linux-kernel - Re: [PATCH v6 08/19] mm: Add readahead address space operation

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Date:   Wed, 19 Feb 2020 08:52:41 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     Eric Biggers <ebiggers@...nel.org>
Cc:     linux-fsdevel@...r.kernel.org, linux-xfs@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net, cluster-devel@...hat.com,
        linux-mm@...ck.org, ocfs2-devel@....oracle.com,
        linux-ext4@...r.kernel.org, linux-erofs@...ts.ozlabs.org,
        linux-btrfs@...r.kernel.org
Subject: Re: [PATCH v6 08/19] mm: Add readahead address space operation

On Tue, Feb 18, 2020 at 07:10:44PM -0800, Eric Biggers wrote:
> > +``readahead``
> > +	Called by the VM to read pages associated with the address_space
> > +	object.  The pages are consecutive in the page cache and are
> > +	locked.  The implementation should decrement the page refcount
> > +	after starting I/O on each page.  Usually the page will be
> > +	unlocked by the I/O completion handler.  If the function does
> > +	not attempt I/O on some pages, the caller will decrement the page
> > +	refcount and unlock the pages for you.	Set PageUptodate if the
> > +	I/O completes successfully.  Setting PageError on any page will
> > +	be ignored; simply unlock the page if an I/O error occurs.
> > +
> 
> This is unclear about how "not attempting I/O" works and how that affects who is
> responsible for putting and unlocking the pages.  How does the caller know which
> pages were not attempted?  Can any arbitrary subset of pages be not attempted,
> or just the last N pages?

Changed to:

``readahead``
        Called by the VM to read pages associated with the address_space
        object.  The pages are consecutive in the page cache and are
        locked.  The implementation should decrement the page refcount
        after starting I/O on each page.  Usually the page will be
        unlocked by the I/O completion handler.  If the filesystem decides
        to stop attempting I/O before reaching the end of the readahead
        window, it can simply return.  The caller will decrement the page
        refcount and unlock the remaining pages for you.  Set PageUptodate
        if the I/O completes successfully.  Setting PageError on any page
        will be ignored; simply unlock the page if an I/O error occurs.