Re: [PATCH] btrfs: Replace kmap() with kmap_local_page() in zstd.c

["Fabio M. De Francesco" <fmdefrancesco@gmail.com>]

On martedì 14 giugno 2022 16:25:21 CEST David Sterba wrote:
> On Tue, Jun 14, 2022 at 01:22:50AM +0200, Fabio M. De Francesco wrote:
> > On lunedì 13 giugno 2022 20:39:13 CEST David Sterba wrote:
> > > On Sat, Jun 11, 2022 at 03:52:03PM +0200, Fabio M. De Francesco 
wrote:
> > > > The use of kmap() is being deprecated in favor of 
kmap_local_page(). 
> > With
> > > > kmap_local_page(), the mapping is per thread, CPU local and not 
> > globally
> > > > visible.
> > > > 
> > > > Therefore, use kmap_local_page() / kunmap_local() in zstd.c because 
in
> > > > this file the mappings are per thread and are not visible in other
> > > > contexts; meanwhile refactor zstd_compress_pages() to comply with 
> > nested
> > > > local mapping / unmapping ordering rules.
> > > > 
> > > > Tested with xfstests on a QEMU + KVM 32 bits VM with 4GB of RAM and
> > > > HIGHMEM64G enabled.
> > > > 
> > > > Cc: Filipe Manana <fdmanana@kernel.org>
> > > > Suggested-by: Ira Weiny <ira.weiny@intel.com>
> > > > Signed-off-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>
> > > > ---
> > > > 
> > > > @@ -477,15 +479,16 @@ int zstd_compress_pages(struct list_head *ws, 
> > struct address_space *mapping,
> > > >  		/* Check if we need more input */
> > > >  		if (workspace->in_buf.pos == workspace->in_buf.size) {
> > > >  			tot_in += PAGE_SIZE;
> > > > -			kunmap(in_page);
> > > > +			kunmap_local(workspace->out_buf.dst);
> > > > +			kunmap_local((void *)workspace->in_buf.src);
> > > 
> > > Why is the cast needed?
> > 
> > As I wrote in an email I sent some days ago ("[RFC PATCH] btrfs: 
Replace 
> > kmap() with kmap_local_page() in zstd.c")[1] I get a series of errors 
like 
> > the following:
> > 
> > /usr/src/git/kernels/linux/fs/btrfs/zstd.c:547:33: warning: passing 
> > argument 1 of '__kunmap_local' discards 'const' qualifier from pointer 
> > target type [-Wdiscarded-qualifiers]
> >   547 |   kunmap_local(workspace->in_buf.src);
> >       |                ~~~~~~~~~~~~~~~~~^~~~
> > /usr/src/git/kernels/linux/include/linux/highmem-internal.h:284:17: 
note: 
> > in definition of macro 'kunmap_local'
> >   284 |  __kunmap_local(__addr);     \
> >       |                 ^~~~~~
> > /usr/src/git/kernels/linux/include/linux/highmem-internal.h:92:41: 
note: 
> > expected 'void *' but argument is of type 'const void *'
> >    92 | static inline void __kunmap_local(void *vaddr)
> >       |                                   ~~~~~~^~~~~
> > 
> > Therefore, this is a (bad?) hack to make these changes compile.
> 
> I think it's a bad practice and that API that does not modify parameters
> should declare the pointers const. Type casts should be used in
> justified cases and not to paper over fixable issues.
> 
> > A better solution is changing the prototype of __kunmap_local(); I
> > suppose that Andrew won't object, but who knows?
> > 
> > (+Cc Andrew Morton).
> > 
> > I was waiting for your comments. At now I've done about 15 conversions 
> > across the kernel but it's the first time I had to pass a pointer to 
const 
> > void to kunmap_local(). Therefore, I was not sure if changing the API 
were 
> > better suited (however I have already discussed this with Ira).
> 
> IMHO it should be fixed in the API.
> 
I agree with you in full.

At the same time when you sent this email I submitted a patch to change 
kunmap_local() and kunmap_atomic().

After Andrew takes them I'll send v2 of this patch to zstd.c without those 
unnecessary casts.

Thanks for your review,

Fabio