Btrfs devel - Sep 2014 - [PATCH v3] btrfs-progs: fix page align issue for lzo compress in restore

When runing restore under lzo compression, "bad compress length"
problems are encountered.
It is because there is a page align problem with the @decompress_lzo,
as follows:
		|------| |----|-| |------|...|------|
		  page         ^    page       page
			       |
			  3 bytes left

	When lzo compress pages im RAM, lzo will ensure that
	the 4 bytes len will be in one page as a whole.
	There is a situation that 3 (or less) bytes are left
	at the end of a page, and then the 4 bytes len is
	stored at the start of the next page.
	But the @decompress_lzo doesn't goto the start of
	the next page and continue to read the next 4 bytes
	which is across two pages, so a random value is fetched
	as a "bad compress length".

So we check page alignment every time before we are going to
fetch the next @len and after the former piece of data is decompressed.
If the current page that we reach has less than 4 bytes left,
then we should fetch the next @len at the start of next page.

Signed-off-by: Gui Hecheng <guihc.fnst@cn.fujitsu.com>
Reviewed-by: Marc Dietrich <marvin24@gmx.de>
---
changelog
	v1->v2: adopt alignment check method suggested by Marc
	v2->v3: make code more readable
---
 cmds-restore.c | 27 ++++++++++++++++++++++++++-
 1 file changed, 26 insertions(+), 1 deletion(-)

diff --git a/cmds-restore.c b/cmds-restore.c
index 38a131e..5094b05 100644
--- a/cmds-restore.c
+++ b/cmds-restore.c
@@ -57,6 +57,9 @@ static int dry_run = 0;
 
 #define LZO_LEN 4
 #define PAGE_CACHE_SIZE 4096
+#define PAGE_CACHE_MASK (~(PAGE_CACHE_SIZE - 1))
+#define PAGE_CACHE_ALIGN(addr) (((addr) + PAGE_CACHE_SIZE - 1)	\
+							& PAGE_CACHE_MASK)
 #define lzo1x_worst_compress(x) ((x) + ((x) / 16) + 64 + 3)
 
 static int decompress_zlib(char *inbuf, char *outbuf, u64 compress_len,
@@ -93,6 +96,28 @@ static inline size_t read_compress_length(unsigned char *buf)
 	return le32_to_cpu(dlen);
 }
 
+static void align_if_need(size_t *tot_in, size_t *in_len)
+{
+	int tot_in_aligned;
+	int bytes_left;
+
+	tot_in_aligned = PAGE_CACHE_ALIGN(*tot_in);
+	bytes_left = tot_in_aligned - *tot_in;
+
+	if (bytes_left >= LZO_LEN)
+		return;
+
+	/*
+	 * The LZO_LEN bytes is guaranteed to be
+	 * in one page as a whole, so if a page
+	 * has fewer than LZO_LEN bytes left,
+	 * the LZO_LEN bytes should be fetched
+	 * at the start of the next page
+	 */
+	*in_len += bytes_left;
+	*tot_in = tot_in_aligned;
+}
+
 static int decompress_lzo(unsigned char *inbuf, char *outbuf, u64 compress_len,
 			  u64 *decompress_len)
 {
@@ -135,8 +160,8 @@ static int decompress_lzo(unsigned char *inbuf, char
*outbuf, u64 compress_len,
 		}
 		out_len += new_len;
 		outbuf += new_len;
+		align_if_need(&tot_in, &in_len);
 		inbuf += in_len;
-		tot_in += in_len;
 	}
 
 	*decompress_len = out_len;
-- 
1.8.1.4

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