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SubjectRe: [bisected] RAID1 direct IO redirecting sector loop since 6.0
On Wed, 2 Nov 2022 01:08:26 +0300
Dmitrii Tcvetkov <me@demsh.org> wrote:

> On Tue, 1 Nov 2022 15:18:20 -0600
> Keith Busch <kbusch@kernel.org> wrote:
> > Oh shoot, sorry about that! Should have been this:
> >
> > @@ -703,6 +702,7 @@ void disk_stack_limits(struct gendisk *disk,
> > struct block_device *bdev, pr_notice("%s: Warning: Device %pg is
> > misaligned\n", disk->disk_name, bdev);
> >
> > + blk_queue_dma_alignment(t, queue_logical_block_size(t) - 1);
> > disk_update_readahead(disk);
> > }
> > EXPORT_SYMBOL(disk_stack_limits);
>
> This didn't change behaviour, second guest still hangs.

Managed to write a program in C, which allows to reproduce this without
Qemu.

# cat test.c
#define _GNU_SOURCE

#include <stdlib.h>
#include <unistd.h>
#include <stdio.h>
#include <fcntl.h>
#include <string.h>
#include <errno.h>
#include <pthread.h>

#define THREADCOUNT 8
#define PATHLIMIT 256
#define BUFSIZE 4096

#define LV1 "/dev/lvmraid/zfs"
#define LV2 "/dev/lvmraid/wrk"

struct params {
char path[PATHLIMIT];
char buffer[BUFSIZE];
};


struct params alloc_params(char *path) {
struct params out;

if (strlen(path) >= PATHLIMIT) {
printf("supplied path too long\n");
abort();
}

strncpy(&out.path[0], path, PATHLIMIT);
memset(&out.buffer, 0, BUFSIZE);
return out;
}

void *worker(void *data) {
struct params *p = (struct params *) data;
int counter = 0;
ssize_t n = 0;

int fd = open(p->path, O_RDONLY|O_DIRECT|O_CLOEXEC);
if (fd == -1) return NULL;

while (counter < 2048) {
pread(fd, p->buffer, BUFSIZE, 0);
counter++;
}

close(fd);
return NULL;
}

int main(void) {
struct params parray[THREADCOUNT] = {
alloc_params(LV1),
alloc_params(LV1),
alloc_params(LV1),
alloc_params(LV1),
alloc_params(LV2),
alloc_params(LV2),
alloc_params(LV2),
alloc_params(LV2),
};
pthread_t threads[THREADCOUNT];

for (int i = 0; i < THREADCOUNT; i++) {
int ret = pthread_create(&threads[i], NULL, worker, (void *)
&parray[i]); if (ret!=0) {
printf("failed to create thread: %d", ret);
abort();
}
}
for (int i = 0; i < THREADCOUNT; i++) {
int ret = pthread_join(threads[i], NULL);
if(ret!=0) {
printf("failed to join thread: %d", ret);
abort();
}
}

return 0;
}

# gcc -O2 -pthread test.c
# for i in $(seq 1 64); do ./a.out;done

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