Re: [PATCH] mm: don't warn if the node is offlined

[Zach O'Keefe]

On Mon, Oct 31, 2022 at 3:08 PM Michal Hocko <mhocko@xxxxxxxx> 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.

Thanks,
Zach


> [...]
>
> > 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@xxxxxxxxxxxxxxxxxxxxxxxxx
> > Signed-off-by: Yang Shi <shy828301@xxxxxxxxx>
> > Cc: Zach O'Keefe <zokeefe@xxxxxxxxxx>
> > Cc: Michal Hocko <mhocko@xxxxxxxx>
>
> 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@xxxxxxxx>
>
> > ---
> >  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