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Message-ID: <CAHc6FU7HvPqpBSRQJfbgBgQpjFzYfWfTwq+F-tOUq+-Jc8efQg@mail.gmail.com>
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
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