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

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

[Fabio M. De Francesco]

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>
---

Thanks to Ira Weiny for his invaluable help and persevering support.
Thanks also to Filipe Manana for identifying a fundamental detail I had overlooked:
https://lore.kernel.org/lkml/20220611093411.GA3779054@falcondesktop/ 

tweed32:/usr/lib/xfstests # ./check -g compress
FSTYP         -- btrfs
PLATFORM      -- Linux/i686 tweed32 5.19.0-rc1-vanilla-debug+ #22 SMP PREEMPT_DYNAMIC Sat Jun 11 14:31:39 CEST 2022
MKFS_OPTIONS  -- /dev/loop1
MOUNT_OPTIONS -- /dev/loop1 /mnt/scratch

btrfs/024 1s ...  0s
btrfs/026 3s ...  3s
btrfs/037 1s ...  1s
btrfs/038 1s ...  0s
btrfs/041 0s ...  1s
btrfs/062 34s ...  35s
btrfs/063 18s ...  18s
btrfs/067 33s ...  32s
btrfs/068 10s ...  10s
btrfs/070       [not run] btrfs and this test needs 5 or more disks in SCRATCH_DEV_POOL
btrfs/071       [not run] btrfs and this test needs 5 or more disks in SCRATCH_DEV_POOL
btrfs/072 33s ...  33s
btrfs/073 17s ...  15s
btrfs/074 36s ...  32s
btrfs/076 0s ...  0s
btrfs/103 1s ...  1s
btrfs/106 1s ...  1s
btrfs/109 1s ...  0s
btrfs/113 0s ...  1s
btrfs/138 43s ...  46s
btrfs/149 1s ...  1s
btrfs/183 1s ...  1s
btrfs/205 1s ...  1s
btrfs/234 3s ...  3s
btrfs/246 0s ...  0s
btrfs/251 1s ...  1s
Ran: btrfs/024 btrfs/026 btrfs/037 btrfs/038 btrfs/041 btrfs/062 btrfs/063 btrfs/067 btrfs/068 btrfs/070 btrfs/071 btrfs/072 btrfs/073 btrfs/074 btrfs/076 btrfs/103 btrfs/106 btrfs/109 btrfs/113 btrfs/138 btrfs/149 btrfs/183 btrfs/205 btrfs/234 btrfs/246 btrfs/251
Not run: btrfs/070 btrfs/071
Passed all 26 tests

 fs/btrfs/zstd.c | 42 +++++++++++++++++++++++-------------------
 1 file changed, 23 insertions(+), 19 deletions(-)

diff --git a/fs/btrfs/zstd.c b/fs/btrfs/zstd.c
index 0fe31a6f6e68..4d50eb3edce5 100644
--- a/fs/btrfs/zstd.c
+++ b/fs/btrfs/zstd.c
@@ -391,6 +391,8 @@ int zstd_compress_pages(struct list_head *ws, struct address_space *mapping,
 	*out_pages = 0;
 	*total_out = 0;
 	*total_in = 0;
+	workspace->in_buf.src = NULL;
+	workspace->out_buf.dst = NULL;
 
 	/* Initialize the stream */
 	stream = zstd_init_cstream(&params, len, workspace->mem,
@@ -403,7 +405,7 @@ int zstd_compress_pages(struct list_head *ws, struct address_space *mapping,
 
 	/* map in the first page of input data */
 	in_page = find_get_page(mapping, start >> PAGE_SHIFT);
-	workspace->in_buf.src = kmap(in_page);
+	workspace->in_buf.src = kmap_local_page(in_page);
 	workspace->in_buf.pos = 0;
 	workspace->in_buf.size = min_t(size_t, len, PAGE_SIZE);
 
@@ -415,7 +417,7 @@ int zstd_compress_pages(struct list_head *ws, struct address_space *mapping,
 		goto out;
 	}
 	pages[nr_pages++] = out_page;
-	workspace->out_buf.dst = kmap(out_page);
+	workspace->out_buf.dst = kmap_local_page(out_page);
 	workspace->out_buf.pos = 0;
 	workspace->out_buf.size = min_t(size_t, max_out, PAGE_SIZE);
 
