* Intermittent kernel WARN in mm.h get_page() when forcefully removing a xen_netfront interface from xenstore
@ 2026-07-15 3:49 Aaron Rainbolt
2026-07-15 11:02 ` Pedro Falcato
0 siblings, 1 reply; 3+ messages in thread
From: Aaron Rainbolt @ 2026-07-15 3:49 UTC (permalink / raw)
To: xen-devel; +Cc: jgross, linux-kernel, linux-mm, marmarek, adrelanos
[-- Attachment #1: Type: text/plain, Size: 10815 bytes --]
A user in the Qubes OS issue tracker reported that they could cause
kernel WARNs in some of their AppVMs when forcibly terminating a NetVM
that those AppVMs were connected to. [1] Typically, a message such as
`vif vif-0: xenbus: device forcefully removed from xenstore` appears,
then a "WARNING" appears immediately thereafter indicating an issue in
`xennet_disconnect_backend`, with the line of code throwing the warning
being somewhere in `include/linux/mm.h`. Originally it was thought that
the warning was simply normal behavior when a device is forcefully
removed, but it seemed very weird that memory management code would be
in charge of triggering that warning. Furthermore, I cannot reproduce
the issue reliably; for some users, it happens immediately after
shutting down the NetVM, whereas for me, I had to use multiple tricks
including pausing and unpausing a VM to trigger it. After some
fiddling, I was able to reproduce the issue using the latest stable
7.1.3 kernel:
1. On a Whonix-Workstation 18 AppVM (such as anon-whonix), build kernel
7.1.3 using the steps from
https://kernel-team.pages.debian.net/kernel-handbook/ch-common-tasks.html#s-common-size,
ensuring that debug info is enabled. Use Debian's kernel config from
the linux-base-7.1.3+deb14-amd64_7.1.3-1_amd64.deb package as the
starting kernel config.
2. Install the new kernel into a cloned Whonix-Workstation 18 template.
3. Install pvgrub in dom0: `sudo qubes-dom0-update --action=install
grub2-xen-pvh`
4. Create a new AppVM named `anon-whonix-kerntest` based on this
template.
5. Set the kernel for `anon-whonix-kerntest` to `pvgrub2-pvh`.
6. Start `anon-whonix-kerntest`.
7. Run `uname -r` in `anon-whonix-kerntest`, ensure that the kernel
version displayed is 7.1.3.
8. Open a root QTerminal window in `anon-whonix-kerntest` by running
the following in dom0: `qvm-run -u root anon-whonix-kerntest
qterminal`.
9. In the root QTerminal window, run `sysctl kernel.warn_limit=5` to
prevent an immediate kernel panic when the kernel warn occurs.
10. Open Tor Browser in `anon-whonix-kerntest`, and attempt to navigate
to a fairly resource-intensive web page like Reddit's front page.
11. While the page is still loading, run `qvm-pause
anon-whonix-kerntest` in dom0 to freeze the AppVM.
12. In dom0, run `qvm-kill sys-whonix`.
13. Wait a few seconds, then run `qvm-unpause anon-whonix-kerntest` to
unfreeze the AppVM.
14. In the root QTerminal window of `anon-whonix-kerntest`, run `dmesg
-w` to watch for kernel messages.
