[Xen-devel] LVM2 Snapshot Problems

[Michael Vrable <mvrable@xxxxxxxxxxx>]

Thus is recounted my quest for LVM snapshots in Xen:

As I mentioned earlier, I've attempted to get snapshots working with
LVM2 in Xen.  I haven't yet been successful, but it almost works...

What I've done so far: I'm using a recent version of Xen 2.0.
(Currently testing with the xen-unstable.tgz snapshot from last night.)
I'm running XenoLinux 2.4.26 in domain 0, with the device mapper patches
from http://sources.redhat.com/dm/ applied to the kernel.  Domain 0 is
running a mostly up-to-date Debian testing installation with the LVM2
tools installed.

I've been successfully using this setup to create and destroy LVM
logical volumes for the other guest domains, so the basics of LVM are
working.  Snapshots aren't, however.

I attempted to create a snapshot of an existing logical volume.  The
command I used was
    lvcreate -L 256M -s -n xen1-snap vg1/xen1
and it terminates with a segmentation fault.  The system logs show:

    Aug  9 17:45:17 localhost kernel: Unable to handle kernel NULL pointer 
dereference at virtual address 00000018
    Aug  9 17:45:17 localhost kernel:  printing eip:
    Aug  9 17:45:17 localhost kernel: c02c8db3
    Aug  9 17:45:17 localhost kernel: *pde=00000000(55555000)
    Aug  9 17:45:17 localhost kernel: Oops: 0002
    Aug  9 17:45:17 localhost kernel: CPU:    0
    Aug  9 17:45:17 localhost kernel: EIP:    0819:[alloc_area+99/128]    Not 
tainted
    Aug  9 17:45:17 localhost kernel: EFLAGS: 00213246
    Aug  9 17:45:17 localhost kernel: eax: 00000000   ebx: 00000000   ecx: 
00000204
      edx: 011f5000
    Aug  9 17:45:17 localhost kernel: esi: 00000002   edi: 00000000   ebp: 
c3267860
      esp: c2325e88
    Aug  9 17:45:17 localhost kernel: ds: 0821   es: 0821   ss: 0821
    Aug  9 17:45:17 localhost kernel: Process lvcreate (pid: 339, 
stackpage=c2325000)<1>
    Aug  9 17:45:17 localhost kernel: Stack: c4481000 000001f2 00000063 
c3267860 fffffff4 00000010 c2ba390c c02c95bb
    Aug  9 17:45:17 localhost kernel:        c3267860 000001f0 c2ba390c 
00000200 c2ba38c0 c4418164 c02c7c6b c2ba390c
    Aug  9 17:45:17 localhost kernel:        00000010 00000000 00000000 
00000003 c2ba38d4 50325ee4 c2ba38c0 c4418170
    Aug  9 17:45:17 localhost kernel: Call Trace: 
[dm_create_persistent+155/320] [snapshot_ctr+923/1072] 
[dm_table_add_target+249/336] [populate_table+130/224] [table_load+104/304]
    Aug  9 17:45:17 localhost kernel:    [ctl_ioctl+235/336] [table_load+0/304] 
[sys_ioctl+201/592] [system_call+47/51]
    Aug  9 17:45:17 localhost kernel:

The offending line of code is in the alloc_area function (in
drivers/md/dm-exception-store.c):

        int r = -ENOMEM;
        size_t i, len, nr_pages;
        struct page *page, *last = NULL;

        len = ps->chunk_size << SECTOR_SHIFT;

        /*
         * Allocate the chunk_size block of memory that will hold
         * a single metadata area.
         */
        ps->area = vmalloc(len);
        if (!ps->area)
                return r;

        nr_pages = sectors_to_pages(ps->chunk_size);

        /*
         * We lock the pages for ps->area into memory since
         * they'll be doing a lot of io.  We also chain them
         * together ready for dm-io.
         */
        for (i = 0; i < nr_pages; i++) {
                page = vmalloc_to_page(ps->area + (i * PAGE_SIZE));
                LockPage(page);
                if (last)
                        last->list.next = &page->list;
                last = page;
        }

LockPage is the line causing the fault, so it appears that
vmalloc_to_page is returning NULL.

I've had success using a similarly-patched 2.4.26 kernel running on the
bare hardware, not in Xen--this is where I was doing my testing of
snapshot functionality last week.

I don't know the cause of the trouble with snapshots under Xen.  It
wouldn't surprise me if it were a bug in the device mapper code that is
somehow being triggered when running under Xen.  I'm posting here in the
hope that maybe someone does have an idea what the problem is, or
failing that, to at least give an update on my attempts to use LVM
snapshots with Xen.

I'm hoping that the device mapper snapshot functionality in Linux 2.6.8,
when it is released, will work.  But I'd like to get snapshots working
with 2.4 if possible.  If I do get this working, I'll send a post with
instructions for doing so.

Is anyone else using LVM2 and Xen?

--Michael Vrable


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