On 09/15/2017 12:18 AM, Hugo Mills wrote: > As far as I know, both of these are basically known issues, with no > good solution, other than not using O_DIRECT. Certainly the first > issue is one I recognise. The second isn't one I recognise directly, > but is unsurprising to me. > > There have been discussions -- including developers -- on this list > as recent as a month or so ago. The general outcome seems to be that > any problems with O_DIRECT are not going to be fixed.
I missed this thread; could you point it to me ? If csum and O_DIRECT are not reliable, why not disallow one of them: i.e allow O_DIRECT only on nodatasum files... ZFS (on linux) do not support O_DIRECT at all... In fact most of the applications which benefit from O_DIRECT (it comes to me VM e DB), are the ones which need also nodatasum to have good performance. One of the strongest point of BTRFS was the checksums; but these are not effective when the file is opened with O_DIRECT; worse there are cases where the file is corrupted and the application got -EIO; not mentioning that the dmesg is filled by "csum failed ...." > > Hugo. > > On Fri, Sep 15, 2017 at 12:00:19AM +0200, Goffredo Baroncelli wrote: >> Hi all, >> >> I discovered two bugs when O_DIRECT is used... >> >> 1) a corrupted file doesn't return -EIO when O_DIRECT is used >> >> Normally BTRFS prevents to access the contents of a corrupted file; however >> I was able read the content of a corrupted file simply using O_DIRECT >> >> # in a new btrfs filesystem, create a file >> $ sudo mkfs.btrfs -f /dev/sdd5 >> $ mount /dev/sdd5 t >> $ (while true; do echo -n "abcefg" ; done )| sudo dd of=t/abcd >> bs=$((16*1024)) iflag=fullblock count=1024 >> >> # corrupt the file >> $ sudo filefrag -v t/abcd >> Filesystem type is: 9123683e >> File size of t/abcd is 16777216 (4096 blocks of 4096 bytes) >> ext: logical_offset: physical_offset: length: expected: flags: >> 0: 0.. 3475: 70656.. 74131: 3476: >> 1: 3476.. 4095: 74212.. 74831: 620: 74132: >> last,eof >> t/abcd: 2 extents found >> $ sudo umount t >> $ sudo ~/btrfs/btrfs-progs/btrfs-corrupt-block -l $((70656*4096)) -b 10 >> /dev/sdd5 >> mirror 1 logical 289406976 physical 289406976 device /dev/sdd5 >> corrupting 289406976 copy 1 >> >> # try to access the file; expected result: -EIO >> $ sudo mount /dev/sdd5 t >> $ dd if=t/abcd | hexdump -c | head >> dd: error reading 't/abcd': Input/output error >> 0+0 records in >> 0+0 records out >> 0 bytes copied, 0.000477413 s, 0.0 kB/s >> >> >> # try to access the file using O_DIRECT; expected result: -EIO, instead the >> file is accessible >> $ dd if=t/abcd iflag=direct bs=4096 | hexdump -c | head >> 0000000 001 001 001 001 001 001 001 001 001 001 001 001 001 001 001 001 >> * >> 0001000 f g a b c e f g a b c e f g a b >> 0001010 c e f g a b c e f g a b c e f g >> 0001020 a b c e f g a b c e f g a b c e >> 0001030 f g a b c e f g a b c e f g a b >> 0001040 c e f g a b c e f g a b c e f g >> 0001050 a b c e f g a b c e f g a b c e >> 0001060 f g a b c e f g a b c e f g a b >> 0001070 c e f g a b c e f g a b c e f g >> >> (dmesg report the checksum mismatch) >> [13265.085645] BTRFS warning (device sdd5): csum failed root 5 ino 257 off 0 >> csum 0x98f94189 expected csum 0x0ab6be80 mirror 1 >> >> Note the first 4k filled by 0x01 !!!!! >> >> Conclusion: even if the file is corrupted and normally BTRFS prevent to >> access it, using O_DIRECT >> a) no error is returned to the caller >> b) instead of the page stored on the disk, it is returned a page filled with >> 0x01 (according also with the function __readpage_endio_check()) >> >> >> 2) The second bug, is a more severe bug. If during a writing of a buffer >> with O_DIRECT, the buffer is updated at the same time by a second process, >> the checksum may be incorrect. >> >> At the end of the email there is the code which shows the problem: two >> process share the same memory: the first write it to the disk, the second >> update the buffer