15. Wait a few seconds, then start `sys-whonix`. Qubes OS will reattach
the network when this is done (there is the equivalent of `xl
network-attach` done by the Qubes code automatically). The following
messages should appear in dmesg:
[ 426.609004] vif vif-0: xenbus: device forcefully removed from xenstore
[ 426.609242] ------------[ cut here ]------------
[ 426.609254] WARNING: ./include/linux/mm.h:2097 at xennet_disconnect_backend+0x1c7/0x520 [xen_netfront], CPU#0: xenwatch/64
[ 426.609269] Modules linked in: snd_seq_dummy(E) snd_hrtimer(E) snd_seq(E) snd_seq_device(E) snd_timer(E) snd(E) soundcore(E) cfg80211(E) nls_ascii(E) nls_cp437(E) vfat(E) fat(E) rfkill(E) xenfs(E) nft_reject_inet(E) nf_reject_ipv4(E) nf_reject_ipv6(E) nft_reject(E) nft_ct(E) nf_conntrack(E) nf_defrag_ipv6(E) nf_defrag_ipv4(E) nf_tables(E) binfmt_misc(E) intel_rapl_msr(E) intel_rapl_common(E) intel_uncore_frequency_common(E) intel_pmc_ssram_telemetry(E) intel_vsec(E) aesni_intel(E) gf128mul(E) xen_netfront(E) xen_privcmd(E) xen_gntdev(E) xen_gntalloc(E) xen_blkback(E) drm(E) xen_evtchn(E) jitterentropy_rng(E) libsha3(E) loop(E) configfs(E) efi_pstore(E) nfnetlink(E) ip_tables(E) x_tables(E) autofs4(E) ext4(E) crc16(E) mbcache(E) jbd2(E) dm_snapshot(E) dm_bufio(E) dm_mod(E) xen_blkfront(E)
[ 426.609334] CPU: 0 UID: 0 PID: 64 Comm: xenwatch Tainted: G E 7.1.3 #5 PREEMPT(lazy)
[ 426.609340] Tainted: [E]=UNSIGNED_MODULE
[ 426.609343] RIP: 0010:xennet_disconnect_backend+0x1c7/0x520 [xen_netfront]
[ 426.609349] Code: 94 dd a0 10 00 00 48 8b 4a 08 48 89 ce 83 e6 01 48 83 ee 01 48 09 f1 48 21 ca 0f b6 4a 33 81 f9 f5 00 00 00 0f 85 ea fe ff ff <0f> 0b 49 81 ff 00 01 00 00 0f 82 f8 fe ff ff 4c 89 fe 48 c7 c7 e0
[ 426.609352] RSP: 0018:ffffce3d810c3c98 EFLAGS: 00010246
[ 426.609356] RAX: 0000000000000010 RBX: 0000000000000000 RCX: 00000000000000f5
[ 426.609358] RDX: fffffb0d80ca9800 RSI: 0000000000000000 RDI: ffffffff88c8a8a0
[ 426.609360] RBP: ffff8dec51df0000 R08: ffff8debf4dd5a80 R09: ffff8debf4dd5a80
[ 426.609362] R10: 0000000000000020 R11: 00000000000000bf R12: 0000000000000000
[ 426.609364] R13: ffff8dec8ef30b80 R14: ffff8dec197fcac0 R15: 0000000000000000
[ 426.609366] FS: 0000000000000000(0000) GS:ffff8ded2d129000(0000) knlGS:0000000000000000
[ 426.609369] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 426.609370] CR2: 00005faa1a7d3737 CR3: 00000000b420a003 CR4: 0000000000770ef0
[ 426.609377] PKRU: 55555554
[ 426.609379] Call Trace:
[ 426.609384] <TASK>
[ 426.609395] xennet_remove+0x9f/0x380 [xen_netfront]
[ 426.609399] ? kfree+0x26e/0x3d0
[ 426.609407] xenbus_dev_remove+0x71/0x100
[ 426.609414] device_release_driver_internal+0x19c/0x200
[ 426.609419] bus_remove_device+0xe8/0x1c0
[ 426.609425] ? device_remove_attrs+0xa1/0x100
[ 426.609430] device_del+0x160/0x3e0
[ 426.609433] ? __pfx_xenwatch_thread+0x10/0x10
[ 426.609436] device_unregister+0x17/0x70
[ 426.609439] xenbus_dev_changed.cold+0x3e/0x5b
[ 426.609445] ? __pfx_xenwatch_thread+0x10/0x10
[ 426.609447] xenwatch_thread+0x92/0x1c0
[ 426.609451] ? __pfx_autoremove_wake_function+0x10/0x10
[ 426.609466] kthread+0xe3/0x120
[ 426.609470] ? __pfx_kthread+0x10/0x10
[ 426.609473] ret_from_fork+0x2ba/0x340
[ 426.609478] ? __pfx_kthread+0x10/0x10
[ 426.609480] ret_from_fork_asm+0x1a/0x30
[ 426.609491] </TASK>
[ 426.609493] ---[ end trace 0000000000000000 ]---
(Note that I have not been able to reproduce the issue reliably; these
steps worked for me once, but they may not work every time.)
