Date:   Wed, 9 Nov 2022 11:33:35 -0800
From:   Yang Shi <shy828301@...il.com>
To:     Michal Hocko <mhocko@...e.com>
Cc:     zokeefe@...gle.com, akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        syzbot+0044b22d177870ee974f@...kaller.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

On Tue, Nov 8, 2022 at 2:15 PM Michal Hocko <mhocko@...e.com> wrote:
>
> 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@...kaller.appspotmail.com
> > Signed-off-by: Yang Shi <shy828301@...il.com>
> > Suggested-by: Michal Hocko <mhocko@...e.com>
> > Cc: Zach O'Keefe <zokeefe@...gle.com>
>
> Acked-by: Michal Hocko <mhocko@...e.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

this_node sounds good to me.

> 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

Just because I thought "if (gfp_mask & (__GFP_THISNODE|__GFP_NOWARN))
!= (__GFP_THISNODE|__GFP_NOWARN))" is too long, so I added gfp to
store the intermediate result.

>
>         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

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