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@xxxxxxxxx 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.
