Commit 1c3dc173 authored by Qu Wenruo's avatar Qu Wenruo Committed by David Sterba

btrfs: rework btrfs_decompress_buf2page()

There are several bugs inside the function btrfs_decompress_buf2page()

- @start_byte doesn't take bvec.bv_offset into consideration
  Thus it can't handle case where the target range is not page aligned.

- Too many helper variables
  There are tons of helper variables, @buf_offset, @current_buf_start,
  @start_byte, @prev_start_byte, @working_bytes, @bytes.
  This hurts anyone who wants to read the function.

- No obvious main cursor for the iteartion
  A new problem caused by previous problem.

- Comments for parameter list makes no sense
  Like @buf_start is the offset to @buf, or offset inside the full
  decompressed extent? (Spoiler alert, the later case)
  And @total_out acts more like @buf_start + @size_of_buf.

  The worst is @disk_start.
  The real meaning of it is the file offset of the full decompressed
  extent.

This patch will rework the whole function by:

- Add a proper comment with ASCII art to explain the parameter list

- Rework parameter list
  The old @buf_start is renamed to @decompressed, to show how many bytes
  are already decompressed inside the full decompressed extent.
  The old @total_out is replaced by @buf_len, which is the decompressed
  data size.
  For old @disk_start and @bio, just pass @compressed_bio in.

- Use single main cursor
  The main cursor will be @cur_file_offset, to show what's the current
  file offset.
  Other helper variables will be declared inside the main loop, and only
  minimal amount of helper variables:
  * offset_inside_decompressed_buf:	The only real helper
  * copy_start_file_offset:		File offset we start memcpy
  * bvec_file_offset:			File offset of current bvec

