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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.
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