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

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Date:   Fri, 3 Jul 2020 12:41:08 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Andreas Gruenbacher <agruenba@...hat.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Dave Chinner <david@...morbit.com>,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC v2 1/2] fs: Add IOCB_NOIO flag for generic_file_read_iter

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.

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