linux-kernel - Re: [RFC] Bypass filesystems for reading cached pages

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Message-ID: <CAHc6FU7R2vMZ9+aXLsQ+ubECbfrBTR+yh03b_T++PRxd479vsQ@mail.gmail.com>
Date:   Wed, 24 Jun 2020 14:35:40 +0200
From:   Andreas Gruenbacher <agruenba@...hat.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     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] Bypass filesystems for reading cached pages

On Mon, Jun 22, 2020 at 8:13 PM Matthew Wilcox <willy@...radead.org> wrote:
> On Mon, Jun 22, 2020 at 04:35:05PM +0200, Andreas Gruenbacher wrote:
> > I'm fine with not moving that functionality into the VFS. The problem
> > I have in gfs2 is that taking glocks is really expensive. Part of that
> > overhead is accidental, but we definitely won't be able to fix it in
> > the short term. So something like the IOCB_CACHED flag that prevents
> > generic_file_read_iter from issuing readahead I/O would save the day
> > for us. Does that idea stand a chance?
>
> For the short-term fix, is switching to a trylock in gfs2_readahead()
> acceptable?

Well, it's the only thing we can do for now, right?

> diff --git a/fs/gfs2/aops.c b/fs/gfs2/aops.c
> index 72c9560f4467..6ccd478c81ff 100644
> --- a/fs/gfs2/aops.c
> +++ b/fs/gfs2/aops.c
> @@ -600,7 +600,7 @@ static void gfs2_readahead(struct readahead_control *rac)
>         struct gfs2_inode *ip = GFS2_I(inode);
>         struct gfs2_holder gh;
>
> -       gfs2_holder_init(ip->i_gl, LM_ST_SHARED, 0, &gh);
> +       gfs2_holder_init(ip->i_gl, LM_ST_SHARED, LM_FLAG_TRY, &gh);
>         if (gfs2_glock_nq(&gh))
>                 goto out_uninit;
>         if (!gfs2_is_stuffed(ip))

Thanks,
Andreas