commit ca0246bb97c2 ("z3fold: fix possible reclaim races") introduced
the PAGE_CLAIMED flag "to avoid racing on a z3fold 'headless' page
release." By atomically testing and setting the bit in each of
z3fold_free() and z3fold_reclaim_page(), a double-free was avoided.
However, commit 746d179b0e66 ("z3fold: stricter locking and more careful
reclaim") appears to have unintentionally broken this behavior by moving
the PAGE_CLAIMED check in z3fold_reclaim_page() to after the page lock
gets taken, which only happens for non-headless pages. For headless
pages, the check is now skipped entirely and races can occur again.
I have observed such a race on my system:
page:00000000ffbd76b7 refcount:0 mapcount:0 mapping:0000000000000000
index:0x0 pfn:0x165316
flags: 0x2ffff0000000000()
raw: 02ffff0000000000 ffffea0004535f48 ffff8881d553a170 0000000000000000
raw: 0000000000000000 0000000000000011 00000000ffffffff 0000000000000000
page dumped because: VM_BUG_ON_PAGE(page_ref_count(page) == 0)
------------[ cut here ]------------
kernel BUG at include/linux/mm.h:707!
invalid opcode: 0000 [#1] PREEMPT SMP KASAN PTI
CPU: 2 PID: 291928 Comm: kworker/2:0 Tainted: G B
5.10.7-arch1-1-kasan #1
Hardware name: Gigabyte Technology Co., Ltd. H97N-WIFI/H97N-WIFI, BIOS F9b
03/03/2016
Workqueue: zswap-shrink shrink_worker
RIP: 0010:__free_pages+0x10a/0x130
Code: c1 e7 06 48 01 ef 45 85 e4 74 d1 44 89 e6 31 d2 41 83 ec 01 e8 e7 b0
ff ff eb da 48 c7 c6 e0 32 91 88 48 89 ef e8 a6 89 f8 ff <0f> 0b 4c 89 e7 e8 fc
79 07 00 e9 33 ff ff ff 48 89 ef e8 ff 79 07
RSP: 0000:ffff88819a2ffb98 EFLAGS: 00010296
RAX: 0000000000000000 RBX: ffffea000594c5a8 RCX: 0000000000000000
RDX: 1ffffd4000b298b7 RSI: 0000000000000000 RDI: ffffea000594c5b8
RBP: ffffea000594c580 R08: 000000000000003e R09: ffff8881d5520bbb
R10: ffffed103aaa4177 R11: 0000000000000001 R12: ffffea000594c5b4
R13: 0000000000000000 R14: ffff888165316000 R15: ffffea000594c588
FS: 0000000000000000(0000) GS:ffff8881d5500000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f7c8c3654d8 CR3: 0000000103f42004 CR4: 00000000001706e0
Call Trace:
z3fold_zpool_shrink+0x9b6/0x1240
? sugov_update_single+0x357/0x990
? sched_clock+0x5/0x10
? sched_clock_cpu+0x18/0x180
? z3fold_zpool_map+0x490/0x490
? _raw_spin_lock_irq+0x88/0xe0
shrink_worker+0x35/0x90
process_one_work+0x70c/0x1210
? pwq_dec_nr_in_flight+0x15b/0x2a0
worker_thread+0x539/0x1200
? __kthread_parkme+0x73/0x120
? rescuer_thread+0x1000/0x1000
kthread+0x330/0x400
? __kthread_bind_mask+0x90/0x90
ret_from_fork+0x22/0x30
Modules linked in: rfcomm ebtable_filter ebtables ip6table_filter
ip6_tables iptable_filter ccm algif_aead des_generic libdes ecb algif_skcipher
cmac bnep md4 algif_hash af_alg vfat fat intel_rapl_msr intel_rapl_common
x86_pkg_temp_thermal intel_powerclamp coretemp kvm_intel iwlmvm
hid_logitech_hidpp kvm at24 mac80211 snd_hda_codec_realtek iTCO_wdt
snd_hda_codec_generic intel_pmc_bxt snd_hda_codec_hdmi ledtrig_audio
iTCO_vendor_support mei_wdt mei_hdcp snd_hda_intel snd_intel_dspcfg libarc4
soundwire_intel irqbypass iwlwifi soundwire_generic_allocation rapl
soundwire_cadence intel_cstate snd_hda_codec intel_uncore btusb joydev mousedev
snd_usb_audio pcspkr btrtl uvcvideo nouveau btbcm i2c_i801 btintel snd_hda_core
videobuf2_vmalloc i2c_smbus snd_usbmidi_lib videobuf2_memops bluetooth
snd_hwdep soundwire_bus snd_soc_rt5640 videobuf2_v4l2 cfg80211 snd_soc_rl6231
videobuf2_common snd_rawmidi lpc_ich alx videodev mdio snd_seq_device
snd_soc_core mc ecdh_generic mxm_wmi mei_me
hid_logitech_dj wmi snd_compress e1000e ac97_bus mei ttm rfkill
snd_pcm_dmaengine ecc snd_pcm snd_timer snd soundcore mac_hid acpi_pad
pkcs8_key_parser it87 hwmon_vid crypto_user fuse ip_tables x_tables ext4
crc32c_generic crc16 mbcache jbd2 dm_crypt cbc encrypted_keys trusted tpm
rng_core usbhid dm_mod crct10dif_pclmul crc32_pclmul crc32c_intel
ghash_clmulni_intel aesni_intel crypto_simd cryptd glue_helper xhci_pci
xhci_pci_renesas i915 video intel_gtt i2c_algo_bit drm_kms_helper syscopyarea
sysfillrect sysimgblt fb_sys_fops cec drm agpgart
---[ end trace 126d646fc3dc0ad8 ]---
To fix the issue, re-add the earlier test and set in the case where we
have a headless page.
Fixes: 746d179b0e66 ("z3fold: stricter locking and more careful reclaim")
Signed-off-by: Thomas Hebb <tommyh...@gmail.com>
---
I have NOT tested this patch yet beyond compiling it. If the approach
seems good, I'll test it on my system for a period of several days and
see if I can reproduce the crash before sending a v1.
mm/z3fold.c | 16 +++++++++++++++-
1 file changed, 15 insertions(+), 1 deletion(-)
diff --git a/mm/z3fold.c b/mm/z3fold.c
index 0152ad9931a8..8ae944eeb8e2 100644
--- a/mm/z3fold.c
+++ b/mm/z3fold.c
@@ -1350,8 +1350,22 @@ static int z3fold_reclaim_page(struct z3fold_pool *pool,
unsigned int retries)
page = list_entry(pos, struct page, lru);
zhdr = page_address(page);
- if (test_bit(PAGE_HEADLESS, &page->private))
+ if (test_bit(PAGE_HEADLESS, &page->private)) {
+ /*
+ * For non-headless pages, we wait to do this
+ * until we have the page lock to avoid racing
+ * with __z3fold_alloc(). Headless pages don't
+ * have a lock (and __z3fold_alloc() will never
+ * see them), but we still need to test and set
+ * PAGE_CLAIMED to avoid racing with
+ * z3fold_free(), so just do it now before
+ * leaving the loop.
+ */
+ if (test_and_set_bit(PAGE_CLAIMED,
&page->private))
+ continue;
+
break;
+ }
if (kref_get_unless_zero(&zhdr->refcount) == 0) {
zhdr = NULL;
--
2.30.0
