* DIO Write Regression on Preallocated Extents
@ 2012-09-11 17:31 Wade Cline
2012-09-11 18:40 ` Josef Bacik
0 siblings, 1 reply; 2+ messages in thread
From: Wade Cline @ 2012-09-11 17:31 UTC (permalink / raw)
To: linux-btrfs; +Cc: cmm
Hi,
I was doing some fragmentation tests on preallocated extents on Josef's
btrfs-next branch (commit 8fe3b6) with the O_DIRECT flag enabled and
noticed some strange behavior. Writing to a preallocated extent
currently triggers a WARN_ON on in the kernel, triggers a csum error for
what appears to be each block, and causes the remaining preallocated
(and not written-to) area to read 1's.
The issue seems to have appeared in commit 8d37ef "Btrfs: improve fsync
by filtering extents that we want" but the issues started earlier in
16ecb6 "Btrfs: turbo charge fsync". Prior to 16ecb6, direct I/O on
preallocated extents was not generating false data, after 16ecb6 direct
I/O was not writing -any- data, and after 8d37ef direct I/O was
generating false data.
The tests were done using a simple program I wrote (below) which
preallocates a 128k file and performs a 4096-byte write.
Steps to reproduce:
mkfs.btrfs /dev/sdb
mount -t btrfs /dev/sdb /mnt/btrfs
cd /mnt/btrfs
gcc -o main <../main.c>
./main single direct
od -A x -t x2 testfile
It seems that the issue in part occurs in unpin_extent_cache:
WARN_ON(!em || em->start != start);
printk() shows that the function expects the extent_map for the write
performed but instead receives the extent_map for the entire
preallocated extent. The extent_map is then marked as read:
if (test_bit(EXTENT_FLAG_PREALLOC, &em->flags)) {
prealloc = true;
clear_bit(EXTENT_FLAG_PREALLOC, &em->flags);
}
which also causes fragmentation issues from further writes. I am not
sure where the 1's are generated, though; using dd to check the on-disk
contents shows that the data is uninitialized.
I don't currently have a patch for this, but thought it was important
enough to bring up. Below are the od output, the main.c code I used to
produce this error, and the kernel message. Let me know if you have any
questions.
Regards,
Wade
od output:
------------------------------
000000 0101 0101 0101 0101 0101 0101 0101 0101
*
07a000 ffff ffff ffff ffff ffff ffff ffff ffff
*
07b000 0101 0101 0101 0101 0101 0101 0101 0101
*
0f4240
main.c:
------------------------------
#define _GNU_SOURCE
#include <sys/types.h>
#include <sys/stat.h>
#include <errno.h>
#include <fcntl.h>
#include <stdint.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <unistd.h>
#define FILE_LENGTH 1000000
#define FILE_NAME "testfile"
#define BLOCK_SIZE 4096
// Modes for running the program.
enum mode {
// Write once to the middle of the file.
SINGLE_WRITE,
// Write twice to the middle of the file.
DOUBLE_WRITE,
// Write twice, sequentially, to the middle of the file.
BUDDY_WRITE,
// Write once at the 1/2 and 3/4 markers in the file.
// Obviously, the non-sequentiality is opposite of the "buddy write".
ENEMY_WRITE,
// Write once at the 1/2 and 1/4 markers in the file.
NEMESIS_WRITE
};
void print_usage() {
fprintf(stderr, "<binary file> <single|double|buddy|enemy|nemesis>
<buffered|direct|buffered-sync|direct-sync>\n\n");
exit(EXIT_FAILURE);
}
int main(int argc, char* argv[]) {
int fd;
int open_flags;
int temp;
int mode;
int i;
uint8_t* buffer;
// Validate parameters.
if (argc < 3) {
fprintf(stderr, "Expected at two arguments.\n");
print_usage();
}
// Parse first argument.
if (strcmp(argv[1], "single") == 0) {
mode = SINGLE_WRITE;
}
else if (strcmp(argv[1], "double") == 0) {
mode = DOUBLE_WRITE;
}
else if (strcmp(argv[1], "buddy") == 0) {
mode = BUDDY_WRITE;
}
else if (strcmp(argv[1], "enemy") == 0) {
mode = ENEMY_WRITE;
}
else if (strcmp(argv[1], "nemesis") == 0) {
mode = NEMESIS_WRITE;
}
else {
fprintf(stderr, "Unable to parse write mode '%s'.\n", argv[1]);
print_usage();
}
// Parse second argument.
open_flags |= O_RDWR | O_CREAT;
if (strcmp(argv[2], "buffered") == 0) {
; // Do nothing.