`./include/linux/mm.h:2097` looks like this:
```
static inline void get_page(struct page *page)
{
struct folio *folio = page_folio(page);
if (WARN_ON_ONCE(folio_test_slab(folio))) /* <-- 2097 */
return;
if (WARN_ON_ONCE(folio_test_large_kmalloc(folio)))
return;
folio_get(folio);
}
```
Following the stack trace, all of these line numbers should be accurate
if working with an unmodified 7.1.3 kernel tree:
$ scripts/faddr2line ./drivers/net/xen-netfront.ko xennet_disconnect_backend+0x1c7/0x520
xennet_disconnect_backend+0x1c7/0x520:
get_page at /home/user/linux-7.1.3/./include/linux/mm.h:2097 (discriminator 1)
(inlined by) xennet_release_tx_bufs at /home/user/linux-7.1.3/drivers/net/xen-netfront.c:1435 (discriminator 1)
(inlined by) xennet_disconnect_backend at /home/user/linux-7.1.3/drivers/net/xen-netfront.c:1839 (discriminator 1)
$ scripts/faddr2line ./drivers/net/xen-netfront.ko xennet_remove+0x9f/0x380
xennet_remove+0x9f/0x380:
xennet_remove at /home/user/linux-7.1.3/drivers/net/xen-netfront.c:2664
$ scripts/faddr2line ./vmlinux kfree+0x26e/0x3d0
kfree+0x26e/0x3d0:
stat at mm/slub.c:383
(inlined by) slab_free at mm/slub.c:6258
(inlined by) kfree at mm/slub.c:6566
$ scripts/faddr2line ./vmlinux xenbus_dev_remove+0x71/0x100
xenbus_dev_remove+0x71/0x100:
xenbus_dev_remove at drivers/xen/xenbus/xenbus_probe.c:353
$ scripts/faddr2line ./vmlinux device_release_driver_internal+0x19c/0x200
device_release_driver_internal+0x19c/0x200:
__device_release_driver at drivers/base/dd.c:1354
(inlined by) device_release_driver_internal at drivers/base/dd.c:1375
$ scripts/faddr2line ./vmlinux bus_remove_device+0xe8/0x1c0
bus_remove_device+0xe8/0x1c0:
kset_put at include/linux/kobject.h:193
(inlined by) subsys_put at drivers/base/base.h:74
(inlined by) bus_remove_device at drivers/base/bus.c:671
$ scripts/faddr2line ./vmlinux device_remove_attrs+0xa1/0x100
device_remove_attrs+0xa1/0x100:
device_remove_attrs at drivers/base/core.c:2976
$ scripts/faddr2line ./vmlinux device_del+0x160/0x3e0
device_del+0x160/0x3e0:
device_del at drivers/base/core.c:3896
$ scripts/faddr2line ./vmlinux __pfx_xenwatch_thread+0x10/0x10
__pfx_xenwatch_thread+0x10/0x10:
xenwatch_thread at drivers/xen/xenbus/xenbus_xs.c:844
$ scripts/faddr2line ./vmlinux device_unregister+0x17/0x70
device_unregister+0x17/0x70:
put_device at drivers/base/core.c:3813
(inlined by) device_unregister at drivers/base/core.c:3937
$ scripts/faddr2line ./vmlinux xenbus_dev_changed.cold+0x3e/0x5b
xenbus_dev_changed.cold+0x3e/0x5b:
xenbus_cleanup_devices at drivers/xen/xenbus/xenbus_probe.c:451
(inlined by) xenbus_dev_changed at drivers/xen/xenbus/xenbus_probe.c:648
$ scripts/faddr2line ./vmlinux __pfx_xenwatch_thread+0x10/0x10
__pfx_xenwatch_thread+0x10/0x10:
xenwatch_thread at drivers/xen/xenbus/xenbus_xs.c:844
$ scripts/faddr2line ./vmlinux xenwatch_thread+0x92/0x1c0
xenwatch_thread+0x92/0x1c0:
xenwatch_thread at drivers/xen/xenbus/xenbus_xs.c:870
$ scripts/faddr2line ./vmlinux __pfx_autoremove_wake_function+0x10/0x10
