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Message-ID: <11997068.O9o76ZdvQC@opensuse>
Date: Tue, 14 Jun 2022 01:42:06 +0200
From: "Fabio M. De Francesco" <fmdefrancesco@...il.com>
To: dsterba@...e.cz
Cc: 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 martedì 14 giugno 2022 01:22:50 CEST 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.
> 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).
>
> > I see that it leads to a warning but we pass a
> > const buffer and that breaks the API contract as in kunmap it would be
> > accessed as non-const and potentially changed without warning or
> > compiler error. If kunmap_local does not touch the buffer
>
> Yes, correct, kunmap_local() does _not_ touch the buffer.
>
> > and 'const
> > void*' would work too, then it should be fixed.
>
> I'll send an RFC patch for changing __kunmap_local() and the other
> functions of the calls chain down to kunmap_local_indexed().
> Furthermore, changes to kunmap_local_indexed() prototype require also
> changes to __kunmap_atomic() (if I recall correctly...).
>
> Thanks for your review,
>
> Fabio
>
Sorry, I forgot to paste a link:
[1] https://lore.kernel.org/lkml/20220611020451.28170-1-fmdefrancesco@gmail.com/
Fabio
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