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Date:   Tue, 1 Nov 2022 08:54:23 +0100
From:   Michal Hocko <mhocko@suse.com>
To:     Zach O'Keefe <zokeefe@google.com>
Cc:     Yang Shi <shy828301@gmail.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
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On Mon 31-10-22 17:05:06, Zach O'Keefe wrote:
> On Mon, Oct 31, 2022 at 3:08 PM Michal Hocko <mhocko@suse.com> wrote:
> >
> > 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?
> 
> TBH -- I did not look closely at the syzbot reproducer (let alone
> attempt to run it) and assumed this was the case. Taking a quick look,
> at least memory hot remove is enabled:
> 
> CONFIG_ARCH_ENABLE_MEMORY_HOTPLUG=y
> CONFIG_ARCH_ENABLE_MEMORY_HOTREMOVE=y
> CONFIG_MEMORY_HOTPLUG=y
> CONFIG_MEMORY_HOTPLUG_DEFAULT_ONLINE=y
> CONFIG_MEMORY_HOTREMOVE=y
> 
> But looking at the C reproducer, I don't immediately see anywhere
> where we offline nodes. I'll try to run this tomorrow to make sure I'm
> not missing something real here.

Looking slightly closer at hpage_collapse_scan_file I think that it is
possible that xas_for_each simply doesn't find any entries in the page
cache and with khugepaged_max_ptes_none == HPAGE_PMD_NR we can fall back
to collapse_file even without any real entries.

But the mere possibility of the hotplug race should be a sufficient
ground to remove those WARN_ONs


Thanks!
-- 
Michal Hocko
SUSE Labs