I have been stressing out the FIEMAP ioctl() quite a bit recently, while working on the wiper.sh SSD TRIM utility (part of the hdparm package). For an hour or so today, things got very confusing. My wiper.sh script stopped working correctly, and I narrowed it down to FIEMAP's incorrect use of the "LAST" flag. Normally, FIEMAP signals the final extent of a file by setting the "LAST" bit (bit-0) in flags. When the application sees this flag, it knows it should not need to issue any more FIEMAP calls. But suddenly, for a couple of hours today, FIEMAP began setting the "LAST" flag on every single FIEMAP call, tricking my code into thinking that the file being queried was a lot smaller than it really was. No strace(), because I still hadn't figured-out what was going on, but I did save this trace from debug code inside hdparm. The file being FIEMAP'd was a 50GB+ file, filling the ext4 filesystem to near 100% capacity. I have no idea what caused FIEMAP to misbehave, but after a couple of hours it suddenly started working correctly again. This was all observed on 32-bit x86 Linux-2.6.29.4, and no, it is not reproduceable. A command-line "sync" was done before the file was opened for FIEMAP. The output below shows the sequence of FIEMAP calls on the single massive file. Notice that the first call returned the "LAST" flag set, despite there being many many more extents after that bunch. Cheers ioctl(FIEMAP) returned 57 extents log=0 phy=6543114240 len=16777216 flags=0x800 log=16777216 phy=48435822592 len=8388608 flags=0x800 log=25165824 phy=48452599808 len=8388608 flags=0x800 ... log=3917479936 phy=70330089472 len=125829120 flags=0x800 log=4043309056 phy=70455918592 len=125829120 flags=0x800 log=4169138176 phy=70581747712 len=125829120 flags=0x801 <<== BUG! ioctl(FIEMAP) returned 204 extents log=4294967296 phy=70707576832 len=125829120 flags=0x800 ... log=29469179904 phy=100050927616 len=125829120 flags=0x800 ioctl(FIEMAP) returned 204 extents log=29595009024 phy=100176756736 len=125829120 flags=0x800 ... log=45743718400 phy=73115267072 len=102400 flags=0x800 ioctl(FIEMAP) returned 204 extents ... ... log=51256500224 phy=19321303040 len=4096 flags=0x800 log=51256504320 phy=19321757696 len=4096 flags=0x800 ioctl(FIEMAP) returned 176 extents log=51256508416 phy=19325272064 len=4096 flags=0x800 ... log=51262967808 phy=102504951808 len=36864 flags=0x800 log=51263004672 phy=102518059008 len=36864 flags=0x800 log=51263041536 phy=107746783232 len=20480 flags=0x801 << CORRECT ioctl(FIEMAP) returned 0 extents -- 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
