Re: [PATCH] mm: __delete_from_page_cache show Bad page if mapped

Sasha Levin
2016-04-18T15:07:23Z

On 03/01/2016 01:45 AM, Hugh Dickins wrote:
> Commit e1534ae95004 ("mm: differentiate page_mapped() from page_mapcount()
> for compound pages") changed the famous BUG_ON(page_mapped(page)) in
> __delete_from_page_cache() to VM_BUG_ON_PAGE(page_mapped(page)): which
> gives us more info when CONFIG_DEBUG_VM=y, but nothing at all when not.
>
> Although it has not usually been very helpul, being hit long after the
> error in question, we do need to know if it actually happens on users'
> systems; but reinstating a crash there is likely to be opposed :)
>
> In the non-debug case, pr_alert("BUG: Bad page cache") plus dump_page(),
> dump_stack(), add_taint() - I don't really believe LOCKDEP_NOW_UNRELIABLE,
> but that seems to be the standard procedure now. Move that, or the
> VM_BUG_ON_PAGE(), up before the deletion from tree: so that the
> unNULLified page->mapping gives a little more information.
>
> If the inode is being evicted (rather than truncated), it won't have
> any vmas left, so it's safe(ish) to assume that the raised mapcount is
> erroneous, and we can discount it from page_count to avoid leaking the
> page (I'm less worried by leaking the occasional 4kB, than losing a
> potential 2MB page with each 4kB page leaked).
>
> Signed-off-by: Hugh Dickins <hughd@google.com>
> ---
> I think this should go into v4.5, so I've written it with an atomic_sub
> on page->_count; Joonsoo has noticed, and kindly agreed to page_ref'ify
> it for mmotm after it's merged.

Hey Hugh,

I seem to be hitting this while fuzzing:

