linux-kernel - Re: [RFC v2 1/2] fs: Add IOCB_NOIO flag for generic_file_read

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Date:   Sun, 5 Jul 2020 17:08:21 +0200
From:   Andreas Gruenbacher <agruenba@...hat.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Dave Chinner <david@...morbit.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC v2 1/2] fs: Add IOCB_NOIO flag for generic_file_read_iter

On Fri, Jul 3, 2020 at 1:41 PM Matthew Wilcox <willy@...radead.org> wrote:
>
> On Fri, Jul 03, 2020 at 11:53:24AM +0200, Andreas Gruenbacher wrote:
> > Add an IOCB_NOIO flag that indicates to generic_file_read_iter that it
> > shouldn't trigger any filesystem I/O for the actual request or for
> > readahead.  This allows to do tentative reads out of the page cache as
> > some filesystems allow, and to take the appropriate locks and retry the
> > reads only if the requested pages are not cached.
> >
> > Signed-off-by: Andreas Gruenbacher <agruenba@...hat.com>
>
> Reviewed-by: Matthew Wilcox (Oracle) <willy@...radead.org>
>
> > @@ -2249,9 +2253,18 @@ EXPORT_SYMBOL_GPL(generic_file_buffered_read);
> >   *
> >   * This is the "read_iter()" routine for all filesystems
> >   * that can use the page cache directly.
> > + *
> > + * The IOCB_NOWAIT flag in iocb->ki_flags indicates that -EAGAIN shall
> > + * be returned when no data can be read without waiting for I/O requests
> > + * to complete; it doesn't prevent readahead.
> > + *
> > + * The IOCB_NOIO flag in iocb->ki_flags indicates that -EAGAIN shall be
> > + * returned when no data can be read without issuing new I/O requests,
> > + * and 0 shall be returned when readhead would have been triggered.
>
> s/shall/may/ -- if we read a previous page then hit a readahead page,
> we'll return a positive value.  If the first page we hit is a readahead
> page, then yes, we'll return zero.

How about this?

 * The IOCB_NOIO flag in iocb->ki_flags indicates that no new I/O
 * requests shall be made for the read or for readahead.  When no data
 * can be read, -EAGAIN shall be returned.  When readahead would be
 * triggered, a short read (possibly of length 0) shall be returned.

> Again, I'm happy for the patch to go in as-is without this nitpick.

Thanks,
Andreas