[PATCH v3 2/2] btrfs: Replace kmap() with kmap_local_page() in zstd.c

From: Fabio M. De Francesco
Date: Thu Jun 16 2022 - 03:16:19 EST

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 QEMU + KVM 32 bits VM with 4GB of RAM and
HIGHMEM64G enabled. These changes passed all tests of the "compress" group.

Cc: Filipe Manana <fdmanana@xxxxxxxxxx>
Suggested-by: Ira Weiny <ira.weiny@xxxxxxxxx>
Signed-off-by: Fabio M. De Francesco <fmdefrancesco@xxxxxxxxx>
---

v2->v3: Remove unnecessary casts to arguments of kunmap_local() now that
this API can take pointers to const void.

v1->v2: No changes.

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


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..5d2ab0bac9d2 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(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(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(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(workspace->in_buf.src);
return ret;
}

--
2.36.1