On Wed, Feb 23, 2011 at 4:59 PM, Yongqiang Yang <xiaoqiangnk@xxxxxxxxx> wrote: > 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. We can call find_get_pages() with nr_pages=1 instead. And we can regulate the range with page->index if it is not the the paging contains start block. > > 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 > -- 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
