Re: kernel BUG in page_add_anon_rmap

[David Hildenbrand]

On 30.01.23 10:03, David Hildenbrand wrote:
> > > > I reproduced on next-20230127 (did not try upstream yet).
> >
> > Upstream's fine; on next-20230127 (with David's repro) it bisects to
> > 5ddaec50023e ("mm/mmap: remove __vma_adjust()").  I think I'd better
> > hand on to Liam, rather than delay you by puzzling over it further myself.
>
> Thanks for identifying the problematic commit! ...
>
> > > > I think two key things are that a) THP are set to "always" and b) we have a
> > > > NUMA setup [I assume].
> > > >
> > > > The relevant bits:
> > > >
> > > > [  439.886738] page:00000000c4de9000 refcount:513 mapcount:2
> > > > mapping:0000000000000000 index:0x20003 pfn:0x14ee03
> > > > [  439.893758] head:000000003d5b75a4 order:9 entire_mapcount:0
> > > > nr_pages_mapped:511 pincount:0
> > > > [  439.899611] memcg:ffff986dc4689000
> > > > [  439.902207] anon flags:
> > > > 0x17ffffc009003f(locked|referenced|uptodate|dirty|lru|active|head|swapbacked|node=0|zone=2|lastcpupid=0x1fffff)
> > > > [  439.910737] raw: 0017ffffc0020000 ffffe952c53b8001 ffffe952c53b80c8
> > > > dead000000000400
> > > > [  439.916268] raw: 0000000000000000 0000000000000000 0000000000000001
> > > > 0000000000000000
> > > > [  439.921773] head: 0017ffffc009003f ffffe952c538b108 ffff986de35a0010
> > > > ffff98714338a001
> > > > [  439.927360] head: 0000000000020000 0000000000000000 00000201ffffffff
> > > > ffff986dc4689000
> > > > [  439.932341] page dumped because: VM_BUG_ON_PAGE(!first && (flags & ((
> > > > rmap_t)((((1UL))) << (0)))))
> > > >
> > > >
> > > > Indeed, the mapcount of the subpage is 2 instead of 1. The subpage is only
> > > > mapped into a single
> > > > page table (no fork() or similar).
> >
> > Yes, that mapcount:2 is weird; and what's also weird is the index:0x20003:
> > what is remove_migration_pte(), in an mbind(0x20002000,...), doing with
> > index:0x20003?
>
> I was assuming the whole folio would get migrated. As you raise below,
> it's all a bit unclear once THP get involved and dealing with mbind()
> and page migration.
>
> > > > I created this reduced reproducer that triggers 100%:
> >
> > Very helpful, thank you.
> >
> > > > #include <stdint.h>
> > > > #include <unistd.h>
> > > > #include <sys/mman.h>
> > > > #include <numaif.h>
> > > >
> > > > int main(void)
> > > > {
> > > > 	mmap((void*)0x20000000ul, 0x1000000ul, PROT_READ|PROT_WRITE|PROT_EXEC,
> > > > 	     MAP_ANONYMOUS|MAP_FIXED|MAP_PRIVATE, -1, 0ul);
> > > > 	madvise((void*)0x20000000ul, 0x1000000ul, MADV_HUGEPAGE);
> > > >
> > > > 	*(uint32_t*)0x20000080 = 0x80000;
> > > > 	mlock((void*)0x20001000ul, 0x2000ul);
> > > > 	mlock((void*)0x20000000ul, 0x3000ul);
> >
> > It's not an mlock() issue in particular: quickly established by
> > substituting madvise(,, MADV_NOHUGEPAGE) for those mlock() calls.
> > Looks like a vma splitting issue now.
>
> Gah, should have tried something like that first before suspecting it's
> mlock related. :)
>
> > > > 	mbind((void*)0x20002000ul, 0x1000ul, MPOL_LOCAL, NULL, 0x7fful,
> > > > 	MPOL_MF_MOVE);
> >
> > I guess it will turn out not to be relevant to this particular syzbug,
> > but what do we expect an mbind() of just 0x1000 of a THP to do?
> >
> > It's a subject I've wrestled with unsuccessfully in the past: I found
> > myself arriving at one conclusion (split THP) in one place, and a contrary
> > conclusion (widen range) in another place, and never had time to work out
> > one unified answer.
>
> I'm aware of a similar issue with long-term page pinning: we might want
> to pin a 4k portion of a THP, but will end up blocking the whole THP
> from getting migrated/swapped/split/freed/ ... until we unpin (ever?). I
> wrote a reproducer [1] a while ago to show how you can effectively steal
> most THP in the system using comparatively small memlock limit using
> io_uring ...

Correction, my reproducer already triggers a compund page split to 
really only pin a 4k page, to then free the remaining 4k pages of the 
previous THP. As a single 4k page is allocated and pinned, we cannot get 
a THP at these physical memory locations until the page is unpinned.

--
Thanks,

David / dhildenb