Re: [RFC PATCH] z3fold: prevent reclaim/free race for headless pages

[Tom Hebb]

On Tue, Feb 16, 2021 at 1:21 AM Greg Kroah-Hartman
<gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
>
> On Tue, Feb 16, 2021 at 12:44:40AM -0800, Thomas Hebb wrote:
> > 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")
>
> This commit id is not in Linus's tree :(

Oops, I was building from a 5.10 stable tree and accidentally grabbed
the ID of the pick there. This is the correct line, will fix in v1:

Fixes: dcf5aedb24f8 ("z3fold: stricter locking and more careful reclaim")

(And sorry for my initial failure to reply-all; I contribute to
projects that use mailing lists just infrequently enough for me to
forget all the common mistakes, meaning I repeat them every time.)

> thanks,
>
> greg k-h