linux-kernel - Re: [PATCH 1/4] fs/btrfs: Convert kmap to kmap_local

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Date:   Tue, 16 Mar 2021 12:04:43 +0100
From:   David Sterba <dsterba@...e.cz>
To:     Ira Weiny <ira.weiny@...el.com>
Cc:     dsterba@...e.cz, Chris Mason <clm@...com>,
        Josef Bacik <josef@...icpanda.com>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 1/4] fs/btrfs: Convert kmap to kmap_local_page() using
 coccinelle

On Fri, Mar 12, 2021 at 12:03:14PM -0800, Ira Weiny wrote:
> On Fri, Mar 12, 2021 at 07:58:39PM +0100, David Sterba wrote:
> > On Tue, Feb 16, 2021 at 06:48:23PM -0800, ira.weiny@...el.com wrote:
> > > --- a/fs/btrfs/lzo.c
> > > +++ b/fs/btrfs/lzo.c
> > > @@ -118,7 +118,7 @@ int lzo_compress_pages(struct list_head *ws, struct address_space *mapping,
> > >  	struct workspace *workspace = list_entry(ws, struct workspace, list);
> > >  	int ret = 0;
> > >  	char *data_in;
> > > -	char *cpage_out;
> > > +	char *cpage_out, *sizes_ptr;
> > >  	int nr_pages = 0;
> > >  	struct page *in_page = NULL;
> > >  	struct page *out_page = NULL;
> > > @@ -258,10 +258,9 @@ int lzo_compress_pages(struct list_head *ws, struct address_space *mapping,
> > >  	}
> > >  
> > >  	/* store the size of all chunks of compressed data */
> > > -	cpage_out = kmap(pages[0]);
> > > -	write_compress_length(cpage_out, tot_out);
> > > -
> > > -	kunmap(pages[0]);
> > > +	sizes_ptr = kmap_local_page(pages[0]);
> > > +	write_compress_length(sizes_ptr, tot_out);
> > > +	kunmap_local(sizes_ptr);
> > 
> > Why is not cpage_out reused for this mapping? I don't see any reason for
> > another temporary variable, cpage_out is not used at this point.
> 
> For this patch that is true.  However, I'm trying to convert the other kmaps as
> well.  To do that I'll need cpage_out preserved for the final kunmap_local().
> 
> Unfortunately, the required nesting ordering of kmap_local_page() makes
> converting the other calls hacky at best.  I'm not sure what to do about them.
> The best I've come up with is doing a hacky extra unmap/remap to preserve the
> nesting.
> 
> Anyway, I'd prefer to leave this additional temp variable but I can certainly
> change if it you want.  The other conversions may never work/land.  :-/

Ok, no problem keeping the variable then. I've added a note to changelog
why it's there. The whole conversion sounds tricky so adding trivial
helper code is no big deal.