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[dm-devel] Re: bug in dm-loop? - was:Re: Re: device mapper integrated loops - and one more year !



Hi Bryn,

I'll also get the version on kernel.org updated - thanks for catching this!

thanks very much for that quick patch - it works as expected and the kernel oops went away!

since i want to use dm-loop for having a large number of loop-devices, i did some more "hardcore testing". ;)

so, i wrote a little test script which created a lot of dummy-files and turning them into a dm-loop target.

this worked very well - must have been around 3000 loop-targets when i got this one:

# device-mapper: reload ioctl failed: not enough memory available

and in dmesg:

Jan 22 22:42:06 localhost device-mapper: loop: Finalized extent map of 56 bytes, 2 entries. Jan 22 22:42:08 localhost dmlosetup: page allocation failure. order:4, mode:0xd0
Jan 22 22:42:08 localhost  [<c015e1a2>] __alloc_pages+0x272/0x2f0
Jan 22 22:42:09 localhost dmlosetup: page allocation failure. order:4, mode:0xd0
Jan 22 22:42:09 localhost  [<c015e1a2>] __alloc_pages+0x272/0x2f0
Jan 22 22:42:10 localhost dmlosetup: page allocation failure. order:4, mode:0xd0
Jan 22 22:42:10 localhost  [<c015e1a2>] __alloc_pages+0x272/0x2f0
Jan 22 22:42:10 localhost dmlosetup: page allocation failure. order:4, mode:0xd0
Jan 22 22:42:10 localhost  [<c015e1a2>] __alloc_pages+0x272/0x2f0
Jan 22 22:42:10 localhost dmlosetup: page allocation failure. order:4, mode:0xd0
Jan 22 22:42:10 localhost  [<c015e1a2>] __alloc_pages+0x272/0x2f0
Jan 22 22:42:12 localhost dmlosetup: page allocation failure. order:4, mode:0xd0 Jan 22 22:42:13 localhost dmlosetup: page allocation failure. order:4, mode:0xd0
Jan 22 22:42:13 localhost  [<c015e1a2>] __alloc_pages+0x272/0x2f0
Jan 22 22:42:13 localhost dmlosetup: page allocation failure. order:4, mode:0xd0
Jan 22 22:42:13 localhost  [<c015e1a2>] __alloc_pages+0x272/0x2f0
Jan 22 22:42:13 localhost dmlosetup: page allocation failure. order:4, mode:0xd0
Jan 22 22:42:13 localhost  [<c015e1a2>] __alloc_pages+0x272/0x2f0


this was on a system with 256 MB of ram.

i assume, that i ran out of some kernel-buffer memory.

ok, not a real problem for me since i don`t expect having that much number of iso images on my cd-roms server to be loopback mounted, but - anyway - could you probably give a comment on this ?

is this some "natural limitation" due to dm-loop or device mapper consuming kernel-memory, or could this be another bug ?

regards
roland





----- Original Message ----- From: "Bryn M. Reeves" <breeves redhat com>
To: <devzero web de>
Cc: <agk redhat com>; <dm-devel redhat com>
Sent: Monday, January 22, 2007 11:00 AM
Subject: Re: bug in dm-loop? - was:Re: Re: device mapper integrated loops - and one more year !


-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1

devzero web de wrote:

i was doing some testing of dm-loop with latest dmsetup on a 2.6.20-rc5 system.

i did some basic test, which seems to work ok.

i can now map a file to become /dev/mapper/loop0

dmlosetup loop0 test.img

-> device-mapper: loop: Finalized extent map of 1232 bytes, 44 entries.

root localhost ~ # ls -la /dev/mapper/loop0
brw------- 1 root root 252, 0 21. Jan 17:25 /dev/mapper/loop0

Great! Am glad this much is working for you!


while fiddling around a little bit, i accidentally tried to map that file a second time - anyway, i would expect this is quite ok and should work.

dmlosetup loop1 test.img

this bails out with the following error, leaving trace in dmesg:

device-mapper: loop: file is already in use: /root/device-mapper/dmsetup/test.dat BUG: unable to handle kernel NULL pointer dereference at virtual address 00000000
 printing eip:
d097589c
*pde = 00000000
Oops: 0000 [#1]

The dm-loop target only allows a file to be mapped once. That said, it
should just return the error, not oops the kernel!

This is fixed in my CVS copy of this patch - I'm attaching a patch that
you can apply on top of the one that you already have that includes this
fix plus a couple of other minor fixes/enhancements.

I'll also get the version on kernel.org updated - thanks for catching this!

after this, if i do
dmlosetup -d loop1
dmlosetup -d loop0

i cannot "rmmod dm-loop" , now telling me "device is in use", but it isn`t !

so, this looks like a bug to me, leaving dm-loop in an inconsitent state.