__pfx_autoremove_wake_function+0x10/0x10:
autoremove_wake_function at kernel/sched/wait.c:402
$ scripts/faddr2line ./vmlinux kthread+0xe3/0x120
kthread+0xe3/0x120:
kthread at kernel/kthread.c:436
$ scripts/faddr2line ./vmlinux __pfx_kthread+0x10/0x10
__pfx_kthread+0x10/0x10:
kthread at kernel/kthread.c:381
$ scripts/faddr2line ./vmlinux ret_from_fork+0x2ba/0x340
ret_from_fork+0x2ba/0x340:
ret_from_fork at arch/x86/kernel/process.c:164
$ scripts/faddr2line ./vmlinux __pfx_kthread+0x10/0x10
__pfx_kthread+0x10/0x10:
kthread at kernel/kthread.c:381
$ scripts/faddr2line ./vmlinux ret_from_fork_asm+0x1a/0x30
ret_from_fork_asm+0x1a/0x30:
ret_from_fork_asm at arch/x86/entry/entry_64.S:255
--
Aaron
[1] https://github.com/QubesOS/qubes-issues/issues/10969
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^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: Intermittent kernel WARN in mm.h get_page() when forcefully removing a xen_netfront interface from xenstore
2026-07-15 3:49 Intermittent kernel WARN in mm.h get_page() when forcefully removing a xen_netfront interface from xenstore Aaron Rainbolt
@ 2026-07-15 11:02 ` Pedro Falcato
2026-07-15 11:16 ` Jürgen Groß
0 siblings, 1 reply; 3+ messages in thread
From: Pedro Falcato @ 2026-07-15 11:02 UTC (permalink / raw)
To: Aaron Rainbolt
Cc: xen-devel, jgross, linux-kernel, linux-mm, marmarek, adrelanos,
Matthew Wilcox, Vlastimil Babka
+Cc Willy, Vlastimil
On Tue, Jul 14, 2026 at 11:49:12PM -0400, Aaron Rainbolt wrote:
> A user in the Qubes OS issue tracker reported that they could cause
> kernel WARNs in some of their AppVMs when forcibly terminating a NetVM
> that those AppVMs were connected to. [1] Typically, a message such as
> `vif vif-0: xenbus: device forcefully removed from xenstore` appears,
> then a "WARNING" appears immediately thereafter indicating an issue in
> `xennet_disconnect_backend`, with the line of code throwing the warning
> being somewhere in `include/linux/mm.h`. Originally it was thought that
> the warning was simply normal behavior when a device is forcefully
> removed, but it seemed very weird that memory management code would be
> in charge of triggering that warning. Furthermore, I cannot reproduce
> the issue reliably; for some users, it happens immediately after
> shutting down the NetVM, whereas for me, I had to use multiple tricks
> including pausing and unpausing a VM to trigger it. After some
> fiddling, I was able to reproduce the issue using the latest stable
> 7.1.3 kernel:
>
> 1. On a Whonix-Workstation 18 AppVM (such as anon-whonix), build kernel
> 7.1.3 using the steps from
> https://kernel-team.pages.debian.net/kernel-handbook/ch-common-tasks.html#s-common-size,
> ensuring that debug info is enabled. Use Debian's kernel config from
> the linux-base-7.1.3+deb14-amd64_7.1.3-1_amd64.deb package as the
> starting kernel config.
> 2. Install the new kernel into a cloned Whonix-Workstation 18 template.
