linux-kernel - Re: [RFC 2/4] fs: Add IOCB_NOIO flag for generic_file_read

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Date:   Thu, 2 Jul 2020 21:58:20 +0200
From:   Andreas Gruenbacher <agruenba@...hat.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Dave Chinner <david@...morbit.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC 2/4] fs: Add IOCB_NOIO flag for generic_file_read_iter

On Thu, Jul 2, 2020 at 8:06 PM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Thu, Jul 2, 2020 at 9:51 AM Andreas Gruenbacher <agruenba@...hat.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.
>
> Of course, if you want to avoid both new reads to be submitted _and_
> avoid waiting for existing pending reads, you should just set both
> flags, and you get the semantics you want. So for your case, this may
> not make any difference.

Indeed, in the gfs2 case, waiting for existing pending reads should be
fine. I'll send an update after some testing.

Thanks,
Andreas