[ 817.413969] BUG: Bad rss-counter state mm:ffff8801b2a1d000 idx:0 val:1
[ 817.413974] BUG: Bad rss-counter state mm:ffff8801b2a1d000 idx:1 val:2
[ 817.413977] BUG: non-zero nr_ptes on freeing mm: 2
[ 817.606318] page:ffffea0002fb6cc0 count:3 mapcount:1 mapping:ffff8801b58129e8 index:0x0
[ 817.606328] flags: 0x1fffff8000002d(locked|referenced|uptodate|lru)
[ 817.606333] page dumped because: VM_BUG_ON_PAGE(page_mapped(page))
[ 817.606336] page->mem_cgroup:ffff8801d1ce1be0
[ 817.606489] ------------[ cut here ]------------
[ 817.606493] kernel BUG at mm/filemap.c:196!
[ 817.606509] invalid opcode: 0000 [#1] PREEMPT SMP KASAN
[ 817.606519] Modules linked in:
[ 817.606529] CPU: 0 PID: 23148 Comm: modprobe Tainted: G B 4.6.0-rc3-next-20160412-sasha-00023-g0b02d6d-dirty #2998
[ 817.606535] task: ffff8801acc31000 ti: ffff8801acc38000 task.ti: ffff8801acc38000
[ 817.606573] RIP: __delete_from_page_cache (mm/filemap.c:196 (discriminator 1))
[ 817.606577] RSP: 0000:ffff8801acc3f660 EFLAGS: 00010082
[ 817.606582] RAX: 0000000000000000 RBX: ffffea0002fb6cc0 RCX: 0000000000000000
[ 817.606586] RDX: 0000000000000000 RSI: 0000000000000082 RDI: ffffed0035987eb2
[ 817.606591] RBP: ffff8801acc3f7a0 R08: ffff8801d3ddfbf7 R09: ffff8801d3ddfbf5
[ 817.606596] R10: 000000000013000d R11: 00000000e949c223 R12: ffff8801b58129e8
[ 817.606601] R13: ffff8800c7296620 R14: ffff8801acc3f778 R15: ffffea0002b298a0
[ 817.606612] FS: 00007f5159f41700(0000) GS:ffff8801d4200000(0000) knlGS:0000000000000000
[ 817.606617] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 817.606622] CR2: 000055c523664d20 CR3: 00000001c28ec000 CR4: 00000000000406f0
[ 817.606629] Stack:
[ 817.606638] ffff8800cb5d43c0 0000000000000282 1ffff10035987ed3 ffffea0002fb6cc8
[ 817.606646] ffff8801acc31000 ffffea0002fb6ce0 0000000000000000 0000000041b58ab3
[ 817.606654] ffffffffa967ddb8 ffffffff9d637a20 ffffffff9d3e26e0 0000000000000000
[ 817.606655] Call Trace:
[ 817.606717] delete_from_page_cache (include/linux/spinlock.h:362 mm/filemap.c:264)
[ 817.606726] truncate_inode_page (mm/truncate.c:165)
[ 817.606735] truncate_inode_pages_range (mm/truncate.c:290)
[ 817.606817] truncate_inode_pages_final (mm/truncate.c:447)
[ 817.606826] v9fs_evict_inode (fs/9p/vfs_inode.c:455)
[ 817.606837] evict (fs/inode.c:548)
[ 817.606855] iput (fs/inode.c:1483 fs/inode.c:1510)
[ 817.606872] __dentry_kill (fs/dcache.c:345 fs/dcache.c:532)
[ 817.606917] dput (fs/dcache.c:786)
[ 817.606938] __fput (fs/file_table.c:227)
[ 817.606947] ____fput (fs/file_table.c:245)
[ 817.606958] task_work_run (kernel/task_work.c:117 (discriminator 1))
[ 817.606969] do_exit (kernel/exit.c:749)
[ 817.607031] do_group_exit (kernel/exit.c:862)
[ 817.607048] SyS_exit_group (kernel/exit.c:889)
[ 817.607056] do_syscall_64 (arch/x86/entry/common.c:350)
[ 817.607076] entry_SYSCALL64_slow_path (arch/x86/entry/entry_64.S:251)
[ 817.607151] Code: e7 e8 8c 7d 19 00 e8 97 6f f6 ff 48 89 df e8 af 00 07 00 84 c0 74 22 e8 86 6f f6 ff 48 c7 c6 c0 5f 30 a7 48 89 df e8 87 8d 09 00 <0f> 0b 48 c7 c7 60 3a 42
ab e8 cb c7 a8 01 e8 64 6f f6 ff 48 8b
All code
========
0: e7 e8 out %eax,$0xe8
2: 8c 7d 19 mov %?,0x19(%rbp)
5: 00 e8 add %ch,%al
7: 97 xchg %eax,%edi
8: 6f outsl %ds:(%rsi),(%dx)
9: f6 ff idiv %bh
b: 48 89 df mov %rbx,%rdi
e: e8 af 00 07 00 callq 0x700c2
13: 84 c0 test %al,%al
15: 74 22 je 0x39
17: e8 86 6f f6 ff callq 0xfffffffffff66fa2
1c: 48 c7 c6 c0 5f 30 a7 mov $0xffffffffa7305fc0,%rsi
23: 48 89 df mov %rbx,%rdi
26: e8 87 8d 09 00 callq 0x98db2
2b:* 0f 0b ud2 <-- trapping instruction
2d: 48 c7 c7 60 3a 42 ab mov $0xffffffffab423a60,%rdi
34: e8 cb c7 a8 01 callq 0x1a8c804
39: e8 64 6f f6 ff callq 0xfffffffffff66fa2
3e: 48 8b 00 mov (%rax),%rax

Code starting with the faulting instruction
===========================================
0: 0f 0b ud2
2: 48 c7 c7 60 3a 42 ab mov $0xffffffffab423a60,%rdi
9: e8 cb c7 a8 01 callq 0x1a8c7d9
e: e8 64 6f f6 ff callq 0xfffffffffff66f77
13: 48 8b 00 mov (%rax),%rax
[ 817.607160] RIP __delete_from_page_cache (mm/filemap.c:196 (discriminator 1))
[ 817.607162] RSP <ffff8801acc3f660>