File cache failure with read-only volumes.

[bga]

I finally managed to get my IDE controller to work and mounted my Zeta partition under Haiku. Just to be safe, I mounted it read-only. To my surprise, I can not read any file on the volume. Here is what I get:

/Zeta 1.2/home> cat readat_test.cpp cat: readat_test.cpp: Read-only file system

Syslog shows:

KERN: file_cache: read pages failed: Read-only file system

And strace:

/Zeta 1.2/home> strace cat readat_test
[   728] _kern_image_relocated(0x681) (215 us)
[   728] --- SIGCONT (Continued) ---
[   728] _kern_create_sem(0x0, "exit stack lock") = 0x00003856 (15 us)
[   728] _kern_create_sem(0x1, "fork lock") = 0x00003857 (15 us)
[   728] _kern_get_system_info(0x7ffeecb4, 0x1fc) = 0x00000000 No error (7 us)
[   728] _kern_reserve_heap_address_range([0x18000000], 0x1, 0x48000000) = 0x00000000 No error (7 us)
[   728] _kern_create_area("heap", 0x2c29f0, 0x1, 0x32000, 0x0, 0x33) = 0x00001d6a (22 us)
[   728] _kern_acquire_sem(0x3857) = 0x00000000 No error (0 us)
[   728] _kern_release_sem(0x3857) = 0x00000000 No error (1 us)
[   728] _kern_create_sem(0x1, "env lock") = 0x00003858 (1 us)
[   728] _kern_acquire_sem(0x3857) = 0x00000000 No error (0 us)
[   728] _kern_release_sem(0x3857) = 0x00000000 No error (0 us)
[   728] _kern_create_sem(0x0, "user group") = 0x00003859 (15 us)
[   728] _kern_set_area_protection(0x1d64, 0x15) = 0x00000000 No error (14 us)
[   728] _kern_set_area_protection(0x1d66, 0x15) = 0x00000000 No error (23 us)
[   728] _kern_release_sem(0x3849) = 0x00000000 No error (0 us)
[   728] _kern_release_sem(0x3856) = 0x00000000 No error (0 us)
[   728] _kern_acquire_sem(0x3858) = 0x00000000 No error (0 us)
[   728] _kern_release_sem(0x3858) = 0x00000000 No error (0 us)
[   728] _kern_read_stat(0x1, (nil), false, 0x7ffeef3c, 0x3c) = 0x00000000 No error (1 us)
[   728] _kern_open(0xffffffff, "readat_test", 0x0, 0x0) = 0x00000003 (19 us)
[   728] _kern_read_stat(0x3, (nil), false, 0x7ffeef3c, 0x3c) = 0x00000000 No error (1 us)
[   728] _kern_read(0x3, 0xffffffffffffffff, 0x1802d000, 0x1000) = 0x80006008 (159 us)
/bin/cat: [   728] _kern_write(0x2, 0xffffffffffffffff, 0x7ffec85c, 0xa) = 0x0000000a (4 us)
readat_test[   728] _kern_write(0x2, 0xffffffffffffffff, 0x7ffec85c, 0xb) = 0x0000000b (3 us)
: Read-only file system[   728] _kern_write(0x2, 0xffffffffffffffff, 0x7ffec3fc, 0x17) = 0x00000017 (8 us)

[   728] _kern_write(0x2, 0xffffffffffffffff, 0x2b518b, 0x1) = 0x00000001 (3 us)
[   728] _kern_close(0x3) = 0x00000000 No error (2 us)
[   728] _kern_close(0x1) = 0x00000000 No error (0 us)
[   728] _kern_close(0x2) = 0x00000000 No error (1 us)
[   728] _kern_release_sem(0x3856) = 0x00000000 No error (0 us)
[   728] _kern_exit_team(0x1) (2 us)

It seems the problem is the _kern_write() calls in a read-only volume. But why does reading a file result in a write operation?

[marcusoverhagen]

I think _kern_write(0x2, ...) is a write to stdout.

[bga]

Makes sense. Still, I do not understand why I am getting a read-only error when reading a file (more specifically, reading pages from the file system.

[bga]

I tracked down this to the bfs_io() code:

static status_t bfs_io(fs_volume* _volume, fs_vnode* _node, void* _cookie, io_request* request) {

Volume* volume = (Volume*)_volume->private_volume; Inode* inode = (Inode*)_node->private_node;

if (volume->IsReadOnly())

return B_READ_ONLY_DEVICE;

if (inode->FileCache() == NULL)

RETURN_ERROR(B_BAD_VALUE);

We lock the node here and will unlock it in the "finished" hook. rw_lock_read_lock(&inode->Lock());

return do_iterative_fd_io(volume->Device(), request,

iterative_io_get_vecs_hook, iterative_io_finished_hook, inode);

}

Why does it return an error if the volume is read only? I do not know enough about this code to know if this is necessary or not but, worse case scenario, I think vfs_vnode_io() should check for this error from the io() VFS call and fall back to the synchronous version instead of immediatelly propagating this error back (with results in this bug).

[bga]

I went ahead and removed the check for read-only devices in bfs_io() in my local tree. Now I can read files from the volume even if it is read-only and I did not notice any side-effects (but then, again, I didn't do much).

[axeld]

Thanks, fixed in hrev26958.