@@ -450,9 +452,9 @@ int zstd_compress_pages(struct list_head *ws, struct address_space *mapping,
 		if (workspace->out_buf.pos == workspace->out_buf.size) {
 			tot_out += PAGE_SIZE;
 			max_out -= PAGE_SIZE;
-			kunmap(out_page);
+			kunmap_local(workspace->out_buf.dst);
 			if (nr_pages == nr_dest_pages) {
-				out_page = NULL;
+				workspace->out_buf.dst = NULL;
 				ret = -E2BIG;
 				goto out;
 			}
@@ -462,7 +464,7 @@ int zstd_compress_pages(struct list_head *ws, struct address_space *mapping,
 				goto out;
 			}
 			pages[nr_pages++] = out_page;
-			workspace->out_buf.dst = kmap(out_page);
+			workspace->out_buf.dst = kmap_local_page(out_page);
 			workspace->out_buf.pos = 0;
 			workspace->out_buf.size = min_t(size_t, max_out,
 							PAGE_SIZE);
@@ -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);
 			put_page(in_page);
-
 			start += PAGE_SIZE;
 			len -= PAGE_SIZE;
 			in_page = find_get_page(mapping, start >> PAGE_SHIFT);
-			workspace->in_buf.src = kmap(in_page);
+			workspace->in_buf.src = kmap_local_page(in_page);
 			workspace->in_buf.pos = 0;
 			workspace->in_buf.size = min_t(size_t, len, PAGE_SIZE);
+			workspace->out_buf.dst = kmap_local_page(out_page);
 		}
 	}
 	while (1) {
@@ -510,9 +513,9 @@ int zstd_compress_pages(struct list_head *ws, struct address_space *mapping,
 
 		tot_out += PAGE_SIZE;
 		max_out -= PAGE_SIZE;
-		kunmap(out_page);
+		kunmap_local(workspace->out_buf.dst);
 		if (nr_pages == nr_dest_pages) {
-			out_page = NULL;
+			workspace->out_buf.dst = NULL;
 			ret = -E2BIG;
 			goto out;
 		}
@@ -522,7 +525,7 @@ int zstd_compress_pages(struct list_head *ws, struct address_space *mapping,
 			goto out;
 		}
 		pages[nr_pages++] = out_page;
-		workspace->out_buf.dst = kmap(out_page);
+		workspace->out_buf.dst = kmap_local_page(out_page);
 		workspace->out_buf.pos = 0;
 		workspace->out_buf.size = min_t(size_t, max_out, PAGE_SIZE);
 	}
@@ -538,12 +541,12 @@ int zstd_compress_pages(struct list_head *ws, struct address_space *mapping,
 out:
 	*out_pages = nr_pages;
 	/* Cleanup */
-	if (in_page) {
-		kunmap(in_page);
+	if (workspace->out_buf.dst)
+		kunmap_local(workspace->out_buf.dst);
+	if (workspace->in_buf.src) {
+		kunmap_local((void *)workspace->in_buf.src);
 		put_page(in_page);
 	}
-	if (out_page)
-		kunmap(out_page);
 	return ret;
 }
 
@@ -567,7 +570,7 @@ int zstd_decompress_bio(struct list_head *ws, struct compressed_bio *cb)
 		goto done;
 	}
 
-	workspace->in_buf.src = kmap(pages_in[page_in_index]);
+	workspace->in_buf.src = kmap_local_page(pages_in[page_in_index]);
 	workspace->in_buf.pos = 0;
 	workspace->in_buf.size = min_t(size_t, srclen, PAGE_SIZE);
 
