Re: [RFC 2/4] fs: Add IOCB_NOIO flag for generic_file_read_iter

From: Andreas Gruenbacher <agruenba@redhat.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Matthew Wilcox <willy@infradead.org>,
	Dave Chinner <david@fromorbit.com>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	Linux-MM <linux-mm@kvack.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [RFC 2/4] fs: Add IOCB_NOIO flag for generic_file_read_iter
Date: Tue, 7 Jul 2020 16:30:19 +0200	[thread overview]
Message-ID: <CAHc6FU6LmR7m_8UHmB_77jUpYNo-kgCZ-1YTLqya-PPqvvBy7Q@mail.gmail.com> (raw)
In-Reply-To: <CAHk-=wgpsuC6ejzr3pn5ej5Yn5z4xthNUUOvmA7KXHHGynL15Q@mail.gmail.com>

On Thu, Jul 2, 2020 at 8:06 PM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
> On Thu, Jul 2, 2020 at 9:51 AM Andreas Gruenbacher <agruenba@redhat.com> 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.
>
> This looks sane to me, except for this part:
> >                 if (!PageUptodate(page)) {
> > -                       if (iocb->ki_flags & IOCB_NOWAIT) {
> > +                       if (iocb->ki_flags & (IOCB_NOWAIT | IOCB_NOIO)) {
> >                                 put_page(page);
> >                                 goto would_block;
> >                         }
>
> This path doesn't actually initiate reads at all - it waits for
> existing reads to finish.
>
> So I think it should only check for IOCB_NOWAIT.

It turns out that label readpage is reachable from here via goto
page_not_up_to_date / goto page_not_up_to_date_locked. So IOCB_NOIO
needs to be checked somewhere. I'll send an update.

Andreas