Even with all these extensive comments, the final function is still
smaller than the original function, which is definitely a win.
Signed-off-by: default avatarQu Wenruo <wqu@suse.com>
Signed-off-by: default avatarDavid Sterba <dsterba@suse.com>
parent 557023ea
......@@ -1272,96 +1272,82 @@ void __cold btrfs_exit_compress(void)
}
/*
* Copy uncompressed data from working buffer to pages.
* Copy decompressed data from working buffer to pages.
*
* buf_start is the byte offset we're of the start of our workspace buffer.
* @buf: The decompressed data buffer
* @buf_len: The decompressed data length
* @decompressed: Number of bytes that are already decompressed inside the
* compressed extent
* @cb: The compressed extent descriptor
* @orig_bio: The original bio that the caller wants to read for
*
* total_out is the last byte of the buffer
* An easier to understand graph is like below:
*
* |<- orig_bio ->| |<- orig_bio->|
* |<------- full decompressed extent ----->|
* |<----------- @cb range ---->|
* | |<-- @buf_len -->|
* |<--- @decompressed --->|
*
* Note that, @cb can be a subpage of the full decompressed extent, but
* @cb->start always has the same as the orig_file_offset value of the full
* decompressed extent.
*
* When reading compressed extent, we have to read the full compressed extent,
* while @orig_bio may only want part of the range.
* Thus this function will ensure only data covered by @orig_bio will be copied
* to.
*
* Return 0 if we have copied all needed contents for @orig_bio.
* Return >0 if we need continue decompress.
*/
int btrfs_decompress_buf2page(const char *buf, unsigned long buf_start,
unsigned long total_out, u64 disk_start,
struct bio *bio)
int btrfs_decompress_buf2page(const char *buf, u32 buf_len,
struct compressed_bio *cb, u32 decompressed)
{
unsigned long buf_offset;
unsigned long current_buf_start;
unsigned long start_byte;
unsigned long prev_start_byte;
unsigned long working_bytes = total_out - buf_start;
unsigned long bytes;
struct bio_vec bvec = bio_iter_iovec(bio, bio->bi_iter);
/*
* start byte is the first byte of the page we're currently
* copying into relative to the start of the compressed data.
*/
start_byte = page_offset(bvec.bv_page) - disk_start;
/* we haven't yet hit data corresponding to this page */
if (total_out <= start_byte)
return 1;
/*
* the start of the data we care about is offset into
* the middle of our working buffer
*/
if (total_out > start_byte && buf_start < start_byte) {
buf_offset = start_byte - buf_start;
working_bytes -= buf_offset;
} else {
buf_offset = 0;
}
current_buf_start = buf_start;
/* copy bytes from the working buffer into the pages */
while (working_bytes > 0) {
bytes = min_t(unsigned long, bvec.bv_len,
PAGE_SIZE - (buf_offset % PAGE_SIZE));
bytes = min(bytes, working_bytes);
struct bio *orig_bio = cb->orig_bio;
/* Offset inside the full decompressed extent */
u32 cur_offset;
cur_offset = decompressed;
/* The main loop to do the copy */
while (cur_offset < decompressed + buf_len) {
struct bio_vec bvec;
size_t copy_len;
u32 copy_start;
/* Offset inside the full decompressed extent */
u32 bvec_offset;
bvec = bio_iter_iovec(orig_bio, orig_bio->bi_iter);
/*
* cb->start may underflow, but subtracting that value can still
* give us correct offset inside the full decompressed extent.
*/
bvec_offset = page_offset(bvec.bv_page) + bvec.bv_offset - cb->start;
memcpy_to_page(bvec.bv_page, bvec.bv_offset, buf + buf_offset,
bytes);
flush_dcache_page(bvec.bv_page);
/* Haven't reached the bvec range, exit */
if (decompressed + buf_len <= bvec_offset)
return 1;
buf_offset += bytes;
working_bytes -= bytes;
current_buf_start += bytes;
/* check if we need to pick another page */
bio_advance(bio, bytes);
if (!bio->bi_iter.bi_size)
return 0;
bvec = bio_iter_iovec(bio, bio->bi_iter);
prev_start_byte = start_byte;
start_byte = page_offset(bvec.bv_page) - disk_start;
copy_start = max(cur_offset, bvec_offset);
copy_len = min(bvec_offset + bvec.bv_len,
decompressed + buf_len) - copy_start;
ASSERT(copy_len);
/*
* We need to make sure we're only adjusting
* our offset into compression working buffer when
* we're switching pages. Otherwise we can incorrectly
* keep copying when we were actually done.
* Extra range check to ensure we didn't go beyond
* @buf + @buf_len.
*/
if (start_byte != prev_start_byte) {
/*