Unfortunately, after an oops in a kernel module you're pretty much
forced to reboot - the kernel terminates the code path that caused the
fault, leaving a hanging reference count on the module.

furthermore, one question about naming conventions :

dmlosetup testloop1 test.dat
dmlosetup: Could not parse loop_device testloop1
Usage:

losetup [-d|-a] [-e encryption] [-o offset] [-f|loop_device] [file]

Couldn't process command line.


so, it`s mandatory that loop_device needs to begin with "loop...." !?


It is at the moment :) The current command line parameters try to be as
faithful as possible to the nomrmal losetup, where devices are normally
named as (/dev/)loopN. We can't exactly match that without disrupting
users of the regular loop driver though, so this should probably be relaxed.

Thanks for testing!

Bryn.





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--------------------------------------------------------------------------------


--- drivers/md/dm-loop.c.rel 2007-01-22 09:45:04.000000000 +0000
+++ drivers/md/dm-loop.c 2007-01-22 09:45:10.000000000 +0000
@@ -22,7 +22,7 @@
#include "dm-bio-record.h"

#define DM_MSG_PREFIX "loop"
-#define LOOP_MAX_EXTENTS 1024
+#define LOOP_MAX_EXTENTS 2048

#define DMLOOP_READONLY 0x01
#define DMLOOP_SYNC 0x02
@@ -69,7 +69,7 @@ struct loop_c {
};

#ifdef CONFIG_DM_DEBUG
-static void dump_extent(struct extent *e)
+static void dump_extent(unsigned i, struct extent *e)
{
 const char types[] = { 'f', 'd' };

@@ -81,8 +81,8 @@ static void dump_extent(struct extent *e
 return;
 }

- DMDEBUG("start: %8llu len: %4llu %4c.rstart: %8llu",
- e->start, e->len, types[e->type],
+ DMDEBUG(" [%u] start: %8llu len: %4llu %4c.rstart: %8llu",
+ i, e->start, e->len, types[e->type],
 (sector_t)e->data );
}

@@ -97,7 +97,7 @@ static void dump_extent_map(struct exten
 map->nr_extents, map->cur_extent);

 for (i = 0; i < map->nr_extents; i++)
- dump_extent(DMLOOP_EXTENT(i));
+ dump_extent(i, DMLOOP_EXTENT(i));
}

#else /* CONFIG_DM_DEBUG */
@@ -228,16 +228,25 @@ reprobe:
 continue;
 }

+ if (nr_extents == LOOP_MAX_EXTENTS && probe_block < last_block){
+ DMERR("Loopfile contains more than LOOP_MAX_EXTENTS (%d) fragments\n",
+ LOOP_MAX_EXTENTS);
+ goto out_free;
+ }
+
 map->nr_extents = nr_extents;
 map->cur_extent = 0;

 DMDEBUG("created initial extent map, finalizing.");
 map = finalize_map(map);
+ if (!map)
+ return -ENOMEM;
+
+ //dump_extent_map(map);
 DMINFO("Finalized extent map of %u bytes, %d entries.",
 (map->nr_extents * sizeof(struct extent)),
 map->nr_extents);

- dump_extent_map(map);
 lc->blkbits = blkbits;
 lc->map = map;

@@ -408,7 +417,7 @@ static struct file *loop_get_file(char *
{
 struct file *filp;
 struct inode *inode;
- int r;
+ int r = 0;

 filp = filp_open(loop_path,
 ((*flags & DMLOOP_READONLY) ? O_RDONLY : O_RDWR) |
@@ -434,6 +443,7 @@ static struct file *loop_get_file(char *

 if (IS_SWAPFILE(inode)) {
 DMERR("file is already in use: %s", loop_path);
+ r = -EBUSY;
 goto out;
 }

@@ -461,7 +471,7 @@ static int loop_setup_size(struct loop_c
 lc->size = i_size_read(inode);
 lc->blkbits = inode->i_blkbits;

- if (lc->offset & (1 << lc->blkbits - 1)) {
+ if (lc->offset & ((1 << lc->blkbits) - 1)) {
 DMERR("Backing file offset of %lld bytes not a multiple of "
 "filesystem blocksize (%d)", lc->offset,
 1 << lc->blkbits);
@@ -565,8 +575,11 @@ static int loop_ctr(struct dm_target *ti
 goto out_putf;

 r = setup_loop_extents(lc);
- if (r) {
- ti->error = "Could not create extent map";
+ if (r == -EINVAL) {
+ ti->error = "Could not create extent map - invalid argument";
+ goto out_putf;
+ } else if (r == -ENOMEM) {
+ ti->error = "Could not allocate extent map";
 goto out_putf;
 }




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