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[172.10.233.147]) by smtp.gmail.com with ESMTPSA id z20-20020a4ad1b4000000b0051762747277sm790223oor.41.2023.01.28.22.49.38 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sat, 28 Jan 2023 22:49:39 -0800 (PST) Date: Sat, 28 Jan 2023 22:49:31 -0800 (PST) From: Hugh Dickins X-X-Sender: hugh@ripple.attlocal.net To: "Liam R. Howlett" cc: Hugh Dickins , David Hildenbrand , Matthew Wilcox , Sanan Hasanov , "akpm@linux-foundation.org" , "linux-mm@kvack.org" , "linux-kernel@vger.kernel.org" , "contact@pgazz.com" , "syzkaller@googlegroups.com" , Huang Ying Subject: Re: kernel BUG in page_add_anon_rmap In-Reply-To: <713c6242-be65-c212-b790-2b908627c1b4@google.com> Message-ID: <9d8fb9c-1b81-67cd-e55b-34517388e1ab@google.com> References: <713c6242-be65-c212-b790-2b908627c1b4@google.com> MIME-Version: 1.0 Content-Type: multipart/mixed; boundary="-1463760895-1811275884-1674974979=:23545" Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org This message is in MIME format. The first part should be readable text, while the remaining parts are likely unreadable without MIME-aware tools. ---1463760895-1811275884-1674974979=:23545 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: QUOTED-PRINTABLE 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 sy= zbot. > > >> > > >> We enhanced the coverage of the configuration file using our tool, > > >> klocalizer. > > >> > > >> Kernel Branch:=C2=A06.2.0-rc5-next-20230124 > > >> Kernel > > >> config:=C2=A0https://drive.google.com/file/d/1MZSgIF4R9QfikEuF5siUIZ= VPce-GiJQK/view?usp=3Dsharing > > >> Reproducer:=C2=A0https://drive.google.com/file/d/1H5KWkT9VVMWTUVVgIa= Zi6J-fmukRx-BM/view?usp=3Dsharing > > >> > > >> Thank you! > > >> > > >> Best regards, > > >> Sanan Hasanov This is a very interesting find: the thanks go to you. > > >> > > >> head: 0000000000020000 0000000000000000 00000004ffffffff ffff8881002= b8000 > > >> page dumped because: VM_BUG_ON_PAGE(!first && (flags & (( rmap_t)(((= (1UL))) > > >> << (0))))) > > >> ------------[ cut here ]------------ > > >=20 > > > 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 thi= s > > > is a multi-page folio, but more than that is hard to tell. > > >=20 > > >> kernel BUG at mm/rmap.c:1248! > > >=20 > > > That tracks with VM_BUG_ON_PAGE(!first && (flags & RMAP_EXCLUSIVE), p= age); > > >=20 > > >> invalid opcode: 0000 [#1] PREEMPT SMP KASAN > > >> CPU: 7 PID: 14932 Comm: syz-executor.1 Not tainted 6.2.0-rc5-next-20= 230124 > > >> #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: > > >> > > >> remove_migration_pte+0xaa6/0x1390 mm/migrate.c:261 > > >=20 > > > if (folio_test_anon(folio)) > > > page_add_anon_rmap(new, vma, pvmw.ad= dress, > > > rmap_flags); > > >=20 > > > Earlier in that function, we had: > > > if (folio_test_anon(folio) && > > > !is_readable_migration_entry(entry)) > > > rmap_flags |=3D RMAP_EXCLUSIVE; > > >=20 > > > 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. > > >=20 > > > All right, back to rmap.c: > > >=20 > > > first =3D atomic_inc_and_test(&page->_mapcount); > > >=20 > > > So first is clearly false (ie _mapcount was not -1), implying somebod= y > > > 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 Januar= y. > > > Huang has some more changes to migrate.c that I don't feel qualified > > > to judge. > > >=20 > > > Nothing's jumping out at me as obviously wrong. Is it possible to > > > do a bisect? > >=20 > > 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. > >=20 > > I think two key things are that a) THP are set to "always" and b) we ha= ve a > > NUMA setup [I assume]. > >=20 > > The relevant bits: > >=20 > > [ 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|swapb= acked|node=3D0|zone=3D2|lastcpupid=3D0x1fffff) > > [ 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))))) > >=20 > >=20 > > Indeed, the mapcount of the subpage is 2 instead of 1. The subpage is o= nly > > 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. > >=20 > > I created this reduced reproducer that triggers 100%: Very helpful, thank you. > >=20 > >=20 > > #include > > #include > > #include > > #include > >=20 > > int main(void) > > { > > =09mmap((void*)0x20000000ul, 0x1000000ul, PROT_READ|PROT_WRITE|PROT_EXE= C, > > =09 MAP_ANONYMOUS|MAP_FIXED|MAP_PRIVATE, -1, 0ul); > > =09madvise((void*)0x20000000ul, 0x1000000ul, MADV_HUGEPAGE); > >=20 > > =09*(uint32_t*)0x20000080 =3D 0x80000; > > =09mlock((void*)0x20001000ul, 0x2000ul); > > =09mlock((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. > > =09mbind((void*)0x20002000ul, 0x1000ul, MPOL_LOCAL, NULL, 0x7fful, > > =09MPOL_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 > > =09return 0; > > } > >=20 > > We map a large-enough are for a THP and then populate a fresh anon THP = (PMD > > mapped) > > to write to it. > >=20 > > The first mlock() will trigger PTE-mapping the THP and mlocking that su= bpage. > > The second mlock() seems to cause the issue. The final mbind() triggers= page > > migration. > >=20 > > 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. > >=20 > > Somehow, the double mlock() of the same page seems to affect our mapcou= nt. > >=20 > > CCing Hugh. >=20 > 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. >=20 > Hugh ---1463760895-1811275884-1674974979=:23545--