}
else if (strcmp(argv[2], "direct") == 0) {
open_flags |= O_DIRECT;
}
else if (strcmp(argv[2], "buffered-sync") == 0) {
open_flags |= O_SYNC;
}
else if (strcmp(argv[2], "direct-sync") == 0) {
open_flags |= O_DIRECT | O_SYNC;
}
else {
fprintf(stderr, "Unable to parse: %s.\n", argv[2]);
print_usage();
}
// Initialize the buffer.
if (posix_memalign((void**)&buffer, 4096, BLOCK_SIZE) != 0) {
perror("posix_memalign() FAILED.");
}
memset(buffer, 255, sizeof(uint8_t) * BLOCK_SIZE);
// Remove any old file.
temp = unlink(FILE_NAME);
if (temp != 0 && errno != ENOENT) {
perror("Unable to remove old file.");
}
// Create and open the file.
fd = open(FILE_NAME, open_flags, 0777);
if (fd == -1) {
perror("Unable to open file.");
}
// Pre-allocate space for the file.
if (fallocate(fd, 0, 0, FILE_LENGTH) == -1) {
perror("Unable to allocate space for file.");
}
// Seek to the middle of the file.
if (lseek(fd, (FILE_LENGTH/2) - ((FILE_LENGTH/2) % BLOCK_SIZE),
SEEK_SET) == -1) {
perror("Unable to seek to middle of file.");
}
// Write to the middle of the file.
if (write(fd, buffer, BLOCK_SIZE) < BLOCK_SIZE) {
perror("Not all content written to disk.");
}
// Do a buddy, double, or enemy write if applicable.
if (mode == BUDDY_WRITE) {
// Buddeh write.
if (write(fd, buffer, BLOCK_SIZE) < BLOCK_SIZE) {
perror("Computer does not want to be my friend.");
}
}
else if (mode == DOUBLE_WRITE) {
// Muck with buffer contents.
for (i = 0; i < BLOCK_SIZE; i++) {
buffer[i] += i;
}
// Move back.
if (lseek(fd, -BLOCK_SIZE, SEEK_CUR) == -1) {
perror("Unable to move back in file.");
}
// Double write.
if (write(fd, buffer, BLOCK_SIZE) < BLOCK_SIZE) {
perror("Unable to double-write.");
}
}
else if (mode == ENEMY_WRITE) {
// Move a quarter further into the file.
if (lseek(fd, (FILE_LENGTH/4) - ((FILE_LENGTH/4) % BLOCK_SIZE),
SEEK_CUR) == -1) {
perror("Unable to perform enemy seek.");
}
// Enemy write.
if (write(fd, buffer, BLOCK_SIZE) < BLOCK_SIZE) {
perror("Evil writing plans have been FOILED AGAIN!");
}
}
else if (mode == NEMESIS_WRITE) {
// Move a quarter back into the file.
if (lseek(fd, (FILE_LENGTH/4) - ((FILE_LENGTH/4) % BLOCK_SIZE),
SEEK_SET) == -1) {
perror("Unable to perform nemesis seek.");
}
// Nemesis write.
if (write(fd, buffer, BLOCK_SIZE) < BLOCK_SIZE) {
perror("Nemesis write phailed.\n");
}
}
// Flush contents to disk.
if (fsync(fd) == -1) {
perror("Unable to flush contents to disk.");
}
// Close the file.
if (close(fd) == -1) {
perror("Unable to close file.");
}
// Exit successfully.
exit(EXIT_SUCCESS);
}
Kernel Message:
------------------------------
[ 126.041359] Btrfs loaded
[ 126.087911] device fsid 857c1198-4429-4422-8ef3-ad453650f141 devid 1
transid 4 /dev/sdb
[ 126.211396] btrfs: disk space caching is enabled
[ 133.171808] ------------[ cut here ]------------
[ 133.171864] WARNING: at fs/btrfs/extent_map.c:258
unpin_extent_cache+0x53/0xfa [btrfs]()
[ 133.171869] Hardware name: KVM
[ 133.171871] Modules linked in: btrfs zlib_deflate libcrc32c [last
unloaded: btrfs]
[ 133.171888] Pid: 5107, comm: btrfs-endio-wri Not tainted 3.6.0-rc3+ #33
[ 133.171891] Call Trace:
[ 133.171907] [<ffffffff8103144b>] warn_slowpath_common+0x80/0x98
[ 133.171915] [<ffffffff81031478>] warn_slowpath_null+0x15/0x17
[ 133.171951] [<ffffffffa01ae725>] unpin_extent_cache+0x53/0xfa [btrfs]
[ 133.171990] [<ffffffffa01a47bf>] btrfs_finish_ordered_io+0x1e9/0x30d
[btrfs]
[ 133.171999] [<ffffffff8103eebb>] ? del_timer_sync+0x3c/0x49
[ 133.172043] [<ffffffffa01a48f3>] finish_ordered_fn+0x10/0x12 [btrfs]
[ 133.172179] [<ffffffffa01bf12f>] worker_loop+0xaa/0x36b [btrfs]
[ 133.172235] [<ffffffffa01bf085>] ? btrfs_queue_worker+0x15b/0x15b
[btrfs]
[ 133.172280] [<ffffffffa01bf085>] ? btrfs_queue_worker+0x15b/0x15b
[btrfs]
[ 133.172304] [<ffffffff8104d1a0>] kthread+0x6a/0x72
[ 133.172330] [<ffffffff8153eff4>] kernel_thread_helper+0x4/0x10
[ 133.172353] [<ffffffff8104d136>] ?
