request from page_cache_sync_readahead(). It just
> reads some, and this is you hinting that you need the next bit. Am I
> right?
The first previous patch just let the caller to do the readahead for the
whole hunk and set PG_readahead to a page as the start of readahead
window. So when we meet with a page with PG_readahead flag set, we know
it's time to move our readahead window so a new readahead is issued here.
Fengguang Wu has a document named "On the Design of a New Linux
Readahead Framework", you can refer to it and hope I read it clearly.
Regards,
Tao
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