* make sure our new page is covered by this
* working buffer
*/
if (total_out <= start_byte)
return 1;
ASSERT(copy_start - decompressed < buf_len);
memcpy_to_page(bvec.bv_page, bvec.bv_offset,
buf + copy_start - decompressed, copy_len);
flush_dcache_page(bvec.bv_page);
cur_offset += copy_len;
/*
* the next page in the biovec might not be adjacent
* to the last page, but it might still be found
* inside this working buffer. bump our offset pointer
*/
if (total_out > start_byte &&
current_buf_start < start_byte) {
buf_offset = start_byte - buf_start;
working_bytes = total_out - start_byte;
current_buf_start = buf_start + buf_offset;
}
}
bio_advance(orig_bio, copy_len);
/* Finished the bio */
if (!orig_bio->bi_iter.bi_size)
return 0;
}
return 1;
}
......
......@@ -86,9 +86,8 @@ int btrfs_compress_pages(unsigned int type_level, struct address_space *mapping,
unsigned long *total_out);
int btrfs_decompress(int type, unsigned char *data_in, struct page *dest_page,
unsigned long start_byte, size_t srclen, size_t destlen);
int btrfs_decompress_buf2page(const char *buf, unsigned long buf_start,
unsigned long total_out, u64 disk_start,
struct bio *bio);
int btrfs_decompress_buf2page(const char *buf, u32 buf_len,
struct compressed_bio *cb, u32 decompressed);
blk_status_t btrfs_submit_compressed_write(struct btrfs_inode *inode, u64 start,
unsigned int len, u64 disk_start,
......
......@@ -293,8 +293,6 @@ int lzo_decompress_bio(struct list_head *ws, struct compressed_bio *cb)
unsigned long tot_len;
char *buf;
struct page **pages_in = cb->compressed_pages;
u64 disk_start = cb->start;
struct bio *orig_bio = cb->orig_bio;
data_in = page_address(pages_in[0]);
tot_len = read_compress_length(data_in);
......@@ -391,14 +389,14 @@ int lzo_decompress_bio(struct list_head *ws, struct compressed_bio *cb)
buf_start = tot_out;
tot_out += out_len;
ret2 = btrfs_decompress_buf2page(workspace->buf, buf_start,
tot_out, disk_start, orig_bio);
ret2 = btrfs_decompress_buf2page(workspace->buf, out_len,
cb, buf_start);
if (ret2 == 0)
break;
}
done:
if (!ret)
zero_fill_bio(orig_bio);
zero_fill_bio(cb->orig_bio);
return ret;
}
......
......@@ -275,8 +275,6 @@ int zlib_decompress_bio(struct list_head *ws, struct compressed_bio *cb)
unsigned long total_pages_in = DIV_ROUND_UP(srclen, PAGE_SIZE);
unsigned long buf_start;
struct page **pages_in = cb->compressed_pages;
u64 disk_start = cb->start;
struct bio *orig_bio = cb->orig_bio;
data_in = page_address(pages_in[page_in_index]);
workspace->strm.next_in = data_in;
......@@ -314,9 +312,8 @@ int zlib_decompress_bio(struct list_head *ws, struct compressed_bio *cb)
if (buf_start == total_out)
break;
ret2 = btrfs_decompress_buf2page(workspace->buf, buf_start,
total_out, disk_start,
orig_bio);
ret2 = btrfs_decompress_buf2page(workspace->buf,
total_out - buf_start, cb, buf_start);
if (ret2 == 0) {
ret = 0;
goto done;
......@@ -336,8 +333,7 @@ int zlib_decompress_bio(struct list_head *ws, struct compressed_bio *cb)
data_in = page_address(pages_in[page_in_index]);
workspace->strm.next_in = data_in;
tmp = srclen - workspace->strm.total_in;
workspace->strm.avail_in = min(tmp,
PAGE_SIZE);
workspace->strm.avail_in = min(tmp, PAGE_SIZE);
}
}
if (ret != Z_STREAM_END)
......@@ -347,7 +343,7 @@ int zlib_decompress_bio(struct list_head *ws, struct compressed_bio *cb)
done:
zlib_inflateEnd(&workspace->strm);
if (!ret)
zero_fill_bio(orig_bio);
zero_fill_bio(cb->orig_bio);
return ret;
}
......
......@@ -540,8 +540,6 @@ int zstd_decompress_bio(struct list_head *ws, struct compressed_bio *cb)
{
struct workspace *workspace = list_entry(ws, struct workspace, list);
struct page **pages_in = cb->compressed_pages;
u64 disk_start = cb->start;
struct bio *orig_bio = cb->orig_bio;
size_t srclen = cb->compressed_len;
ZSTD_DStream *stream;
int ret = 0;
......@@ -582,7 +580,7 @@ int zstd_decompress_bio(struct list_head *ws, struct compressed_bio *cb)
workspace->out_buf.pos = 0;
ret = btrfs_decompress_buf2page(workspace->out_buf.dst,
buf_start, total_out, disk_start, orig_bio);
total_out - buf_start, cb, buf_start);
if (ret == 0)
break;
......@@ -607,7 +605,7 @@ int zstd_decompress_bio(struct list_head *ws, struct compressed_bio *cb)
}
}
ret = 0;
zero_fill_bio(orig_bio);
zero_fill_bio(cb->orig_bio);
done:
return ret;
}
......
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