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Re: [dm-devel] [PATCH 1/7] dm-bufio: return success or failure on prefetch




On Mon, 13 Jan 2014, Mike Snitzer wrote:

> On Sat, Jan 11 2014 at 12:29pm -0500,
> Mikulas Patocka <mpatocka redhat com> wrote:
> 
> > This patch makes dm_bufio_prefetch return 1 on success and 0 on failure.
> > A prefetch failure may happen in case of temporary memory shortage.
> > 
> > Signed-off-by: Mikulas Patocka <mpatocka redhat com>
> > 
> > ---
> >  drivers/md/dm-bufio.c |    8 ++++++--
> >  drivers/md/dm-bufio.h |    4 ++--
> >  2 files changed, 8 insertions(+), 4 deletions(-)
> > 
> > Index: linux-3.13-rc7/drivers/md/dm-bufio.c
> > ===================================================================
> > --- linux-3.13-rc7.orig/drivers/md/dm-bufio.c	2014-01-09 16:11:31.000000000 +0100
> > +++ linux-3.13-rc7/drivers/md/dm-bufio.c	2014-01-09 16:16:43.000000000 +0100
> > @@ -1068,10 +1068,11 @@ void *dm_bufio_new(struct dm_bufio_clien
> >  }
> >  EXPORT_SYMBOL_GPL(dm_bufio_new);
> >  
> > -void dm_bufio_prefetch(struct dm_bufio_client *c,
> > +int dm_bufio_prefetch(struct dm_bufio_client *c,
> >  		       sector_t block, unsigned n_blocks)
> >  {
> >  	struct blk_plug plug;
> > +	int success = 1;
> >  
> >  	LIST_HEAD(write_list);
> >  
> 
> I'd prefer to see a bool used here.  Or an actual error code like
> -ENOMEM.
> 
> > @@ -1104,13 +1105,16 @@ void dm_bufio_prefetch(struct dm_bufio_c
> >  			if (!n_blocks)
> >  				goto flush_plug;
> >  			dm_bufio_lock(c);
> > -		}
> > +		} else
> > +			success = 0;
> 
> Why do you continue to do work if a prefetch failed?  Why not return
> early here?

I realized that the patch for the return values was wrong (it would return 
0 if some buffers were already prefetched).

And it's not really needed, if dm_bufio_prefetch doesn't do prefetch 
because of memory shortage, we can do nothing as dm_bufio_read will read 
that buffer anyway.

So, I am sending next 6-patch series, it's the same without the first 
patch.

Mikulas


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