Re: [PATCH] mm: Fix a race during split THP

[Michal Hocko]

Hi,
I have noticed this fix and I do not see it in the mmotm tree.
Is there anything blocking this patch or it simply fall through cracks?

On Fri 09-10-20 15:36:47, Huang, Ying wrote:
> From: Huang Ying <ying.huang@xxxxxxxxx>
> 
> It is reported that the following bug is triggered if the HDD is used as swap
> device,
> 
> [ 5758.157556] BUG: kernel NULL pointer dereference, address: 0000000000000007
> [ 5758.165331] #PF: supervisor write access in kernel mode
> [ 5758.171161] #PF: error_code(0x0002) - not-present page
> [ 5758.176894] PGD 0 P4D 0
> [ 5758.179721] Oops: 0002 [#1] SMP PTI
> [ 5758.183614] CPU: 10 PID: 316 Comm: kswapd1 Kdump: loaded Tainted: G S               --------- ---  5.9.0-0.rc3.1.tst.el8.x86_64 #1
> [ 5758.196717] Hardware name: Intel Corporation S2600CP/S2600CP, BIOS SE5C600.86B.02.01.0002.082220131453 08/22/2013
> [ 5758.208176] RIP: 0010:split_swap_cluster+0x47/0x60
> [ 5758.213522] Code: c1 e3 06 48 c1 eb 0f 48 8d 1c d8 48 89 df e8 d0 20 6a 00 80 63 07 fb 48 85 db 74 16 48 89 df c6 07 00 66 66 66 90 31 c0 5b c3 <80> 24 25 07 00 00 00 fb 31 c0 5b c3 b8 f0 ff ff ff 5b c3 66 0f 1f
> [ 5758.234478] RSP: 0018:ffffb147442d7af0 EFLAGS: 00010246
> [ 5758.240309] RAX: 0000000000000000 RBX: 000000000014b217 RCX: ffffb14779fd9000
> [ 5758.248281] RDX: 000000000014b217 RSI: ffff9c52f2ab1400 RDI: 000000000014b217
> [ 5758.256246] RBP: ffffe00c51168080 R08: ffffe00c5116fe08 R09: ffff9c52fffd3000
> [ 5758.264208] R10: ffffe00c511537c8 R11: ffff9c52fffd3c90 R12: 0000000000000000
> [ 5758.272172] R13: ffffe00c51170000 R14: ffffe00c51170000 R15: ffffe00c51168040
> [ 5758.280134] FS:  0000000000000000(0000) GS:ffff9c52f2a80000(0000) knlGS:0000000000000000
> [ 5758.289163] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [ 5758.295575] CR2: 0000000000000007 CR3: 0000000022a0e003 CR4: 00000000000606e0
> [ 5758.303538] Call Trace:
> [ 5758.306273]  split_huge_page_to_list+0x88b/0x950
> [ 5758.311433]  deferred_split_scan+0x1ca/0x310
> [ 5758.316202]  do_shrink_slab+0x12c/0x2a0
> [ 5758.320491]  shrink_slab+0x20f/0x2c0
> [ 5758.324482]  shrink_node+0x240/0x6c0
> [ 5758.328469]  balance_pgdat+0x2d1/0x550
> [ 5758.332652]  kswapd+0x201/0x3c0
> [ 5758.336157]  ? finish_wait+0x80/0x80
> [ 5758.340147]  ? balance_pgdat+0x550/0x550
> [ 5758.344525]  kthread+0x114/0x130
> [ 5758.348126]  ? kthread_park+0x80/0x80
> [ 5758.352214]  ret_from_fork+0x22/0x30
> [ 5758.356203] Modules linked in: fuse zram rfkill sunrpc intel_rapl_msr intel_rapl_common sb_edac x86_pkg_temp_thermal intel_powerclamp coretemp mgag200 iTCO_wdt crct10dif_pclmul iTCO_vendor_support drm_kms_helper crc32_pclmul ghash_clmulni_intel syscopyarea sysfillrect sysimgblt fb_sys_fops cec rapl joydev intel_cstate ipmi_si ipmi_devintf drm intel_uncore i2c_i801 ipmi_msghandler pcspkr lpc_ich mei_me i2c_smbus mei ioatdma ip_tables xfs libcrc32c sr_mod sd_mod cdrom t10_pi sg igb ahci libahci i2c_algo_bit crc32c_intel libata dca wmi dm_mirror dm_region_hash dm_log dm_mod
> [ 5758.412673] CR2: 0000000000000007
> [    0.000000] Linux version 5.9.0-0.rc3.1.tst.el8.x86_64 (mockbuild@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx) (gcc (GCC) 8.3.1 20191121 (Red Hat 8.3.1-5), GNU ld version 2.30-79.el8) #1 SMP Wed Sep 9 16:03:34 EDT 2020
> 
> After further digging it's found that the following race condition exists in the
> original implementation,
> 
> CPU1                                                             CPU2
> ----                                                             ----
> deferred_split_scan()
>   split_huge_page(page) /* page isn't compound head */
>     split_huge_page_to_list(page, NULL)
>       __split_huge_page(page, )
>         ClearPageCompound(head)
>         /* unlock all subpages except page (not head) */
>                                                                  add_to_swap(head)  /* not THP */
>                                                                    get_swap_page(head)
>                                                                    add_to_swap_cache(head, )
>                                                                      SetPageSwapCache(head)
>      if PageSwapCache(head)
>        split_swap_cluster(/* swap entry of head */)
>          /* Deref sis->cluster_info: NULL accessing! */
> 
> So, in split_huge_page_to_list(), PageSwapCache() is called for the already
> split and unlocked "head", which may be added to swap cache in another CPU.  So
> split_swap_cluster() may be called wrongly.
> 
> To fix the race, the call to split_swap_cluster() is moved to
> __split_huge_page() before all subpages are unlocked.  So that the
> PageSwapCache() is stable.
> 
> Fixes: 59807685a7e77 ("mm, THP, swap: support splitting THP for THP swap out")
> Reported-and-tested-by: Rafael Aquini <aquini@xxxxxxxxxx>
> Signed-off-by: "Huang, Ying" <ying.huang@xxxxxxxxx>
> Cc: Hugh Dickins <hughd@xxxxxxxxxx>
> Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> ---
>  mm/huge_memory.c | 13 +++++++------
>  1 file changed, 7 insertions(+), 6 deletions(-)
> 
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index cba3812a5c3e..87b0389673dd 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -2478,6 +2478,12 @@ static void __split_huge_page(struct page *page, struct list_head *list,
>  
>  	remap_page(head, nr);
>  
> +	if (PageSwapCache(head)) {
> +		swp_entry_t entry = { .val = page_private(head) };
> +
> +		split_swap_cluster(entry);
> +	}
> +
>  	for (i = 0; i < nr; i++) {
>  		struct page *subpage = head + i;
>  		if (subpage == page)
> @@ -2713,12 +2719,7 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
>  		}
>  
>  		__split_huge_page(page, list, end, flags);
> -		if (PageSwapCache(head)) {
> -			swp_entry_t entry = { .val = page_private(head) };
> -
> -			ret = split_swap_cluster(entry);
> -		} else
> -			ret = 0;
> +		ret = 0;
>  	} else {
>  		if (IS_ENABLED(CONFIG_DEBUG_VM) && mapcount) {
>  			pr_alert("total_mapcount: %u, page_count(): %u\n",
> -- 
> 2.28.0

-- 
Michal Hocko
SUSE Labs