> 3. Install pvgrub in dom0: `sudo qubes-dom0-update --action=install
> grub2-xen-pvh`
> 4. Create a new AppVM named `anon-whonix-kerntest` based on this
> template.
> 5. Set the kernel for `anon-whonix-kerntest` to `pvgrub2-pvh`.
> 6. Start `anon-whonix-kerntest`.
> 7. Run `uname -r` in `anon-whonix-kerntest`, ensure that the kernel
> version displayed is 7.1.3.
> 8. Open a root QTerminal window in `anon-whonix-kerntest` by running
> the following in dom0: `qvm-run -u root anon-whonix-kerntest
> qterminal`.
> 9. In the root QTerminal window, run `sysctl kernel.warn_limit=5` to
> prevent an immediate kernel panic when the kernel warn occurs.
> 10. Open Tor Browser in `anon-whonix-kerntest`, and attempt to navigate
> to a fairly resource-intensive web page like Reddit's front page.
> 11. While the page is still loading, run `qvm-pause
> anon-whonix-kerntest` in dom0 to freeze the AppVM.
> 12. In dom0, run `qvm-kill sys-whonix`.
> 13. Wait a few seconds, then run `qvm-unpause anon-whonix-kerntest` to
> unfreeze the AppVM.
> 14. In the root QTerminal window of `anon-whonix-kerntest`, run `dmesg
> -w` to watch for kernel messages.
> 15. Wait a few seconds, then start `sys-whonix`. Qubes OS will reattach
> the network when this is done (there is the equivalent of `xl
> network-attach` done by the Qubes code automatically). The following
> messages should appear in dmesg:
>
> [ 426.609004] vif vif-0: xenbus: device forcefully removed from xenstore
> [ 426.609242] ------------[ cut here ]------------
> [ 426.609254] WARNING: ./include/linux/mm.h:2097 at xennet_disconnect_backend+0x1c7/0x520 [xen_netfront], CPU#0: xenwatch/64
> [ 426.609269] Modules linked in: snd_seq_dummy(E) snd_hrtimer(E) snd_seq(E) snd_seq_device(E) snd_timer(E) snd(E) soundcore(E) cfg80211(E) nls_ascii(E) nls_cp437(E) vfat(E) fat(E) rfkill(E) xenfs(E) nft_reject_inet(E) nf_reject_ipv4(E) nf_reject_ipv6(E) nft_reject(E) nft_ct(E) nf_conntrack(E) nf_defrag_ipv6(E) nf_defrag_ipv4(E) nf_tables(E) binfmt_misc(E) intel_rapl_msr(E) intel_rapl_common(E) intel_uncore_frequency_common(E) intel_pmc_ssram_telemetry(E) intel_vsec(E) aesni_intel(E) gf128mul(E) xen_netfront(E) xen_privcmd(E) xen_gntdev(E) xen_gntalloc(E) xen_blkback(E) drm(E) xen_evtchn(E) jitterentropy_rng(E) libsha3(E) loop(E) configfs(E) efi_pstore(E) nfnetlink(E) ip_tables(E) x_tables(E) autofs4(E) ext4(E) crc16(E) mbcache(E) jbd2(E) dm_snapshot(E) dm_bufio(E) dm_mod(E) xen_blkfront(E)
> [ 426.609334] CPU: 0 UID: 0 PID: 64 Comm: xenwatch Tainted: G E 7.1.3 #5 PREEMPT(lazy)
> [ 426.609340] Tainted: [E]=UNSIGNED_MODULE
> [ 426.609343] RIP: 0010:xennet_disconnect_backend+0x1c7/0x520 [xen_netfront]
> [ 426.609349] Code: 94 dd a0 10 00 00 48 8b 4a 08 48 89 ce 83 e6 01 48 83 ee 01 48 09 f1 48 21 ca 0f b6 4a 33 81 f9 f5 00 00 00 0f 85 ea fe ff ff <0f> 0b 49 81 ff 00 01 00 00 0f 82 f8 fe ff ff 4c 89 fe 48 c7 c7 e0
> [ 426.609352] RSP: 0018:ffffce3d810c3c98 EFLAGS: 00010246
> [ 426.609356] RAX: 0000000000000010 RBX: 0000000000000000 RCX: 00000000000000f5
> [ 426.609358] RDX: fffffb0d80ca9800 RSI: 0000000000000000 RDI: ffffffff88c8a8a0
> [ 426.609360] RBP: ffff8dec51df0000 R08: ffff8debf4dd5a80 R09: ffff8debf4dd5a80
> [ 426.609362] R10: 0000000000000020 R11: 00000000000000bf R12: 0000000000000000
> [ 426.609364] R13: ffff8dec8ef30b80 R14: ffff8dec197fcac0 R15: 0000000000000000
> [ 426.609366] FS: 0000000000000000(0000) GS:ffff8ded2d129000(0000) knlGS:0000000000000000
> [ 426.609369] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [ 426.609370] CR2: 00005faa1a7d3737 CR3: 00000000b420a003 CR4: 0000000000770ef0
> [ 426.609377] PKRU: 55555554
> [ 426.609379] Call Trace:
This is due (or looks like it...) to
static netdev_tx_t xennet_start_xmit(struct sk_buff *skb, struct net_device *dev)
[...]
