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xen-devel
Re: [Xen-devel] LVM2 Snapshot Problems
Please try adding 'XEN_flush_page_update_queue()' to
mm/vmalloc.c:__vmalloc_area_pages, just before 'return 0':
.....
spin_unlock(&init_mm.page_table_lock);
flush_cache_all();
XEN_flush_page_update_queue();
return 0;
err:
.....
-- Keir
> 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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