Re: mm: shmem: freeing mlocked page

[Dave Jones]

On Tue, Nov 18, 2014 at 10:44:02PM -0500, Sasha Levin wrote:
 > On 11/18/2014 04:58 PM, Andrew Morton wrote:
 
 > >> [ 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?
 > 
 > Trinity can't really log anything because attempts to log syscalls slow everything
 > down to a crawl to the point nothing reproduces.

If the machine is still alive after /proc/sys/kernel/tainted changes,
trinity will dump a trinity-post-mortem.log somewhere[*] that should
contain the last two syscalls each process did. (Even if logging
is disabled).

It's not perfect however, and knowing that we passed a pointer to
a syscall isn't always useful unless we also dump the data that pointer
pointed at.  It's a work in progress. I don't know if I'm going to
get time to improve it any time soon though.

	Dave

[*] wherever cwd happened to be when the main process exited.
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