page = virt_to_page(skb->data); /* skb->data is a slab kmallocation */
/* ... */
info.page = page;
first_tx = xennet_make_first_txreq(&info, offset, len);
/* ... */
queue->grant_tx_page[i] = page;
and then later
static void xennet_disconnect_backend(struct netfront_info *info)
/* ... */
xennet_release_tx_bufs
get_page(queue->grant_tx_page[i]);
gnttab_end_foreign_access(queue->grant_tx_ref[i],
queue->grant_tx_page[i]);
which may trigger get_page() on a slab page:
static inline void get_page(struct page *page)
{
struct folio *folio = page_folio(page);
if (WARN_ON_ONCE(folio_test_slab(folio)))
return;
if (WARN_ON_ONCE(folio_test_large_kmalloc(folio)))
return;
folio_get(folio);
}
and there's your WARN. The code as-is is wrong. get_page() does not stabilize
slab allocations, and the skb is freed right after, which may very well free
the data. This _can_ make sense for other page frags, but I would simply defer
freeing the skb until either the full skb is xmitted, or you successfully
cancel the whole transfer (what I understand is the "foreign access" in this
code). The skb itself should pin the pages.
--
Pedro
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: Intermittent kernel WARN in mm.h get_page() when forcefully removing a xen_netfront interface from xenstore
2026-07-15 11:02 ` Pedro Falcato
@ 2026-07-15 11:16 ` Jürgen Groß
0 siblings, 0 replies; 3+ messages in thread
From: Jürgen Groß @ 2026-07-15 11:16 UTC (permalink / raw)
To: Pedro Falcato, Aaron Rainbolt
Cc: xen-devel, linux-kernel, linux-mm, marmarek, adrelanos,
Matthew Wilcox, Vlastimil Babka
[-- Attachment #1.1.1: Type: text/plain, Size: 7099 bytes --]
On 15.07.26 13:02, Pedro Falcato wrote:
> +Cc Willy, Vlastimil
>
> On Tue, Jul 14, 2026 at 11:49:12PM -0400, Aaron Rainbolt wrote:
>> A user in the Qubes OS issue tracker reported that they could cause
>> kernel WARNs in some of their AppVMs when forcibly terminating a NetVM
>> that those AppVMs were connected to. [1] Typically, a message such as
>> `vif vif-0: xenbus: device forcefully removed from xenstore` appears,
>> then a "WARNING" appears immediately thereafter indicating an issue in
>> `xennet_disconnect_backend`, with the line of code throwing the warning
>> being somewhere in `include/linux/mm.h`. Originally it was thought that
>> the warning was simply normal behavior when a device is forcefully
>> removed, but it seemed very weird that memory management code would be
>> in charge of triggering that warning. Furthermore, I cannot reproduce
>> the issue reliably; for some users, it happens immediately after
>> shutting down the NetVM, whereas for me, I had to use multiple tricks
>> including pausing and unpausing a VM to trigger it. After some
>> fiddling, I was able to reproduce the issue using the latest stable
>> 7.1.3 kernel:
>>
>> 1. On a Whonix-Workstation 18 AppVM (such as anon-whonix), build kernel
>> 7.1.3 using the steps from
>> https://kernel-team.pages.debian.net/kernel-handbook/ch-common-tasks.html#s-common-size,
>> ensuring that debug info is enabled. Use Debian's kernel config from
>> the linux-base-7.1.3+deb14-amd64_7.1.3-1_amd64.deb package as the
>> starting kernel config.
