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Date:   Mon, 31 Oct 2022 23:08:48 +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
Subject: Re: [PATCH] mm: don't warn if the node is offlined
Message-ID: <Y2BHcBCR2FIJgU4w@dhcp22.suse.cz>
References: <20221031183122.470962-1-shy828301@gmail.com>
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On Mon 31-10-22 11:31:22, 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

This is quite weird, isn't it? alloc_charge_hpage is selecting the most
busy node (as per collapse_control). How come this can be an offline
node? Is a parallel memory hotplug happening?

[...]

> It is because khugepaged allocates pages with __GFP_THISNODE, but the
> preferred node is offlined.  The warning was even stronger before commit
> 8addc2d00fe17 ("mm: do not warn on offline nodes unless the specific node
> is explicitly requested").  The commit softened the warning for
> __GFP_THISNODE.
> 
> But this warning seems not quite useful because:
>   * There is no guarantee the node is online for __GFP_THISNODE context
>     for all the callsites.

The original idea IIRC was to catch a buggy code which mishandled node
assignment. But this looks like a perfectly valid code. There is no
synchronization with the memory hotplug so it is possible that memory
gets offline during a longer taking scanning.

I do agree that the warning is not really helpful in this case. It is
actually even harmful for those running in panic-on-warn mode.

>   * Kernel just fails the allocation regardless the warning, and it looks
>     all callsites handle the allocation failure gracefully.
> 
> So, removing the warning seems like the good move.
> 
> Reported-by: syzbot+0044b22d177870ee974f@syzkaller.appspotmail.com
> Signed-off-by: Yang Shi <shy828301@gmail.com>
> Cc: Zach O'Keefe <zokeefe@google.com>
> Cc: Michal Hocko <mhocko@suse.com>

Unless I am wrong in my above statement I would appreciate extending the
changelog to describe the actual code is correct so the warning is
harmful.

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

> ---
>  include/linux/gfp.h | 2 --
>  1 file changed, 2 deletions(-)
> 
> diff --git a/include/linux/gfp.h b/include/linux/gfp.h
> index ef4aea3b356e..594d6dee5646 100644
> --- a/include/linux/gfp.h
> +++ b/include/linux/gfp.h
> @@ -218,7 +218,6 @@ static inline struct page *
>  __alloc_pages_node(int nid, gfp_t gfp_mask, unsigned int order)
>  {
>  	VM_BUG_ON(nid < 0 || nid >= MAX_NUMNODES);
> -	VM_WARN_ON((gfp_mask & __GFP_THISNODE) && !node_online(nid));
>  
>  	return __alloc_pages(gfp_mask, order, nid, NULL);
>  }
> @@ -227,7 +226,6 @@ static inline
>  struct folio *__folio_alloc_node(gfp_t gfp, unsigned int order, int nid)
>  {
>  	VM_BUG_ON(nid < 0 || nid >= MAX_NUMNODES);
> -	VM_WARN_ON((gfp & __GFP_THISNODE) && !node_online(nid));
>  
>  	return __folio_alloc(gfp, order, nid, NULL);
>  }
> -- 
> 2.26.3

-- 
Michal Hocko
SUSE Labs