linux-kernel - Re: [PATCH] btrfs: Replace kmap() with kmap_local

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Message-ID: <20220614142521.GN20633@twin.jikos.cz>
Date:   Tue, 14 Jun 2022 16:25:21 +0200
From:   David Sterba <dsterba@...e.cz>
To:     "Fabio M. De Francesco" <fmdefrancesco@...il.com>
Cc:     dsterba@...e.cz, Chris Mason <clm@...com>,
        Josef Bacik <josef@...icpanda.com>,
        David Sterba <dsterba@...e.com>,
        Nick Terrell <terrelln@...com>, linux-btrfs@...r.kernel.org,
        linux-kernel@...r.kernel.org, Filipe Manana <fdmanana@...nel.org>,
        Ira Weiny <ira.weiny@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] btrfs: Replace kmap() with kmap_local_page() in zstd.c

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@...nel.org>
> > > Suggested-by: Ira Weiny <ira.weiny@...el.com>
> > > Signed-off-by: Fabio M. De Francesco <fmdefrancesco@...il.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.