>> 2. Install the new kernel into a cloned Whonix-Workstation 18 template.
>> 3. Install pvgrub in dom0: `sudo qubes-dom0-update --action=install
>> grub2-xen-pvh`
>> 4. Create a new AppVM named `anon-whonix-kerntest` based on this
>> template.
>> 5. Set the kernel for `anon-whonix-kerntest` to `pvgrub2-pvh`.
>> 6. Start `anon-whonix-kerntest`.
>> 7. Run `uname -r` in `anon-whonix-kerntest`, ensure that the kernel
>> version displayed is 7.1.3.
>> 8. Open a root QTerminal window in `anon-whonix-kerntest` by running
>> the following in dom0: `qvm-run -u root anon-whonix-kerntest
>> qterminal`.
>> 9. In the root QTerminal window, run `sysctl kernel.warn_limit=5` to
>> prevent an immediate kernel panic when the kernel warn occurs.
>> 10. Open Tor Browser in `anon-whonix-kerntest`, and attempt to navigate
>> to a fairly resource-intensive web page like Reddit's front page.
>> 11. While the page is still loading, run `qvm-pause
>> anon-whonix-kerntest` in dom0 to freeze the AppVM.
>> 12. In dom0, run `qvm-kill sys-whonix`.
>> 13. Wait a few seconds, then run `qvm-unpause anon-whonix-kerntest` to
>> unfreeze the AppVM.
>> 14. In the root QTerminal window of `anon-whonix-kerntest`, run `dmesg
>> -w` to watch for kernel messages.
>> 15. Wait a few seconds, then start `sys-whonix`. Qubes OS will reattach
>> the network when this is done (there is the equivalent of `xl
>> network-attach` done by the Qubes code automatically). The following
>> messages should appear in dmesg:
>>
>> [ 426.609004] vif vif-0: xenbus: device forcefully removed from xenstore
>> [ 426.609242] ------------[ cut here ]------------
>> [ 426.609254] WARNING: ./include/linux/mm.h:2097 at xennet_disconnect_backend+0x1c7/0x520 [xen_netfront], CPU#0: xenwatch/64
>> [ 426.609269] Modules linked in: snd_seq_dummy(E) snd_hrtimer(E) snd_seq(E) snd_seq_device(E) snd_timer(E) snd(E) soundcore(E) cfg80211(E) nls_ascii(E) nls_cp437(E) vfat(E) fat(E) rfkill(E) xenfs(E) nft_reject_inet(E) nf_reject_ipv4(E) nf_reject_ipv6(E) nft_reject(E) nft_ct(E) nf_conntrack(E) nf_defrag_ipv6(E) nf_defrag_ipv4(E) nf_tables(E) binfmt_misc(E) intel_rapl_msr(E) intel_rapl_common(E) intel_uncore_frequency_common(E) intel_pmc_ssram_telemetry(E) intel_vsec(E) aesni_intel(E) gf128mul(E) xen_netfront(E) xen_privcmd(E) xen_gntdev(E) xen_gntalloc(E) xen_blkback(E) drm(E) xen_evtchn(E) jitterentropy_rng(E) libsha3(E) loop(E) configfs(E) efi_pstore(E) nfnetlink(E) ip_tables(E) x_tables(E) autofs4(E) ext4(E) crc16(E) mbcache(E) jbd2(E) dm_snapshot(E) dm_bufio(E) dm_mod(E) xen_blkfront(E)
>> [ 426.609334] CPU: 0 UID: 0 PID: 64 Comm: xenwatch Tainted: G E 7.1.3 #5 PREEMPT(lazy)
>> [ 426.609340] Tainted: [E]=UNSIGNED_MODULE
>> [ 426.609343] RIP: 0010:xennet_disconnect_backend+0x1c7/0x520 [xen_netfront]
>> [ 426.609349] Code: 94 dd a0 10 00 00 48 8b 4a 08 48 89 ce 83 e6 01 48 83 ee 01 48 09 f1 48 21 ca 0f b6 4a 33 81 f9 f5 00 00 00 0f 85 ea fe ff ff <0f> 0b 49 81 ff 00 01 00 00 0f 82 f8 fe ff ff 4c 89 fe 48 c7 c7 e0
