Re: mm: shmem: freeing mlocked page

[Andrew Morton <akpm@linux-foundation.org>]

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.

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

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>