@@ -603,14 +606,15 @@ int zstd_decompress_bio(struct list_head *ws, struct compressed_bio *cb)
 			break;
 
 		if (workspace->in_buf.pos == workspace->in_buf.size) {
-			kunmap(pages_in[page_in_index++]);
+			kunmap_local((void *)workspace->in_buf.src);
+			page_in_index++;
 			if (page_in_index >= total_pages_in) {
 				workspace->in_buf.src = NULL;
 				ret = -EIO;
 				goto done;
 			}
 			srclen -= PAGE_SIZE;
-			workspace->in_buf.src = kmap(pages_in[page_in_index]);
+			workspace->in_buf.src = kmap_local_page(pages_in[page_in_index]);
 			workspace->in_buf.pos = 0;
 			workspace->in_buf.size = min_t(size_t, srclen, PAGE_SIZE);
 		}
@@ -619,7 +623,7 @@ int zstd_decompress_bio(struct list_head *ws, struct compressed_bio *cb)
 	zero_fill_bio(cb->orig_bio);
 done:
 	if (workspace->in_buf.src)
-		kunmap(pages_in[page_in_index]);
+		kunmap_local((void *)workspace->in_buf.src);
 	return ret;
 }
 
-- 
2.36.1

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

[David Sterba]

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? 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 and 'const
void*' would work too, then it should be fixed.

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

[Fabio M. De Francesco]

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.
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

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

[Fabio M. De Francesco]

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@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.
> 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

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

[David Sterba]

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.

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

[Fabio M. De Francesco]

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

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

[Ira Weiny]

On Tue, Jun 14, 2022 at 06:28:48PM +0200, Fabio M. De Francesco wrote:
> 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 
> > 

[snip]

> > > 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.

David,

Would you be willing to take this through your tree as a pre-patch to the kmap
changes in btrfs?

That would be easier for Fabio and probably you and Andrew in the long run.

Ira

> 
> Thanks for your review,
> 
> Fabio
> 
> 
> 

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

[Fabio M. De Francesco]

On martedì 14 giugno 2022 19:07:34 CEST Ira Weiny wrote:
> On Tue, Jun 14, 2022 at 06:28:48PM +0200, Fabio M. De Francesco wrote:
> > 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 
> > > 
> 
> [snip]
> 
> > > > 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.
> 
> David,
> 
> Would you be willing to take this through your tree as a pre-patch to the 
kmap
> changes in btrfs?
> 
> That would be easier for Fabio and probably you and Andrew in the long 
run.
> 
> Ira

David,

Please drop the first version of the changes[1] to the API and instead take 
my second version.[2] I've only reworked the commit message (and subject, 
but this was involuntary) to make explicit that the fundamental reason 
behind these changes are semantic correctness.

Thanks,

Fabio

[1] https://lore.kernel.org/lkml/20220614142531.16478-1-fmdefrancesco@gmail.com/

[2] https://lore.kernel.org/lkml/20220615051256.31466-1-fmdefrancesco@gmail.com/

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

[David Sterba]

On Tue, Jun 14, 2022 at 10:07:34AM -0700, Ira Weiny wrote:
> On Tue, Jun 14, 2022 at 06:28:48PM +0200, Fabio M. De Francesco wrote:
> > 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 
> 
> > > > 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.
> 
> David,
> 
> Would you be willing to take this through your tree as a pre-patch to the kmap
> changes in btrfs?
> 
> That would be easier for Fabio and probably you and Andrew in the long run.

Yes, no problem. I could probably send all the kmap conversions after
tests so it does not need to wait until the next cycle.

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

[David Sterba]

On Tue, Jun 14, 2022 at 10:07:34AM -0700, Ira Weiny wrote:
> On Tue, Jun 14, 2022 at 06:28:48PM +0200, Fabio M. De Francesco wrote:
> > 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 
> > > 
> 
> [snip]
> 
> > > > 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.
> 
> David,
> 
> Would you be willing to take this through your tree as a pre-patch to the kmap
> changes in btrfs?
> 
> That would be easier for Fabio and probably you and Andrew in the long run.

Yes, no problem if the patch has an ack. I could probably send all the
kmap conversions after tests so it does not need to wait until the next
cycle.

