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Date:   Tue, 8 Nov 2022 23:15:04 +0100
From:   Michal Hocko <mhocko@suse.com>
To:     Yang Shi <shy828301@gmail.com>
Cc:     zokeefe@google.com, akpm@linux-foundation.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org,
        syzbot+0044b22d177870ee974f@syzkaller.appspotmail.com
Subject: Re: [mm-unstable v3 PATCH 3/3] mm: replace VM_WARN_ON to pr_warn if
 the node is offline with __GFP_THISNODE
Message-ID: <Y2rU6BHD9eePn8IN@dhcp22.suse.cz>
References: <20221108184357.55614-1-shy828301@gmail.com>
 <20221108184357.55614-3-shy828301@gmail.com>
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On Tue 08-11-22 10:43:57, Yang Shi wrote:
> Syzbot reported the below splat:
> 
> WARNING: CPU: 1 PID: 3646 at include/linux/gfp.h:221 __alloc_pages_node
> include/linux/gfp.h:221 [inline]
> WARNING: CPU: 1 PID: 3646 at include/linux/gfp.h:221
> hpage_collapse_alloc_page mm/khugepaged.c:807 [inline]
> WARNING: CPU: 1 PID: 3646 at include/linux/gfp.h:221
> alloc_charge_hpage+0x802/0xaa0 mm/khugepaged.c:963
> Modules linked in:
> CPU: 1 PID: 3646 Comm: syz-executor210 Not tainted
> 6.1.0-rc1-syzkaller-00454-ga70385240892 #0
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
> Google 10/11/2022
> RIP: 0010:__alloc_pages_node include/linux/gfp.h:221 [inline]
> RIP: 0010:hpage_collapse_alloc_page mm/khugepaged.c:807 [inline]
> RIP: 0010:alloc_charge_hpage+0x802/0xaa0 mm/khugepaged.c:963
> Code: e5 01 4c 89 ee e8 6e f9 ae ff 4d 85 ed 0f 84 28 fc ff ff e8 70 fc
> ae ff 48 8d 6b ff 4c 8d 63 07 e9 16 fc ff ff e8 5e fc ae ff <0f> 0b e9
> 96 fa ff ff 41 bc 1a 00 00 00 e9 86 fd ff ff e8 47 fc ae
> RSP: 0018:ffffc90003fdf7d8 EFLAGS: 00010293
> RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
> RDX: ffff888077f457c0 RSI: ffffffff81cd8f42 RDI: 0000000000000001
> RBP: ffff888079388c0c R08: 0000000000000001 R09: 0000000000000000
> R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
> R13: dffffc0000000000 R14: 0000000000000000 R15: 0000000000000000
> FS:  00007f6b48ccf700(0000) GS:ffff8880b9b00000(0000)
> knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 00007f6b48a819f0 CR3: 00000000171e7000 CR4: 00000000003506e0
> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> Call Trace:
>  <TASK>
>  collapse_file+0x1ca/0x5780 mm/khugepaged.c:1715
>  hpage_collapse_scan_file+0xd6c/0x17a0 mm/khugepaged.c:2156
>  madvise_collapse+0x53a/0xb40 mm/khugepaged.c:2611
>  madvise_vma_behavior+0xd0a/0x1cc0 mm/madvise.c:1066
>  madvise_walk_vmas+0x1c7/0x2b0 mm/madvise.c:1240
>  do_madvise.part.0+0x24a/0x340 mm/madvise.c:1419
>  do_madvise mm/madvise.c:1432 [inline]
>  __do_sys_madvise mm/madvise.c:1432 [inline]
>  __se_sys_madvise mm/madvise.c:1430 [inline]
>  __x64_sys_madvise+0x113/0x150 mm/madvise.c:1430
>  do_syscall_x64 arch/x86/entry/common.c:50 [inline]
>  do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80
>  entry_SYSCALL_64_after_hwframe+0x63/0xcd
> RIP: 0033:0x7f6b48a4eef9
> Code: 28 00 00 00 75 05 48 83 c4 28 c3 e8 b1 15 00 00 90 48 89 f8 48 89
> f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01
> f0 ff ff 73 01 c3 48 c7 c1 b8 ff ff ff f7 d8 64 89 01 48
> RSP: 002b:00007f6b48ccf318 EFLAGS: 00000246 ORIG_RAX: 000000000000001c
> RAX: ffffffffffffffda RBX: 00007f6b48af0048 RCX: 00007f6b48a4eef9
> RDX: 0000000000000019 RSI: 0000000000600003 RDI: 0000000020000000
> RBP: 00007f6b48af0040 R08: 0000000000000000 R09: 0000000000000000
> R10: 0000000000000000 R11: 0000000000000246 R12: 00007f6b48aa53a4
> R13: 00007f6b48bffcbf R14: 00007f6b48ccf400 R15: 0000000000022000
>  </TASK>
> 
> It is because khugepaged allocates pages with __GFP_THISNODE, but the
> preferred node is bogus.  The previous patch fixed the khugepaged
> code to avoid allocating page from non-existing node.  But it is still
> racy against memory hotremove.  There is no synchronization with the
> memory hotplug so it is possible that memory gets offline during a
> longer taking scanning.
> 
> So this warning still seems not quite helpful because:
>   * There is no guarantee the node is online for __GFP_THISNODE context
>     for all the callsites.
>   * Kernel just fails the allocation regardless the warning, and it looks
>     all callsites handle the allocation failure gracefully.
> 
> Although while the warning has helped to identify a buggy code, it is not
> safe in general and this warning could panic the system with panic-on-warn
> configuration which tends to be used surprisingly often.  So replace
> VM_WARN_ON to pr_warn().  And the warning will be triggered if
> __GFP_NOWARN is set since the allocator would print out warning for such
> case if __GFP_NOWARN is not set.
> 
> Reported-by: syzbot+0044b22d177870ee974f@syzkaller.appspotmail.com
> Signed-off-by: Yang Shi <shy828301@gmail.com>
> Suggested-by: Michal Hocko <mhocko@suse.com>
> Cc: Zach O'Keefe <zokeefe@google.com>

