Re: mm: shmem: freeing mlocked page

[Vlastimil Babka <vbabka@suse.cz>]

On 11/18/2014 10:58 PM, Andrew Morton wrote:
> On Fri, 14 Nov 2014 09:39:40 -0500 Sasha Levin <sasha.levin@oracle.com> wrote:
> 
>> 
>> [ 1026.988043] BUG: Bad page state in process trinity-c374  pfn:23f70
>> [ 1026.989684] page:ffffea0000b3d300 count:0 mapcount:0 mapping:          (null) index:0x5b
>> [ 1026.991151] flags: 0x1fffff8028000c(referenced|uptodate|swapbacked|mlocked)
>> [ 1026.992410] page dumped because: PAGE_FLAGS_CHECK_AT_FREE flag(s) set
>> [ 1026.993479] bad because of flags:
>> [ 1026.994125] flags: 0x200000(mlocked)
> 
> Gee that new page dumping code is nice!
> 
>> [ 1026.994816] Modules linked in:
>> [ 1026.995378] CPU: 7 PID: 7879 Comm: trinity-c374 Not tainted 3.18.0-rc4-next-20141113-sasha-00047-gd1763ce-dirty #1455
>> [ 1026.996123] FAULT_INJECTION: forcing a failure.
>> [ 1026.996123] name failslab, interval 100, probability 30, space 0, times -1
>> [ 1026.999050]  0000000000000000 0000000000000000 0000000000b3d300 ffff88061295bbd8
>> [ 1027.000676]  ffffffff92f71097 0000000000000000 ffffea0000b3d300 ffff88061295bc08
>> [ 1027.002020]  ffffffff8197ef7a ffffea0000b3d300 ffffffff942dd148 dfffe90000000000
>> [ 1027.003359] Call Trace:
>> [ 1027.003831] dump_stack (lib/dump_stack.c:52)
>> [ 1027.004725] bad_page (mm/page_alloc.c:338)
>> [ 1027.005623] free_pages_prepare (mm/page_alloc.c:657 mm/page_alloc.c:763)
>> [ 1027.006761] free_hot_cold_page (mm/page_alloc.c:1438)
>> [ 1027.007772] ? __page_cache_release (mm/swap.c:66)
>> [ 1027.008815] put_page (mm/swap.c:270)
>> [ 1027.009665] page_cache_pipe_buf_release (fs/splice.c:93)
>> [ 1027.010888] __splice_from_pipe (fs/splice.c:784 fs/splice.c:886)
>> [ 1027.011917] ? might_fault (./arch/x86/include/asm/current.h:14 mm/memory.c:3734)
>> [ 1027.012856] ? pipe_lock (fs/pipe.c:69)
>> [ 1027.013728] ? write_pipe_buf (fs/splice.c:1534)
>> [ 1027.014756] vmsplice_to_user (fs/splice.c:1574)
>> [ 1027.015725] ? rcu_read_lock_held (kernel/rcu/update.c:169)
>> [ 1027.016757] ? __fget_light (include/linux/fdtable.h:80 fs/file.c:684)
>> [ 1027.017782] SyS_vmsplice (fs/splice.c:1656 fs/splice.c:1639)
>> [ 1027.018863] tracesys_phase2 (arch/x86/kernel/entry_64.S:529)
>> 
> 
> So what happened here?  Userspace fed some mlocked memory into splice()
> and then, while splice() was running, userspace dropped its reference
> to the memory, leaving splice() with the last reference.  Yet somehow,
> that page was still marked as being mlocked.  I wouldn't expect the
> kernel to permit userspace to drop its reference to the memory without
> first clearing the mlocked state.

I did check a bit and something caught my eye. Both page_remove_rmap() and
page_remove_file_rmap() contain this:

        if (unlikely(PageMlocked(page)))
                clear_page_mlock(page);

So could maybe something mlock the page between the check and clear?

I find lru_cache_add_active_or_unevictable somewhat suspicious. But checking if
these two could race will take some time.

> Is it possible to work out from trinity sources what the exact sequence
> was?  Which syscalls are being used, for example?
> 
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