On Wed, Feb 23, 2011 at 5:18 AM, Chris Mason <chris.mason@xxxxxxxxxx> wrote: > [ resend, sorry if this is a dup ] > > Hi everyone, > > We've had reports on btrfs that cp is giving us files full of zeros > instead of actually copying them. It was tracked down to a bug with > the btrfs fiemap implementation where it was returning holes for > delalloc ranges. > > Newer versions of cp are trusting fiemap to tell it where the holes > are, which does seem like a pretty neat trick. > > I decided to give xfs and ext4 a shot with a few tests cases too, xfs > passed with all the ones btrfs was getting wrong, and ext4 got the basic > delalloc case right. > > # mkfs.ext4 /dev/xxx > # mount /dev/xxx /mnt > # dd if=/dev/zero of=/mnt/foo bs=1M count=1 > # fiemap-test foo > ext: 0 logical: [ 0.. 255] phys: 0.. 255 flags: 0x007 tot: 256 > > Horray! But once we throw a hole in, things go bad: > > # mkfs.ext4 /dev/xxx > # mount /dev/xxx /mnt > # dd if=/dev/zero of=/mnt/foo bs=1M count=1 seek=1 > # fiemap-test foo > < no output > Actually, there is no extent in extent tree now, so ext4_ext_walk_space() will pass ext4_ext_fiemap_cb() a variable of struct ext4_ext_cache with the requested length. But in ext4_ext_fiemap_cb() just the paging contains start block is got via find_get_page(), if find_get_page() return null, ext4_ext_fiemap_cb() thinks the whole request range is empty and it returns request range. In 1st case, find_get_page() will succeed. It seems that we should get no. of pages in page cache if find_get_page() fails, and correct the range to be returned. Right? If right I will send a patch. > > We've got a delalloc extent after the hole and ext4 fiemap didn't find > it. If I run sync to kick the delalloc out: > > # sync > # fiemap-test foo > ext: 0 logical: [ 256.. 511] phys: 34048.. 34303 flags: 0x001 tot: 256 Now, there is a extent in extent tree at least. and ext4_ext_walk_space() will regulate the request range to be passed to ext4_ext_fiemap_cb(). > > fiemap-test is sitting in my /usr/local/bin, and I have no idea how it > got there. It's full of pretty comments so I know it isn't mine, but > you can grab it here: > > http://oss.oracle.com/~mason/fiemap-test.c > > xfsqa has a fiemap program too. > > -chris > -- > To unsubscribe from this list: send the line "unsubscribe linux-ext4" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html > -- Best Wishes Yongqiang Yang -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
