Re: [RFC] Bypass filesystems for reading cached pages

[Matthew Wilcox <willy@infradead.org>]

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?

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))