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

[Wang Yugui]

Hi,

Compiler warning:

fs/btrfs/zstd.c:478:55: warning: passing argument 1 of ‘__kunmap_local’ discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
  478 |                         kunmap_local(workspace->in_buf.src);
./include/linux/highmem-internal.h:284:24: note: in definition of macro ‘kunmap_local’
  284 |         __kunmap_local(__addr);                                 \
      |                        ^~~~~~
./include/linux/highmem-internal.h:200:41: note: expected ‘void *’ but argument is of type ‘const void *’
  200 | static inline void __kunmap_local(void *addr)
      |                                   ~~~~~~^~~~

Best Regards
Wang Yugui (wangyugui@e16-tech.com)
2022/07/14

> 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>
> ---
> 
> Thanks to Ira Weiny for his invaluable help and persevering support.
> Thanks also to Filipe Manana for identifying a fundamental detail I had overlooked:
> https://lore.kernel.org/lkml/20220611093411.GA3779054@falcondesktop/ 
> 
> tweed32:/usr/lib/xfstests # ./check -g compress
> FSTYP         -- btrfs
> PLATFORM      -- Linux/i686 tweed32 5.19.0-rc1-vanilla-debug+ #22 SMP PREEMPT_DYNAMIC Sat Jun 11 14:31:39 CEST 2022
> MKFS_OPTIONS  -- /dev/loop1
> MOUNT_OPTIONS -- /dev/loop1 /mnt/scratch
> 
> btrfs/024 1s ...  0s
> btrfs/026 3s ...  3s
> btrfs/037 1s ...  1s
> btrfs/038 1s ...  0s
> btrfs/041 0s ...  1s
> btrfs/062 34s ...  35s
> btrfs/063 18s ...  18s
> btrfs/067 33s ...  32s
> btrfs/068 10s ...  10s
> btrfs/070       [not run] btrfs and this test needs 5 or more disks in SCRATCH_DEV_POOL
> btrfs/071       [not run] btrfs and this test needs 5 or more disks in SCRATCH_DEV_POOL
> btrfs/072 33s ...  33s
> btrfs/073 17s ...  15s
> btrfs/074 36s ...  32s
> btrfs/076 0s ...  0s
> btrfs/103 1s ...  1s
> btrfs/106 1s ...  1s
> btrfs/109 1s ...  0s
> btrfs/113 0s ...  1s
> btrfs/138 43s ...  46s
> btrfs/149 1s ...  1s
> btrfs/183 1s ...  1s
> btrfs/205 1s ...  1s
> btrfs/234 3s ...  3s
> btrfs/246 0s ...  0s
> btrfs/251 1s ...  1s
> Ran: btrfs/024 btrfs/026 btrfs/037 btrfs/038 btrfs/041 btrfs/062 btrfs/063 btrfs/067 btrfs/068 btrfs/070 btrfs/071 btrfs/072 btrfs/073 btrfs/074 btrfs/076 btrfs/103 btrfs/106 btrfs/109 btrfs/113 btrfs/138 btrfs/149 btrfs/183 btrfs/205 btrfs/234 btrfs/246 btrfs/251
> Not run: btrfs/070 btrfs/071
> Passed all 26 tests
> 
>  fs/btrfs/zstd.c | 42 +++++++++++++++++++++++-------------------
>  1 file changed, 23 insertions(+), 19 deletions(-)
> 
> diff --git a/fs/btrfs/zstd.c b/fs/btrfs/zstd.c
> index 0fe31a6f6e68..4d50eb3edce5 100644
> --- a/fs/btrfs/zstd.c
> +++ b/fs/btrfs/zstd.c
> @@ -391,6 +391,8 @@ int zstd_compress_pages(struct list_head *ws, struct address_space *mapping,
>  	*out_pages = 0;
>  	*total_out = 0;
>  	*total_in = 0;
> +	workspace->in_buf.src = NULL;
> +	workspace->out_buf.dst = NULL;
>  
>  	/* Initialize the stream */
>  	stream = zstd_init_cstream(&params, len, workspace->mem,
> @@ -403,7 +405,7 @@ int zstd_compress_pages(struct list_head *ws, struct address_space *mapping,
>  
>  	/* map in the first page of input data */
>  	in_page = find_get_page(mapping, start >> PAGE_SHIFT);
> -	workspace->in_buf.src = kmap(in_page);
> +	workspace->in_buf.src = kmap_local_page(in_page);
>  	workspace->in_buf.pos = 0;
>  	workspace->in_buf.size = min_t(size_t, len, PAGE_SIZE);
>  
> @@ -415,7 +417,7 @@ int zstd_compress_pages(struct list_head *ws, struct address_space *mapping,
>  		goto out;
>  	}
>  	pages[nr_pages++] = out_page;
> -	workspace->out_buf.dst = kmap(out_page);
> +	workspace->out_buf.dst = kmap_local_page(out_page);
>  	workspace->out_buf.pos = 0;
>  	workspace->out_buf.size = min_t(size_t, max_out, PAGE_SIZE);
>  
