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SubjectRe: [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@intel.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.

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