Re: [PATCH] [RFC] gfs2: add flag REQ_PRIO for metadata read ahead

[Eric Wheeler <bcache@lists.ewheeler.net>]

On Tue, 11 Jul 2017, Coly Li wrote:

> When gfs2 does metadata read ahead, currently flags (REQ_RAHEAD | REQ_META)
> are used to submit bio. Flag REQ_META is just a hint for block trace, not
> for block layer code to handle a bio as metadata request.
> 
> When doing read ahead for metadata, A REQ_PRIO flag on the metadata bio
> is very informative to block layer code. For example, if bcache is used as
> a I/O cache for gfs2, it will be possible for bcache code to cache the
> pre-fetched metadata blocks on cache device as well, which may be
> probably to improve metadata I/O performance if the following requests
> hit the cache.
> 
> This patch adds REQ_PRIO flag when submitting a metadata readahead bio.
> A meta data read ahead bio may come from I/O requests for bitmap,
> directoriesmeta or other general metadata request.
> 

Are there any places in gfs2 where REQ_PRIO should be placed on 
latency-sensitive metadata writes?  They would then writeback in bcache 
after the relevant bcache patch is merged.

--
Eric Wheeler


> Signed-off-by: Coly Li <colyli@suse.de>
> ---
>  fs/gfs2/bmap.c    | 5 +++--
>  fs/gfs2/dir.c     | 4 +++-
>  fs/gfs2/meta_io.c | 4 +++-
>  3 files changed, 9 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/gfs2/bmap.c b/fs/gfs2/bmap.c
> index 9fa3aef9a5b3..fa3ea29f39cf 100644
> --- a/fs/gfs2/bmap.c
> +++ b/fs/gfs2/bmap.c
> @@ -291,8 +291,9 @@ static void gfs2_metapath_ra(struct gfs2_glock *gl,
>  		if (trylock_buffer(rabh)) {
>  			if (!buffer_uptodate(rabh)) {
>  				rabh->b_end_io = end_buffer_read_sync;
> -				submit_bh(REQ_OP_READ, REQ_RAHEAD | REQ_META,
> -						rabh);
> +				submit_bh(REQ_OP_READ,
> +					  REQ_RAHEAD | REQ_META | REQ_PRIO,
> +					  rabh);
>  				continue;
>  			}
>  			unlock_buffer(rabh);
> diff --git a/fs/gfs2/dir.c b/fs/gfs2/dir.c
> index db427658ccd9..0741e4018f8c 100644
> --- a/fs/gfs2/dir.c
> +++ b/fs/gfs2/dir.c
> @@ -1514,7 +1514,9 @@ static void gfs2_dir_readahead(struct inode *inode, unsigned hsize, u32 index,
>  				continue;
>  			}
>  			bh->b_end_io = end_buffer_read_sync;
> -			submit_bh(REQ_OP_READ, REQ_RAHEAD | REQ_META, bh);
> +			submit_bh(REQ_OP_READ,
> +				  REQ_RAHEAD | REQ_META | REQ_PRIO,
> +				  bh);
>  			continue;
>  		}
>  		brelse(bh);
> diff --git a/fs/gfs2/meta_io.c b/fs/gfs2/meta_io.c
> index fabe1614f879..6103d1c816ef 100644
> --- a/fs/gfs2/meta_io.c
> +++ b/fs/gfs2/meta_io.c
> @@ -461,7 +461,9 @@ struct buffer_head *gfs2_meta_ra(struct gfs2_glock *gl, u64 dblock, u32 extlen)
>  		bh = gfs2_getbuf(gl, dblock, CREATE);
>  
>  		if (!buffer_uptodate(bh) && !buffer_locked(bh))
> -			ll_rw_block(REQ_OP_READ, REQ_RAHEAD | REQ_META, 1, &bh);
> +			ll_rw_block(REQ_OP_READ,
> +				    REQ_RAHEAD | REQ_META | REQ_PRIO,
> +				    1, &bh);
>  		brelse(bh);
>  		dblock++;
>  		extlen--;
> -- 
> 2.12.0
> 
> --
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