> @@ -450,9 +452,9 @@ int zstd_compress_pages(struct list_head *ws, struct address_space *mapping,
>  		if (workspace->out_buf.pos == workspace->out_buf.size) {
>  			tot_out += PAGE_SIZE;
>  			max_out -= PAGE_SIZE;
> -			kunmap(out_page);
> +			kunmap_local(workspace->out_buf.dst);
>  			if (nr_pages == nr_dest_pages) {
> -				out_page = NULL;
> +				workspace->out_buf.dst = NULL;
>  				ret = -E2BIG;
>  				goto out;
>  			}
> @@ -462,7 +464,7 @@ int zstd_compress_pages(struct list_head *ws, struct address_space *mapping,
>  				goto out;
>  			}
>  			pages[nr_pages++] = out_page;
> -			workspace->out_buf.dst = kmap(out_page);
> +			workspace->out_buf.dst = kmap_local_page(out_page);
>  			workspace->out_buf.pos = 0;
>  			workspace->out_buf.size = min_t(size_t, max_out,
>  							PAGE_SIZE);
> @@ -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);
>  			put_page(in_page);
> -
>  			start += PAGE_SIZE;
>  			len -= PAGE_SIZE;
>  			in_page = find_get_page(mapping, start >> PAGE_SHIFT);
> -			workspace->in_buf.src = kmap(in_page);
> +			workspace->in_buf.src = kmap_local_page(in_page);
>  			workspace->in_buf.pos = 0;
>  			workspace->in_buf.size = min_t(size_t, len, PAGE_SIZE);
> +			workspace->out_buf.dst = kmap_local_page(out_page);
>  		}
>  	}
>  	while (1) {
> @@ -510,9 +513,9 @@ int zstd_compress_pages(struct list_head *ws, struct address_space *mapping,
>  
>  		tot_out += PAGE_SIZE;
>  		max_out -= PAGE_SIZE;
> -		kunmap(out_page);
> +		kunmap_local(workspace->out_buf.dst);
>  		if (nr_pages == nr_dest_pages) {
> -			out_page = NULL;
> +			workspace->out_buf.dst = NULL;
>  			ret = -E2BIG;
>  			goto out;
>  		}
> @@ -522,7 +525,7 @@ int zstd_compress_pages(struct list_head *ws, struct address_space *mapping,
>  			goto out;
>  		}
>  		pages[nr_pages++] = out_page;
> -		workspace->out_buf.dst = kmap(out_page);
> +		workspace->out_buf.dst = kmap_local_page(out_page);
>  		workspace->out_buf.pos = 0;
>  		workspace->out_buf.size = min_t(size_t, max_out, PAGE_SIZE);
>  	}
> @@ -538,12 +541,12 @@ int zstd_compress_pages(struct list_head *ws, struct address_space *mapping,
>  out:
>  	*out_pages = nr_pages;
>  	/* Cleanup */
> -	if (in_page) {
> -		kunmap(in_page);
> +	if (workspace->out_buf.dst)
> +		kunmap_local(workspace->out_buf.dst);
> +	if (workspace->in_buf.src) {
> +		kunmap_local((void *)workspace->in_buf.src);
>  		put_page(in_page);
>  	}
> -	if (out_page)
> -		kunmap(out_page);
>  	return ret;
>  }
>  
> @@ -567,7 +570,7 @@ int zstd_decompress_bio(struct list_head *ws, struct compressed_bio *cb)
>  		goto done;
>  	}
>  
> -	workspace->in_buf.src = kmap(pages_in[page_in_index]);
> +	workspace->in_buf.src = kmap_local_page(pages_in[page_in_index]);
>  	workspace->in_buf.pos = 0;
>  	workspace->in_buf.size = min_t(size_t, srclen, PAGE_SIZE);
>  
> @@ -603,14 +606,15 @@ int zstd_decompress_bio(struct list_head *ws, struct compressed_bio *cb)
>  			break;
>  
>  		if (workspace->in_buf.pos == workspace->in_buf.size) {
> -			kunmap(pages_in[page_in_index++]);
> +			kunmap_local((void *)workspace->in_buf.src);
> +			page_in_index++;
>  			if (page_in_index >= total_pages_in) {
>  				workspace->in_buf.src = NULL;
>  				ret = -EIO;
>  				goto done;
>  			}
>  			srclen -= PAGE_SIZE;
> -			workspace->in_buf.src = kmap(pages_in[page_in_index]);
> +			workspace->in_buf.src = kmap_local_page(pages_in[page_in_index]);
>  			workspace->in_buf.pos = 0;
>  			workspace->in_buf.size = min_t(size_t, srclen, PAGE_SIZE);
>  		}
> @@ -619,7 +623,7 @@ int zstd_decompress_bio(struct list_head *ws, struct compressed_bio *cb)
>  	zero_fill_bio(cb->orig_bio);
>  done:
>  	if (workspace->in_buf.src)
> -		kunmap(pages_in[page_in_index]);
> +		kunmap_local((void *)workspace->in_buf.src);
>  	return ret;
>  }
>  
> -- 
> 2.36.1

