Re: mm: shmem: freeing mlocked page

[Sasha Levin]

On 11/18/2014 04:58 PM, Andrew Morton wrote:
>> [ 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.
> 
> Is it possible to work out from trinity sources what the exact sequence
> was?  Which syscalls are being used, for example?

Phew, this took a long while but I've bisected it (with good confidence) down
to:

commit a38246260912ba4a0f8b563704a965a7a97cf3c3
Author: Davidlohr Bueso <dave@xxxxxxxxxxxx>
Date:   Wed Dec 3 18:54:27 2014 +1100

    mm/memory.c: share the i_mmap_rwsem

    The unmap_mapping_range family of functions do the unmapping of user pages
    (ultimately via zap_page_range_single) without touching the actual
    interval tree, thus share the lock.

    Signed-off-by: Davidlohr Bueso <dbueso@xxxxxxx>
    Cc: "Kirill A. Shutemov" <kirill@xxxxxxxxxxxxx>
    Acked-by: Hugh Dickins <hughd@xxxxxxxxxx>
    Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
    Cc: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
    Cc: Rik van Riel <riel@xxxxxxxxxx>
    Cc: Srikar Dronamraju <srikar@xxxxxxxxxxxxxxxxxx>
    Acked-by: Mel Gorman <mgorman@xxxxxxx>
    Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>


Thanks,
Sasha
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