>> [ 426.609352] RSP: 0018:ffffce3d810c3c98 EFLAGS: 00010246
>> [ 426.609356] RAX: 0000000000000010 RBX: 0000000000000000 RCX: 00000000000000f5
>> [ 426.609358] RDX: fffffb0d80ca9800 RSI: 0000000000000000 RDI: ffffffff88c8a8a0
>> [ 426.609360] RBP: ffff8dec51df0000 R08: ffff8debf4dd5a80 R09: ffff8debf4dd5a80
>> [ 426.609362] R10: 0000000000000020 R11: 00000000000000bf R12: 0000000000000000
>> [ 426.609364] R13: ffff8dec8ef30b80 R14: ffff8dec197fcac0 R15: 0000000000000000
>> [ 426.609366] FS: 0000000000000000(0000) GS:ffff8ded2d129000(0000) knlGS:0000000000000000
>> [ 426.609369] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>> [ 426.609370] CR2: 00005faa1a7d3737 CR3: 00000000b420a003 CR4: 0000000000770ef0
>> [ 426.609377] PKRU: 55555554
>> [ 426.609379] Call Trace:
>
> This is due (or looks like it...) to
> static netdev_tx_t xennet_start_xmit(struct sk_buff *skb, struct net_device *dev)
> [...]
> page = virt_to_page(skb->data); /* skb->data is a slab kmallocation */
> /* ... */
> info.page = page;
> first_tx = xennet_make_first_txreq(&info, offset, len);
> /* ... */
> queue->grant_tx_page[i] = page;
>
> and then later
>
> static void xennet_disconnect_backend(struct netfront_info *info)
> /* ... */
> xennet_release_tx_bufs
> get_page(queue->grant_tx_page[i]);
> gnttab_end_foreign_access(queue->grant_tx_ref[i],
> queue->grant_tx_page[i]);
>
> which may trigger get_page() on a slab page:
> static inline void get_page(struct page *page)
> {
> struct folio *folio = page_folio(page);
> if (WARN_ON_ONCE(folio_test_slab(folio)))
> return;
> if (WARN_ON_ONCE(folio_test_large_kmalloc(folio)))
> return;
> folio_get(folio);
> }
>
> and there's your WARN. The code as-is is wrong. get_page() does not stabilize
> slab allocations, and the skb is freed right after, which may very well free
> the data. This _can_ make sense for other page frags, but I would simply defer
> freeing the skb until either the full skb is xmitted, or you successfully
> cancel the whole transfer (what I understand is the "foreign access" in this
> code). The skb itself should pin the pages.
>
Thanks for the analysis and hint how to resolve this.
This will require quite some reworks, but should be possible.
Thanks,
Juergen
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^ permalink raw reply [flat|nested] 3+ messages in thread
end of thread, other threads:[~2026-07-15 13:42 UTC | newest]
Thread overview: 3+ messages (download: mbox.gz follow: Atom feed
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2026-07-15 3:49 Intermittent kernel WARN in mm.h get_page() when forcefully removing a xen_netfront interface from xenstore Aaron Rainbolt
2026-07-15 11:02 ` Pedro Falcato
2026-07-15 11:16 ` Jürgen Groß
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