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

[Fabio M. De Francesco]

On giovedì 14 luglio 2022 02:25:20 CEST Wang Yugui wrote:
> Hi,
> 
> Compiler warning:
> 
> fs/btrfs/zstd.c:478:55: warning: passing argument 1 of ‘__kunmap_local’ 
discards ‘const’ qualifier from pointer target type [-Wdiscarded-
qualifiers]
>   478 |                         kunmap_local(workspace->in_buf.src);
> ./include/linux/highmem-internal.h:284:24: note: in definition of macro 
‘kunmap_local’
>   284 |         __kunmap_local(__addr);                                 \
>       |                        ^~~~~~
> ./include/linux/highmem-internal.h:200:41: note: expected ‘void *’ but 
argument is of type ‘const void *’
>   200 | static inline void __kunmap_local(void *addr)
>       |                                   ~~~~~~^~~~
> 
> Best Regards
> Wang Yugui (wangyugui@e16-tech.com)
> 2022/07/14

Thanks for the build test, but you're a little late :)

This patch is superseded by a series of two: please see
[PATCH v6 0/2] btrfs: Replace kmap() with kmap_local_page() in zstd.c
at https://lore.kernel.org/lkml/20220706111520.12858-1-fmdefrancesco@gmail.com/

In that series, patch 1/2 changes __kunmap_{local,atomic}() prototypes to 
take pointers to const void.

Regards,

Fabio

> > 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.
> > 

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

[David Sterba]

On Thu, Jul 14, 2022 at 08:25:20AM +0800, Wang Yugui wrote:
> Hi,
> 
> Compiler warning:
> 
> fs/btrfs/zstd.c:478:55: warning: passing argument 1 of ‘__kunmap_local’ discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
>   478 |                         kunmap_local(workspace->in_buf.src);
> ./include/linux/highmem-internal.h:284:24: note: in definition of macro ‘kunmap_local’
>   284 |         __kunmap_local(__addr);                                 \
>       |                        ^~~~~~
> ./include/linux/highmem-internal.h:200:41: note: expected ‘void *’ but argument is of type ‘const void *’
>   200 | static inline void __kunmap_local(void *addr)
>       |                                   ~~~~~~^~~~

This requires patch
https://lore.kernel.org/all/20220706111520.12858-2-fmdefrancesco@gmail.com/
that adds const to kmap/kunmap API.