linux-kernel - Re: [RFC PATCH 0/7] mm: Make more use of readahead

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Message-ID: <20200901223346.GV14765@casper.infradead.org>
Date:   Tue, 1 Sep 2020 23:33:46 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     David Howells <dhowells@...hat.com>
Cc:     linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 0/7] mm: Make more use of readahead_control

On Tue, Sep 01, 2020 at 08:44:14PM +0100, David Howells wrote:
> Matthew Wilcox <willy@...radead.org> wrote:
> 
> > > Note that I've been
> > > passing the number of pages to read in rac->_nr_pages, and then saving it
> > > and changing it certain points, e.g. page_cache_readahead_unbounded().
> > 
> > I do not like this.  You're essentially mutating the meaning of _nr_pages
> > as the ractl moves down the stack, and that's going to lead to bugs.
> > I'd keep it as a separate argument.
> 
> The meaning isn't specified in linux/pagemap.h.  Can you adjust the comments
> on the struct and on readahead_count() to make it more clear what the value
> represents?

It's intentionally not documented.  This documentation is for the
filesystem author, who should not be looking at it.  Neither should
you :-P