Acked-by: Michal Hocko <mhocko@suse.com>

minor nit

> ---
> v3: * Reverted the old commit from mm-unstable which remove the VM_WARN
>       (patch 1/3).
>     * Incorporated the suggestion from Michal to use pr_warn.
> 
> v2: * Added patch 1/2.
>     * Reworded the commit log per Michal.
> 
>  include/linux/gfp.h | 14 ++++++++++++--
>  1 file changed, 12 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/gfp.h b/include/linux/gfp.h
> index ef4aea3b356e..60a1c70ec85c 100644
> --- a/include/linux/gfp.h
> +++ b/include/linux/gfp.h
> @@ -210,6 +210,16 @@ alloc_pages_bulk_array_node(gfp_t gfp, int nid, unsigned long nr_pages, struct p
>  	return __alloc_pages_bulk(gfp, nid, NULL, nr_pages, NULL, page_array);
>  }
>  
> +static inline void warn_if_node_offline(int nid, gfp_t gfp_mask)
> +{
> +	gfp_t gfp = gfp_mask & (__GFP_THISNODE|__GFP_NOWARN);

I would use rather this_node or similarly descriptive name. gfp sounds
like a pasm but this is only a subset of it. If you really want to
improve the readability then you can restructure the condition a bit

	if (gfp_mask & (__GFP_THISNODE|__GFP_NOWARN)) != (__GFP_THISNODE|__GFP_NOWARN))
		return;
	
	if (node_online(nid)) 
		return;
	
	pr_warn("%pGg allocation from offline node %d\n", &gfp, nid);
	dump_stack();

> +
> +	if ((gfp == (__GFP_THISNODE|__GFP_NOWARN)) && !node_online(nid)) {
> +		pr_warn("%pGg allocation from offline node %d\n", &gfp, nid);
> +		dump_stack();
> +	}
> +}
> +
-- 
Michal Hocko
SUSE Labs