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

From: Filipe Manana
Date: Sat Jun 11 2022 - 05:34:23 EST

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).


> 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
>