Re: [PATCH v2] btrfs-progs: fix page align issue for lzo compress in restore

[Gui Hecheng <guihc.fnst@cn.fujitsu.com>]

On Mon, 2014-09-22 at 10:44 +0200, Marc Dietrich wrote:
> Am Montag, 22. September 2014, 16:29:28 schrieb Gui Hecheng:
> > 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: Marc Dietrich <marvin24@gmx.de>
> > Signed-off-by: Gui Hecheng <guihc.fnst@cn.fujitsu.com>
> > ---
> > changelog
> > 	v1->v2: adopt alignment check method suggested by Marc
> > ---
> >  cmds-restore.c | 27 ++++++++++++++++++++++++++-
> >  1 file changed, 26 insertions(+), 1 deletion(-)
> > 
> > diff --git a/cmds-restore.c b/cmds-restore.c
> > index 38a131e..974f45d 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 += tot_in_aligned - *tot_in;
> 
> in_len += bytes_left; // makes it more readable

Oh, yes, that's my carelessness, Thanks!

> > +	*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;
> 
> otherwise, looks good to me.
> 
> Thanks!
> 
> Marc