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

[Fabio M. De Francesco]

On sabato 11 giugno 2022 11:34:11 CEST Filipe Manana wrote:
> On Sat, Jun 11, 2022 at 04:04:51AM +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 (./check -g compress) on QEMU + KVM 32 bits VM 
with
> > 4GB of RAM and HIGHMEM64G enabled.
> > 
> > Suggested-by: Ira Weiny <ira.weiny@xxxxxxxxx>
> > Signed-off-by: Fabio M. De Francesco <fmdefrancesco@xxxxxxxxx>
> > ---
> > 
> > This is an RFC PATCH because it actually passes all xfstests of group
> > "compress" with the only exception of tests/btrfs/138.
> > 
> > Since I am relatively new to kernel development and know very little 
about
> > fs/btrfs design and code, I would like to ask for the help from anyone 
who
> > knows this filesystem and xfstests better than me.
> > 
> > Can anyone please help me figure out what's wrong and how to fix it?
> > 
> > Please note that there is some discussion for changing 
__kunmap_local().
> > For now I had to cast workspace->in_buf.src to pointer to void,
> > otherwise GCC-12 complains with a series of messages 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)
> >       |                                   ~~~~~~^~~~~
> >  
> > This is what I get from running xfstests of "compress" group...
> > 
> > tweed32:/usr/lib/xfstests # ./check -g compress
> > FSTYP         -- btrfs
> > PLATFORM      -- Linux/i686 tweed32 5.19.0-rc1-vanilla-debug+ #20 SMP 
PREEMPT_DYNAMIC Fri Jun 10 14:15:51 CEST 2022
> > MKFS_OPTIONS  -- /dev/loop1
> > MOUNT_OPTIONS -- /dev/loop1 /mnt/scratch
> > 
> > btrfs/024 0s ...  0s
> > btrfs/026 3s ...  3s
> > btrfs/037 1s ...  1s
> > btrfs/038 0s ...  1s
> > btrfs/041 1s ...  0s
> > btrfs/062 34s ...  34s
> > btrfs/063 18s ...  18s
> > btrfs/067 32s ...  30s
> > 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 34s ...  34s
> > btrfs/073 13s ...  17s
> > btrfs/074 35s ...  33s
> > btrfs/076 0s ...  1s
> > btrfs/103 1s ...  0s
> > btrfs/106 0s ...  1s
> > btrfs/109 0s ...  0s
> > btrfs/113 1s ...  0s
> > btrfs/138 43s ... - output mismatch (see /usr/lib/xfstests/results//
btrfs/138.out.bad)
> >     --- tests/btrfs/138.out     2022-05-11 04:02:17.000000000 +0200
> >     +++ /usr/lib/xfstests/results//btrfs/138.out.bad    2022-06-10 
17:22:14.419547768 +0200
> >     @@ -1,2 +1,3 @@
> >      QA output created by 138
> >     +Checksum mismatch for zstd (expected 
4c99665eb952380c4c2c748a78be4f8a, got d41d8cd98f00b204e9800998ecf8427e)
> >      Silence is golden
> >     ...
> >     (Run 'diff -u /usr/lib/xfstests/tests/btrfs/138.out /usr/lib/
xfstests/results//btrfs/138.out.bad'  to see the entire diff)
> > btrfs/149 1s ...  1s
> > btrfs/183 0s ...  1s
> > btrfs/205 2s ...  1s
> > btrfs/234 2s ...  2s
> > btrfs/246 0s ...  1s
> > 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
> > Failures: btrfs/138
> > Failed 1 of 26 tests
> > 
> > tweed32:/usr/lib/xfstests # cat results/btrfs/138.out.bad 
> > QA output created by 138
> > Checksum mismatch for zstd (expected 4c99665eb952380c4c2c748a78be4f8a, 
got d41d8cd98f00b204e9800998ecf8427e)
> > 
> > tweed32:/usr/lib/xfstests # cat results/btrfs/138.full 
> > btrfs-progs v5.17
> > See http://btrfs.wiki.kernel.org for more information.
> > 
> > Performing full device TRIM /dev/loop1 (12.00GiB) ...
> > NOTE: several default settings have changed in version 5.15, please 
make sure
> >       this does not affect your deployments:
> >       - DUP for metadata (-m dup)
> >       - enabled no-holes (-O no-holes)
> >       - enabled free-space-tree (-R free-space-tree)
> > 
