On Fri, 27 Jan 2023, Hugh Dickins wrote: > On Fri, 27 Jan 2023, David Hildenbrand wrote: > > On 26.01.23 19:57, Matthew Wilcox wrote: > > > On Wed, Jan 25, 2023 at 11:59:16PM +0000, Sanan Hasanov wrote: > > >> Good day, dear maintainers, > > >> > > >> We found a bug using a modified kernel configuration file used by syzbot. > > >> > > >> We enhanced the coverage of the configuration file using our tool, > > >> klocalizer. > > >> > > >> Kernel Branch: 6.2.0-rc5-next-20230124 > > >> Kernel > > >> config: https://drive.google.com/file/d/1MZSgIF4R9QfikEuF5siUIZVPce-GiJQK/view?usp=sharing > > >> Reproducer: https://drive.google.com/file/d/1H5KWkT9VVMWTUVVgIaZi6J-fmukRx-BM/view?usp=sharing > > >> > > >> Thank you! > > >> > > >> Best regards, > > >> Sanan Hasanov This is a very interesting find: the thanks go to you. > > >> > > >> head: 0000000000020000 0000000000000000 00000004ffffffff ffff8881002b8000 > > >> page dumped because: VM_BUG_ON_PAGE(!first && (flags & (( rmap_t)((((1UL))) > > >> << (0))))) > > >> ------------[ cut here ]------------ > > > > > > I know it says "cut here" and you did that, but including just a few > > > lines above that would be so much more helpful. I can infer that this > > > is a multi-page folio, but more than that is hard to tell. > > > > > >> kernel BUG at mm/rmap.c:1248! > > > > > > That tracks with VM_BUG_ON_PAGE(!first && (flags & RMAP_EXCLUSIVE), page); > > > > > >> invalid opcode: 0000 [#1] PREEMPT SMP KASAN > > >> CPU: 7 PID: 14932 Comm: syz-executor.1 Not tainted 6.2.0-rc5-next-20230124 > > >> #1 > > >> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.15.0-1 > > >> 04/01/2014 > > >> RIP: 0010:page_add_anon_rmap+0xddd/0x11c0 mm/rmap.c:1248 > > >> Code: c0 ff 48 8b 34 24 48 89 df e8 1f ff 07 00 49 89 c6 e9 85 f6 ff ff e8 > > >> 52 73 c0 ff 48 c7 c6 c0 3c d8 89 48 89 ef e8 b3 23 f8 ff <0f> 0b e8 3c 73 > > >> c0 ff 48 c7 c6 00 3b d8 89 48 89 ef e8 9d 23 f8 ff > > >> RSP: 0018:ffffc9000c56f7b0 EFLAGS: 00010293 > > >> RAX: 0000000000000000 RBX: ffff88807efc6f30 RCX: 0000000000000000 > > >> RDX: ffff8880464fd7c0 RSI: ffffffff81be733d RDI: fffff520018adedb > > >> RBP: ffffea0000c68080 R08: 0000000000000056 R09: 0000000000000000 > > >> R10: 0000000000000001 R11: 0000000000000001 R12: ffffea0000c68000 > > >> R13: 0000000000000001 R14: ffffea0000c68088 R15: 0000000000000000 > > >> FS: 00007f717898a700(0000) GS:ffff888119f80000(0000) > > >> knlGS:0000000000000000 > > >> CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 > > >> CR2: 00007f7178947d78 CR3: 000000004a9e6000 CR4: 0000000000350ee0 > > >> Call Trace: > > >> <TASK> > > >> remove_migration_pte+0xaa6/0x1390 mm/migrate.c:261 > > > > > > if (folio_test_anon(folio)) > > > page_add_anon_rmap(new, vma, pvmw.address, > > > rmap_flags); > > > > > > Earlier in that function, we had: > > > if (folio_test_anon(folio) && > > > !is_readable_migration_entry(entry)) > > > rmap_flags |= RMAP_EXCLUSIVE; > > > > > > so that also makes sense. We can also infer that RMAP_COMPOUND wasn't > > > set, so we're trying to do just one page from the folio. > > > > > > All right, back to rmap.c: > > > > > > first = atomic_inc_and_test(&page->_mapcount); > > > > > > So first is clearly false (ie _mapcount was not -1), implying somebody > > > else already mapped this page. Not really sure what's going on at > > > this point. Seems unlikely that the folio changes in > > > remove_migration_pte() are responsible since they're from last January. > > > Huang has some more changes to migrate.c that I don't feel qualified > > > to judge. > > > > > > Nothing's jumping out at me as obviously wrong. Is it possible to > > > do a bisect? > > > > 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. > > > > 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? My guess is that the remove-__vma_adjust() commit is not properly updating vm_pgoff into non_vma in some case: so that when remove_migration_pte() looks for where to insert the new pte, it's off by one page. > > > > 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. > > 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. So I do wonder what pte replaces the migration entry when the bug here is fixed: is it a pte pointing into the THP as before, in which case what was the point of "migration"? is it a Copy-On-Bind page? or has the whole THP been migrated? I ought to read through those "estimated mapcount" threads more carefully: might be relevant, but I've not paid enough attention. Hugh > > return 0; > > } > > > > We map a large-enough are for a THP and then populate a fresh anon THP (PMD > > mapped) > > to write to it. > > > > The first mlock() will trigger PTE-mapping the THP and mlocking that subpage. > > The second mlock() seems to cause the issue. The final mbind() triggers page > > migration. > > > > Removing one of the mlock() makes it work. Note that we do a double > > mlock() of the same page -- the one we are then trying to migrate. > > > > Somehow, the double mlock() of the same page seems to affect our mapcount. > > > > CCing Hugh. > > Thanks David - most especially for the reproducer, not tried here yet. > I'll assume this is my bug, and get into it later in the day. > > Hugh
