пн, 14 мая 2018 г. в 20:32, David Sterba <dste...@suse.cz>:
> On Mon, May 14, 2018 at 03:02:10PM +0800, Qu Wenruo wrote:
> > As btrfs(5) specified:
> >
> > Note
> > If nodatacow or nodatasum are enabled, compression is disabled.
> >
> > If NODATASUM or NODATACOW set, we should not compress the extent.
> >
> > And in fact, we have bug report about corrupted compressed extent
> > leading to memory corruption in mail list.
> Link please.
> > Although it's mostly buggy lzo implementation causing the problem, btrfs
> > still needs to be fixed to meet the specification.
> That's very vague, what's the LZO bug? If the input is garbage and lzo
> decompression cannot decompress it, it's not a lzo bug.
> > Reported-by: James Harvey <jamespharve...@gmail.com>
> > Signed-off-by: Qu Wenruo <w...@suse.com>
> > ---
> > fs/btrfs/inode.c | 8 ++++++++
> > 1 file changed, 8 insertions(+)
> >
> > diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
> > index d241285a0d2a..dbef3f404559 100644
> > --- a/fs/btrfs/inode.c
> > +++ b/fs/btrfs/inode.c
> > @@ -396,6 +396,14 @@ static inline int inode_need_compress(struct inode
*inode, u64 start, u64 end)
> > {
> > struct btrfs_fs_info *fs_info = btrfs_sb(inode->i_sb);
> >
> > + /*
> > + * Btrfs doesn't support compression without csum or CoW.
> > + * This should have the highest priority.
> > + */
> > + if (BTRFS_I(inode)->flags & BTRFS_INODE_NODATACOW ||
> > + BTRFS_I(inode)->flags & BTRFS_INODE_NODATASUM)
> > + return 0;
> This is also the wrong place to fix that, NODATASUM or NODATACOW inode
> should never make it to compress_file_range (that calls
> inode_need_compress).
David, i've talk about that some time ago:
https://www.spinics.net/lists/linux-btrfs/msg73137.html
NoCow files can be *easy* compressed.
```
➜ ~ touch test
➜ ~ chattr +C test
➜ ~ lsattr test
---------------C-- test
➜ ~ dd if=/dev/zero of=./test bs=1M count=1
1+0 records in
1+0 records out
1048576 bytes (1.0 MB, 1.0 MiB) copied, 0.00099878 s, 1.0 GB/s
➜ ~ sync
➜ ~ filefrag -v test
Filesystem type is: 9123683e
File size of test is 1048576 (256 blocks of 4096 bytes)
ext: logical_offset: physical_offset: length: expected: flags:
0: 0.. 255: 88592741.. 88592996: 256:
last,eof
test: 1 extent found
➜ ~ btrfs fi def -vrczstd test
test
➜ ~ filefrag -v test
Filesystem type is: 9123683e
File size of test is 1048576 (256 blocks of 4096 bytes)
ext: logical_offset: physical_offset: length: expected: flags:
0: 0.. 31: 3125.. 3156: 32: encoded
1: 32.. 63: 3180.. 3211: 32: 3157: encoded
2: 64.. 95: 3185.. 3216: 32: 3212: encoded
3: 96.. 127: 3188.. 3219: 32: 3217: encoded
4: 128.. 159: 3263.. 3294: 32: 3220: encoded
5: 160.. 191: 3355.. 3386: 32: 3295: encoded
6: 192.. 223: 3376.. 3407: 32: 3387: encoded
7: 224.. 255: 3411.. 3442: 32: 3408:
last,encoded,eof
test: 8 extents found
```
--
Have a nice day,
Timofey.
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