> > Label:              (null)
> > UUID:               06e21efe-2454-4d0c-ab80-f226320e1544
> > Node size:          16384
> > Sector size:        4096
> > Filesystem size:    12.00GiB
> > Block group profiles:
> >   Data:             single            8.00MiB
> >   Metadata:         DUP             256.00MiB
> >   System:           DUP               8.00MiB
> > SSD detected:       no
> > Zoned device:       no
> > Incompat features:  extref, skinny-metadata, no-holes
> > Runtime features:   free-space-tree
> > Checksum:           crc32c
> > Number of devices:  1
> > Devices:
> >    ID        SIZE  PATH
> >     1    12.00GiB  /dev/loop1
> > 
> > 100+0 records in
> > 100+0 records out
> > Testing zlib
> > 100+0 records in
> > 100+0 records out
> > Testing lzo
> > 100+0 records in
> > 100+0 records out
> > Testing zstd
> > dd: error reading '/mnt/scratch/zstd': Input/output error
> > 0+0 records in
> > 0+0 records out
> > 
> > tweed32:/usr/lib/xfstests # cat results/btrfs/138.dmesg 
> > [ 1286.929283] run fstests btrfs/138 at 2022-06-10 17:21:30
> > [ 1287.090289] BTRFS info (device loop0): flagging fs with big metadata 
feature
> > [ 1287.090292] BTRFS info (device loop0): using free space tree
> > [ 1287.090293] BTRFS info (device loop0): has skinny extents
> > [ 1287.215036] BTRFS: device fsid 06e21efe-2454-4d0c-ab80-f226320e1544 
devid 1 transid 6 /dev/loop1 scanned by mkfs.btrfs (19573)
> > [ 1287.226730] BTRFS info (device loop1): flagging fs with big metadata 
feature
> > [ 1287.226733] BTRFS info (device loop1): using free space tree
> > [ 1287.226735] BTRFS info (device loop1): has skinny extents
> > [ 1287.228967] BTRFS info (device loop1): checking UUID tree
> > [ 1321.763502] BTRFS info (device loop1): flagging fs with big metadata 
feature
> > [ 1321.763506] BTRFS info (device loop1): using free space tree
> > [ 1321.763506] BTRFS info (device loop1): has skinny extents
> > [ 1321.779751] BTRFS info (device loop1): setting incompat feature flag 
for COMPRESS_LZO (0x8)
> > [ 1325.730614] BTRFS info (device loop1): flagging fs with big metadata 
feature
> > [ 1325.730617] BTRFS info (device loop1): using free space tree
> > [ 1325.730618] BTRFS info (device loop1): has skinny extents
> > [ 1325.748761] BTRFS info (device loop1): setting incompat feature flag 
for COMPRESS_ZSTD (0x10)
> > [ 1330.663239] BTRFS info (device loop1): flagging fs with big metadata 
feature
> > [ 1330.663243] BTRFS info (device loop1): using free space tree
> > [ 1330.663245] BTRFS info (device loop1): has skinny extents
> > [ 1330.813468] BTRFS info (device loop1): flagging fs with big metadata 
feature
> > [ 1330.813471] BTRFS info (device loop1): using free space tree
> > [ 1330.813472] BTRFS info (device loop1): has skinny extents
> > 
> >  fs/btrfs/zstd.c | 41 ++++++++++++++++++++++-------------------
> >  1 file changed, 22 insertions(+), 19 deletions(-)
> > 
> > diff --git a/fs/btrfs/zstd.c b/fs/btrfs/zstd.c
> > index 0fe31a6f6e68..ccfc098319fd 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 +606,14 @@ 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);
> 
> I haven't tried the patch, but from a quick glance at the diff, one
> clear problem is that page_in_index is no longer incremented anywhere.
> 
> That's probably the reason why the test fails when trying to read the
> file (decompress).
> 

Oh, nice catch. I had completely overlooked that increment :-(
 
Thank you so much,

Fabio

> 
> >  			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 +622,7 @@ 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
> > 
>