kthread_freezable_should_stop+0x46/0x46
[ 133.172370] [<ffffffff8153eff0>] ? gs_change+0xb/0xb
[ 133.172376] ---[ end trace b2a0a5d7740db515 ]---
[ 136.140871] btrfs no csum found for inode 257 start 0
[ 136.140880] btrfs no csum found for inode 257 start 4096
[ 136.140884] btrfs no csum found for inode 257 start 8192
[ 136.140888] btrfs no csum found for inode 257 start 12288
[ 136.154769] btrfs csum failed ino 257 off 0 csum 1816078486 private 0
[ 136.154803] btrfs csum failed ino 257 off 4096 csum 2566472073 private 0
[ 136.154838] btrfs csum failed ino 257 off 8192 csum 2566472073 private 0
[ 136.154873] btrfs csum failed ino 257 off 12288 csum 2566472073
private 0
[ 136.155565] btrfs no csum found for inode 257 start 16384
[ 136.155573] btrfs no csum found for inode 257 start 20480
[ 136.155577] btrfs no csum found for inode 257 start 24576
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[ 136.157315] btrfs no csum found for inode 257 start 331776
[ 136.157319] btrfs no csum found for inode 257 start 335872
[ 136.157324] btrfs no csum found for inode 257 start 339968
[ 136.169414] btrfs csum failed ino 257 off 16384 csum 2566472073
private 0
[ 136.169434] btrfs csum failed ino 257 off 20480 csum 2566472073
private 0
[ 136.169440] btrfs csum failed ino 257 off 24576 csum 2566472073
private 0
[ 136.169445] btrfs csum failed ino 257 off 28672 csum 2566472073
private 0
[ 136.169461] btrfs csum failed ino 257 off 32768 csum 2566472073
private 0
[ 136.169466] btrfs csum failed ino 257 off 36864 csum 2566472073
private 0
[ 136.170275] btrfs no csum found for inode 257 start 344064
[ 136.170277] btrfs no csum found for inode 257 start 348160
[ 136.170278] btrfs no csum found for inode 257 start 352256
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[ 136.170281] btrfs no csum found for inode 257 start 364544
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[ 136.170292] btrfs no csum found for inode 257 start 409600
[ 136.170293] btrfs no csum found for inode 257 start 413696
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[ 136.170297] btrfs no csum found for inode 257 start 430080
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[ 136.170299] btrfs no csum found for inode 257 start 438272
[ 136.170316] btrfs no csum found for inode 257 start 442368
[ 136.170321] btrfs no csum found for inode 257 start 446464
[ 136.170325] btrfs no csum found for inode 257 start 450560
[ 136.170329] btrfs no csum found for inode 257 start 454656
[ 136.170334] btrfs no csum found for inode 257 start 458752
[ 136.170338] btrfs no csum found for inode 257 start 462848
[ 136.170342] btrfs no csum found for inode 257 start 466944
[ 136.170347] btrfs no csum found for inode 257 start 471040
[ 136.170351] btrfs no csum found for inode 257 start 475136
[ 136.170356] btrfs no csum found for inode 257 start 479232
[ 136.170360] btrfs no csum found for inode 257 start 483328
[ 136.170364] btrfs no csum found for inode 257 start 487424
[ 136.170369] btrfs no csum found for inode 257 start 491520
[ 136.170373] btrfs no csum found for inode 257 start 495616
[ 136.170381] btrfs no csum found for inode 257 start 503808
[ 136.170386] btrfs no csum found for inode 257 start 507904
[ 136.170390] btrfs no csum found for inode 257 start 512000
[ 136.170395] btrfs no csum found for inode 257 start 516096
[ 136.170399] btrfs no csum found for inode 257 start 520192
[ 136.170404] btrfs no csum found for inode 257 start 524288
[ 136.170408] btrfs no csum found for inode 257 start 528384
[ 136.170413] btrfs no csum found for inode 257 start 532480
[ 136.170418] btrfs no csum found for inode 257 start 536576
[ 136.170422] btrfs no csum found for inode 257 start 540672
[ 136.170427] btrfs no csum found for inode 257 start 544768
[ 136.170431] btrfs no csum found for inode 257 start 548864
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[ 136.170441] btrfs no csum found for inode 257 start 557056
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[ 136.170454] btrfs no csum found for inode 257 start 569344
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[ 136.170464] btrfs no csum found for inode 257 start 577536
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[ 136.170514] btrfs no csum found for inode 257 start 622592
