Re: [syzbot] WARNING in alloc_charge_hpage

[Zach O'Keefe <zokeefe@google.com>]

On Oct 28 10:37, Yang Shi wrote:
> On Fri, Oct 28, 2022 at 12:49 AM syzbot
> <syzbot+0044b22d177870ee974f@syzkaller.appspotmail.com> wrote:
> >
> > Hello,
> >
> > syzbot found the following issue on:
> >
> > HEAD commit:    a70385240892 Merge tag 'perf_urgent_for_v6.1_rc2' of git:/..
> > git tree:       upstream
> > console+strace: https://syzkaller.appspot.com/x/log.txt?x=13f4726a880000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=ea03ca45176080bc
> > dashboard link: https://syzkaller.appspot.com/bug?extid=0044b22d177870ee974f
> > compiler:       gcc (Debian 10.2.1-6) 10.2.1 20210110, GNU ld (GNU Binutils for Debian) 2.35.2
> > syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=13726f72880000
> > C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=13b57436880000
> >
> > Downloadable assets:
> > disk image: https://storage.googleapis.com/syzbot-assets/5e17f1e83cf3/disk-a7038524.raw.xz
> > vmlinux: https://storage.googleapis.com/syzbot-assets/f8ef729877f7/vmlinux-a7038524.xz
> >
> > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > Reported-by: syzbot+0044b22d177870ee974f@syzkaller.appspotmail.com
> >
> > ------------[ cut here ]------------
> > 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
> 
> It looks like the node requested by the allocation is offlined, and
> the allocation is using __GFP_THISNODE. It doesn't sound like
> khugepaged specific problem IIUC, I don't see other callsites do
> anything to prevent the node from being offlined.
> 
> Shall we remove that warn? The page allocation call will return NULL
> anyway. Or we could just ignore the warn?
> 

Thanks for taking a look, Yang.

As for the warn -- I don't see the value in the warning at present.  As you
mention, I don't think it's a khugepaged-thing (as, at least from skimming,
other callsites don't take care to expect that the node is online).  The
allocation might fail -- for a number of reasons -- and I don't think
__GFP_THISNODE + offline node is so special to deserve a warn.

A different question, is if this particular callsite wants to defend against
that possibility.  My understanding, which I state without data, is that the
performance of accessing remote memory outweighs the benefit of pmd-mapping the
memory -- and so I think we rightly fail here.

So, my vote is to just remove the warn.

Thanks,
Zach

> > 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>
> >
> >
> > ---
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> > syzbot engineers can be reached at syzkaller@googlegroups.com.
> >
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> > syzbot can test patches for this issue, for details see:
> > https://goo.gl/tpsmEJ#testing-patches
> >