linux-kernel - Re: [PATCH RFC] mm: Let readahead submit larger batches of pages in case of ra->ra

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Date:   Fri, 11 Sep 2020 13:35:23 +0200
From:   Bean Huo <beanhuo.cn@...il.com>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, beanhuo@...ron.com,
        Richard Weinberger <richard@....at>
Subject: Re: [PATCH RFC] mm: Let readahead submit larger batches of pages
 in case of ra->ra_pages == 0

On Fri, 2020-09-11 at 10:47 +0100, Christoph Hellwig wrote:
> > Hi Andrew
> > Sorry, I am still not quite understanding your above three
> > questions. 
> > 
> > Based on my shallow understanding, ra_pages is associated with
> > read_ahead_kb. Seems ra_pages controls the maximum read-ahead
> > window
> > size, but it doesn't work when the requested size exceeds
> > ra_pages. 
> > 
> > If I set the read_ahead_kb to 0, also, as Christoph mentioned, MTD
> > forcibly sets ra_pages to 0.  I think the intention is that only
> > wants
> > to disable read-ahead, however, doesn't want
> > generic_file_buffered_read() to split the request and read data
> > with
> > 4KB chunk size separately.
> 
> They way I understood Richard this is intentional.

Hi Christoph
Thanks. understood now, MTD expects this result. Even so, I think this
patch doesn't impact MTD because the flash-based FS only achieved the
readpage. Inside __do_page_cache_readahead will use mapping->a_ops-
>readpage to read data.

Thanks,
Bean