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[ 136.170532] btrfs no csum found for inode 257 start 638976
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[ 136.170615] btrfs no csum found for inode 257 start 712704
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[ 136.170624] btrfs no csum found for inode 257 start 720896
[ 136.170629] btrfs no csum found for inode 257 start 724992
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[ 136.170642] btrfs no csum found for inode 257 start 737280
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[ 136.170670] btrfs no csum found for inode 257 start 761856
[ 136.170674] btrfs no csum found for inode 257 start 765952
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[ 136.170684] btrfs no csum found for inode 257 start 774144
[ 136.170688] btrfs no csum found for inode 257 start 778240
[ 136.170693] btrfs no csum found for inode 257 start 782336
[ 136.170697] btrfs no csum found for inode 257 start 786432
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[ 136.170781] btrfs no csum found for inode 257 start 843776
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[ 136.170801] btrfs no csum found for inode 257 start 860160
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^ permalink raw reply [flat|nested] 2+ messages in thread
* Re: DIO Write Regression on Preallocated Extents
2012-09-11 17:31 DIO Write Regression on Preallocated Extents Wade Cline
@ 2012-09-11 18:40 ` Josef Bacik
0 siblings, 0 replies; 2+ messages in thread
From: Josef Bacik @ 2012-09-11 18:40 UTC (permalink / raw)
To: Wade Cline; +Cc: linux-btrfs@vger.kernel.org, cmm@linux.vnet.ibm.com
On Tue, Sep 11, 2012 at 11:31:23AM -0600, Wade Cline wrote:
> Hi,
>
> I was doing some fragmentation tests on preallocated extents on Josef's
> btrfs-next branch (commit 8fe3b6) with the O_DIRECT flag enabled and
> noticed some strange behavior. Writing to a preallocated extent
> currently triggers a WARN_ON on in the kernel, triggers a csum error for
> what appears to be each block, and causes the remaining preallocated
> (and not written-to) area to read 1's.
>
> The issue seems to have appeared in commit 8d37ef "Btrfs: improve fsync
> by filtering extents that we want" but the issues started earlier in
> 16ecb6 "Btrfs: turbo charge fsync". Prior to 16ecb6, direct I/O on
> preallocated extents was not generating false data, after 16ecb6 direct
> I/O was not writing -any- data, and after 8d37ef direct I/O was
> generating false data.
>
> The tests were done using a simple program I wrote (below) which
> preallocates a 128k file and performs a 4096-byte write.
>
> Steps to reproduce:
>
> mkfs.btrfs /dev/sdb
> mount -t btrfs /dev/sdb /mnt/btrfs
> cd /mnt/btrfs
> gcc -o main <../main.c>
> ./main single direct
> od -A x -t x2 testfile
>
> It seems that the issue in part occurs in unpin_extent_cache:
>
> WARN_ON(!em || em->start != start);
>
> printk() shows that the function expects the extent_map for the write
> performed but instead receives the extent_map for the entire
> preallocated extent. The extent_map is then marked as read:
>
> if (test_bit(EXTENT_FLAG_PREALLOC, &em->flags)) {
> prealloc = true;
> clear_bit(EXTENT_FLAG_PREALLOC, &em->flags);
> }
>
> which also causes fragmentation issues from further writes. I am not
> sure where the 1's are generated, though; using dd to check the on-disk
> contents shows that the data is uninitialized.
>
> I don't currently have a patch for this, but thought it was important
> enough to bring up. Below are the od output, the main.c code I used to
> produce this error, and the kernel message. Let me know if you have any
> questions.
Heh oops sorry about that, I know what's wrong and I thought about it when I was
writing these patches but then I never went back to check to make sure DIO did
the right thing. I will fix it up and run your test to make sure I got it
right. Thanks,
Josef
^ permalink raw reply [flat|nested] 2+ messages in thread
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2012-09-11 17:31 DIO Write Regression on Preallocated Extents Wade Cline
2012-09-11 18:40 ` Josef Bacik
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