continuously. A third process try to read the file, but it >> got time to time -EIO >> >> If you ran my code in a btrfs filesystem you got a lot of >> >> ERROR: read thread; r = 8192, expected = 16384 >> ERROR: read thread; r = 8192, expected = 16384 >> ERROR: read thread; e = 5 - Input/output error >> ERROR: read thread; e = 5 - Input/output error >> >> The firsts lines are related to a shorter read (which may happens). The >> lasts lines are related to a checksum mismatch. The dmesg is filled by lines >> like >> [...] >> [14873.573547] BTRFS warning (device sdd5): csum failed root 5 ino 259 off >> 4096 csum 0x0683c6df expected csum 0x55eb85e6 mirror 1 >> [...] >> >> This is definitely a bug. >> >> I think that using O_DIRECT and updating a page at the same time could >> happen in a VM. In BTRFS this could lead to a wrong checksum. The problem >> is that if BTRFS detects a checksum error during a reading: >> a) if O_DIRECT is not used in the read >> * -EIO is returned >> Definitely BAD >> >> b) if O_DIRECT is used in the read >> * it doesn't return the error to the caller >> * it returns a page filled by 0x01 instead of the data from the disk >> Even worse than a) >> >> Note1: even using O_DIRECT with O_SYNC, the problem still persist. >> Note2: the man page of open(2) is filled by a lot of notes about O_DIRECT, >> but also it stated that using O_DIRECT+fork()+mmap(... MAP_SHARED) is >> legally. >> Note3: even "ZFS on linux" has its trouble with O_DIRECT: if fact ZFS >> doesn't support it; see https://github.com/zfsonlinux/zfs/issues/224 >> >> BR >> G.Baroncelli >> >> ----- cut --- cut --- cut ---- >> >> #define _GNU_SOURCE >> #include <stdio.h> >> #include <stdlib.h> >> #include <sys/mman.h> >> #include <assert.h> >> #include <errno.h> >> #include <sys/types.h> >> #include <sys/stat.h> >> #include <fcntl.h> >> #include <string.h> >> #include <unistd.h> >> >> #define FILESIZE (4096*4) >> >> int fd; >> char *buffer = NULL; >> >> void read_thread(const char *nf) { >> >> void *data = mmap(NULL, FILESIZE, >> PROT_READ|PROT_WRITE, >> MAP_PRIVATE|MAP_ANONYMOUS, -1, 0); >> >> assert(data); >> fprintf(stderr, "read_thread: data = %p\n", data); >> int rfd; >> rfd = open(nf, O_RDONLY); >> >> for(;;) { >> ssize_t r = pread(rfd, data, FILESIZE, 0); >> if (r < 0) { >> int e = errno; >> fprintf(stderr, "ERROR: read thread; e = %d - %s\n", >> e, strerror(e)); >> >> } else if (r != FILESIZE) { >> fprintf(stderr, "ERROR: read thread; r = %ld, expected >> = %d\n", >> r, FILESIZE); >> } >> } >> } >> >> void write_thread(void) { >> >> for(;;) { >> ssize_t r = pwrite(fd, buffer, FILESIZE, 0); >> assert(r == FILESIZE); >> } >> } >> >> void update_thread(void) { >> >> for(;;) { >> int i; >> for (i = 0 ; i < FILESIZE ; i++) >> buffer[i] += i+10; >> } >> } >> >> >> int main(int argc, char **argv) { >> >> if (argc < 2) { >> fprintf(stderr, "usage: %s <fname>\n", argv[0]); >> exit(100); >> } >> >> >> buffer = mmap(NULL, FILESIZE, >> PROT_READ|PROT_WRITE, >> MAP_SHARED|MAP_ANONYMOUS, -1, 0); >> >> assert(buffer); >> fprintf(stderr, "main: data = %p\n", buffer); >> >> fd = open(argv[1], O_RDWR|O_DIRECT|O_CREAT, 0660); >> assert(fd>=0); >> >> ssize_t r = pwrite(fd, buffer, FILESIZE, 0); >> assert(r == FILESIZE); >> >> pid_t child; >> >> child = fork(); >> assert(child >= 0); >> if (child == 0) >> write_thread(); >> fprintf(stderr, "write_thread pid = %d\n", child); >> >> child = fork(); >> assert(child >= 0); >> if (child == 0) >> read_thread(argv[1]); >> fprintf(stderr, "read_thread pid = %d\n", child); >> >> child = fork(); >> assert(child >= 0); >> if (child == 0) >> update_thread(); >> fprintf(stderr, "update_thread pid = %d\n", child); >> >> for(;;) >> sleep(100*100*100); >> >> >> return 0; >> } >> >> ----- cut --- cut --- cut -- > -- gpg @keyserver.linux.it: Goffredo Baroncelli <kreijackATinwind.it> Key fingerprint BBF5 1610 0B64 DAC6 5F7D 17B2 0EDA 9B37 8B82 E0B5 -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
