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btrfs: subpage: make lzo_compress_pages() compatible
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There are several problems in lzo_compress_pages() preventing it from
being subpage compatible:

- No page offset is calculated when reading from inode pages
  For subpage case, we could have @start which is not aligned to
  PAGE_SIZE.

  Thus the destination where we read data from must take offset in page
  into consideration.

- The padding for segment header is bound to PAGE_SIZE
  This means, for subpage case we can skip several corners where on x86
  machines we need to add padding zeros.

The rework will:

- Update the comment to replace "page" with "sector"

- Introduce a new helper, copy_compressed_data_to_page(), to do the copy
  So that we don't need to bother page switching for both input and
  output.

  Now in lzo_compress_pages() we only care about page switching for
  input, while in copy_compressed_data_to_page() we only care about the
  page switching for output.

- Only one main cursor
  For lzo_compress_pages() we use @cur_in as main cursor.
  It will be the file offset we are currently at.

  All other helper variables will be only declared inside the loop.

  For copy_compressed_data_to_page() it's similar, we will have
  @cur_out at the main cursor, which records how many bytes are in the
  output.

Signed-off-by: Qu Wenruo <[email protected]>
Signed-off-by: David Sterba <[email protected]>
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@adam900710 @kdave
adam900710 authored and kdave committed Oct 26, 2021
1 parent 2b83a0e commit d408880
Showing 1 changed file with 134 additions and 136 deletions.
270 changes: 134 additions & 136 deletions fs/btrfs/lzo.c
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Original file line number Diff line number Diff line change
Expand Up @@ -32,19 +32,19 @@
* payload.
* One regular LZO compressed extent can have one or more segments.
* For inlined LZO compressed extent, only one segment is allowed.
* One segment represents at most one page of uncompressed data.
* One segment represents at most one sector of uncompressed data.
*
* 2.1 Segment header
* Fixed size. LZO_LEN (4) bytes long, LE32.
* Records the total size of the segment (not including the header).
* Segment header never crosses page boundary, thus it's possible to
* have at most 3 padding zeros at the end of the page.
* Segment header never crosses sector boundary, thus it's possible to
* have at most 3 padding zeros at the end of the sector.
*
* 2.2 Data Payload
* Variable size. Size up limit should be lzo1x_worst_compress(PAGE_SIZE)
* which is 4419 for a 4KiB page.
* Variable size. Size up limit should be lzo1x_worst_compress(sectorsize)
* which is 4419 for a 4KiB sectorsize.
*
* Example:
* Example with 4K sectorsize:
* Page 1:
* 0 0x2 0x4 0x6 0x8 0xa 0xc 0xe 0x10
* 0x0000 | Header | SegHdr 01 | Data payload 01 ... |
Expand Down Expand Up @@ -112,163 +112,161 @@ static inline size_t read_compress_length(const char *buf)
return le32_to_cpu(dlen);
}

/*
* Will do:
*
* - Write a segment header into the destination
* - Copy the compressed buffer into the destination
* - Make sure we have enough space in the last sector to fit a segment header
* If not, we will pad at most (LZO_LEN (4)) - 1 bytes of zeros.
*
* Will allocate new pages when needed.
*/
static int copy_compressed_data_to_page(char *compressed_data,
size_t compressed_size,
struct page **out_pages,
u32 *cur_out,
const u32 sectorsize)
{
u32 sector_bytes_left;
u32 orig_out;
struct page *cur_page;

/*
* We never allow a segment header crossing sector boundary, previous
* run should ensure we have enough space left inside the sector.
*/
ASSERT((*cur_out / sectorsize) == (*cur_out + LZO_LEN - 1) / sectorsize);

cur_page = out_pages[*cur_out / PAGE_SIZE];
/* Allocate a new page */
if (!cur_page) {
cur_page = alloc_page(GFP_NOFS);
if (!cur_page)
return -ENOMEM;
out_pages[*cur_out / PAGE_SIZE] = cur_page;
}

write_compress_length(page_address(cur_page) + offset_in_page(*cur_out),
compressed_size);
*cur_out += LZO_LEN;

orig_out = *cur_out;

/* Copy compressed data */
while (*cur_out - orig_out < compressed_size) {
u32 copy_len = min_t(u32, sectorsize - *cur_out % sectorsize,
orig_out + compressed_size - *cur_out);

cur_page = out_pages[*cur_out / PAGE_SIZE];
/* Allocate a new page */
if (!cur_page) {
cur_page = alloc_page(GFP_NOFS);
if (!cur_page)
return -ENOMEM;
out_pages[*cur_out / PAGE_SIZE] = cur_page;
}

memcpy(page_address(cur_page) + offset_in_page(*cur_out),
compressed_data + *cur_out - orig_out, copy_len);

*cur_out += copy_len;
}

/*
* Check if we can fit the next segment header into the remaining space
* of the sector.
*/
sector_bytes_left = round_up(*cur_out, sectorsize) - *cur_out;
if (sector_bytes_left >= LZO_LEN || sector_bytes_left == 0)
return 0;

/* The remaining size is not enough, pad it with zeros */
memset(page_address(cur_page) + offset_in_page(*cur_out), 0,
sector_bytes_left);
*cur_out += sector_bytes_left;
return 0;
}

int lzo_compress_pages(struct list_head *ws, struct address_space *mapping,
u64 start, struct page **pages, unsigned long *out_pages,
unsigned long *total_in, unsigned long *total_out)
{
struct workspace *workspace = list_entry(ws, struct workspace, list);
const u32 sectorsize = btrfs_sb(mapping->host->i_sb)->sectorsize;
struct page *page_in = NULL;
int ret = 0;
char *data_in;
char *cpage_out, *sizes_ptr;
int nr_pages = 0;
struct page *in_page = NULL;
struct page *out_page = NULL;
unsigned long bytes_left;
unsigned long len = *total_out;
unsigned long nr_dest_pages = *out_pages;
const unsigned long max_out = nr_dest_pages * PAGE_SIZE;
size_t in_len;
size_t out_len;
char *buf;
unsigned long tot_in = 0;
unsigned long tot_out = 0;
unsigned long pg_bytes_left;
unsigned long out_offset;
unsigned long bytes;
/* Points to the file offset of input data */
u64 cur_in = start;
/* Points to the current output byte */
u32 cur_out = 0;
u32 len = *total_out;

*out_pages = 0;
*total_out = 0;
*total_in = 0;

in_page = find_get_page(mapping, start >> PAGE_SHIFT);
data_in = page_address(in_page);

/*
* store the size of all chunks of compressed data in
* the first 4 bytes
* Skip the header for now, we will later come back and write the total
* compressed size
*/
out_page = alloc_page(GFP_NOFS);
if (out_page == NULL) {
ret = -ENOMEM;
goto out;
}
cpage_out = page_address(out_page);
out_offset = LZO_LEN;
tot_out = LZO_LEN;
pages[0] = out_page;
nr_pages = 1;
pg_bytes_left = PAGE_SIZE - LZO_LEN;

/* compress at most one page of data each time */
in_len = min(len, PAGE_SIZE);
while (tot_in < len) {
ret = lzo1x_1_compress(data_in, in_len, workspace->cbuf,
&out_len, workspace->mem);
if (ret != LZO_E_OK) {
pr_debug("BTRFS: lzo in loop returned %d\n",
ret);
cur_out += LZO_LEN;
while (cur_in < start + len) {
const u32 sectorsize_mask = sectorsize - 1;
u32 sector_off = (cur_in - start) & sectorsize_mask;
u32 in_len;
size_t out_len;

/* Get the input page first */
if (!page_in) {
page_in = find_get_page(mapping, cur_in >> PAGE_SHIFT);
ASSERT(page_in);
}

/* Compress at most one sector of data each time */
in_len = min_t(u32, start + len - cur_in, sectorsize - sector_off);
ASSERT(in_len);
ret = lzo1x_1_compress(page_address(page_in) +
offset_in_page(cur_in), in_len,
workspace->cbuf, &out_len,
workspace->mem);
if (ret < 0) {
pr_debug("BTRFS: lzo in loop returned %d\n", ret);
ret = -EIO;
goto out;
}

/* store the size of this chunk of compressed data */
write_compress_length(cpage_out + out_offset, out_len);
tot_out += LZO_LEN;
out_offset += LZO_LEN;
pg_bytes_left -= LZO_LEN;

tot_in += in_len;
tot_out += out_len;

/* copy bytes from the working buffer into the pages */
buf = workspace->cbuf;
while (out_len) {
bytes = min_t(unsigned long, pg_bytes_left, out_len);

memcpy(cpage_out + out_offset, buf, bytes);

out_len -= bytes;
pg_bytes_left -= bytes;
buf += bytes;
out_offset += bytes;

/*
* we need another page for writing out.
*
* Note if there's less than 4 bytes left, we just
* skip to a new page.
*/
if ((out_len == 0 && pg_bytes_left < LZO_LEN) ||
pg_bytes_left == 0) {
if (pg_bytes_left) {
memset(cpage_out + out_offset, 0,
pg_bytes_left);
tot_out += pg_bytes_left;
}

/* we're done, don't allocate new page */
if (out_len == 0 && tot_in >= len)
break;

if (nr_pages == nr_dest_pages) {
out_page = NULL;
ret = -E2BIG;
goto out;
}

out_page = alloc_page(GFP_NOFS);
if (out_page == NULL) {
ret = -ENOMEM;
goto out;
}
cpage_out = page_address(out_page);
pages[nr_pages++] = out_page;

pg_bytes_left = PAGE_SIZE;
out_offset = 0;
}
}
ret = copy_compressed_data_to_page(workspace->cbuf, out_len,
pages, &cur_out, sectorsize);
if (ret < 0)
goto out;

cur_in += in_len;

/* we're making it bigger, give up */
if (tot_in > 8192 && tot_in < tot_out) {
/*
* Check if we're making it bigger after two sectors. And if
* it is so, give up.
*/
if (cur_in - start > sectorsize * 2 && cur_in - start < cur_out) {
ret = -E2BIG;
goto out;
}

/* we're all done */
if (tot_in >= len)
break;

if (tot_out > max_out)
break;

bytes_left = len - tot_in;
put_page(in_page);

start += PAGE_SIZE;
in_page = find_get_page(mapping, start >> PAGE_SHIFT);
data_in = page_address(in_page);
in_len = min(bytes_left, PAGE_SIZE);
}

if (tot_out >= tot_in) {
ret = -E2BIG;
goto out;
/* Check if we have reached page boundary */
if (IS_ALIGNED(cur_in, PAGE_SIZE)) {
put_page(page_in);
page_in = NULL;
}
}

/* store the size of all chunks of compressed data */
sizes_ptr = page_address(pages[0]);
write_compress_length(sizes_ptr, tot_out);
/* Store the size of all chunks of compressed data */
write_compress_length(page_address(pages[0]), cur_out);

ret = 0;
*total_out = tot_out;
*total_in = tot_in;
*total_out = cur_out;
*total_in = cur_in - start;
out:
*out_pages = nr_pages;

if (in_page)
put_page(in_page);

*out_pages = DIV_ROUND_UP(cur_out, PAGE_SIZE);
return ret;
}

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