Re: Multiple potential races on vma->vm_flags

[Sasha Levin]

On 09/23/2015 09:08 AM, Andrey Konovalov wrote:
> On Wed, Sep 23, 2015 at 3:39 AM, Hugh Dickins <hughd@xxxxxxxxxx> wrote:
>> > This is totally untested, and one of you may quickly prove me wrong;
>> > but I went in to fix your "Bad page state (mlocked)" by holding pte
>> > lock across the down_read_trylock of mmap_sem in try_to_unmap_one(),
>> > then couldn't see why it would need mmap_sem at all, given how mlock
>> > and munlock first assert intention by setting or clearing VM_LOCKED
>> > in vm_flags, then work their way up the vma, taking pte locks.
>> >
>> > Calling mlock_vma_page() under pte lock may look suspicious
>> > at first: but what it does is similar to clear_page_mlock(),
>> > which we regularly call under pte lock from page_remove_rmap().
>> >
>> > I'd rather wait to hear whether this appears to work in practice,
>> > and whether you agree that it should work in theory, before writing
>> > the proper description.  I'd love to lose that down_read_trylock.
> No, unfortunately it doesn't work, I still see "Bad page state (mlocked)".
> 
> It seems that your patch doesn't fix the race from the report below, since pte
> lock is not taken when 'vma->vm_flags &= ~VM_LOCKED;' (mlock.c:425)
> is being executed. (Line numbers are from kernel with your patch applied.)

I've fired up my HZ_10000 patch, and this seems to be a real race that is
somewhat easy to reproduce under those conditions.

Here's a fresh backtrace from my VMs:

[1935109.882343] BUG: Bad page state in process trinity-subchil  pfn:3ca200
[1935109.884000] page:ffffea000f288000 count:0 mapcount:0 mapping:          (null) index:0x1e00 compound_mapcount: 0
[1935109.885772] flags: 0x22fffff80144008(uptodate|head|swapbacked|mlocked)
[1935109.887174] page dumped because: PAGE_FLAGS_CHECK_AT_FREE flag(s) set
[1935109.888197] bad because of flags:
[1935109.888759] flags: 0x100000(mlocked)
[1935109.889525] Modules linked in:
[1935109.890165] CPU: 8 PID: 2615 Comm: trinity-subchil Not tainted 4.3.0-rc2-next-20150923-sasha-00079-gec04207-dirty #2569
[1935109.891876]  1ffffffff6445448 00000000e5dca494 ffff8803f7657708 ffffffffa70402da
[1935109.893504]  ffffea000f288000 ffff8803f7657738 ffffffffa56e522b 022fffff80144008
[1935109.894947]  ffffea000f288020 ffffea000f288000 00000000ffffffff ffff8803f76577a8
[1935109.896413] Call Trace:
[1935109.899102]  [<ffffffffa70402da>] dump_stack+0x4e/0x84
[1935109.899821]  [<ffffffffa56e522b>] bad_page+0x17b/0x210
[1935109.900469]  [<ffffffffa56e85a8>] free_pages_prepare+0xb48/0x1110
[1935109.902127]  [<ffffffffa56ee0d1>] __free_pages_ok+0x21/0x260
[1935109.904435]  [<ffffffffa56ee373>] free_compound_page+0x63/0x80
[1935109.905614]  [<ffffffffa581b51e>] free_transhuge_page+0x6e/0x80
[1935109.906752]  [<ffffffffa5709f76>] __put_compound_page+0x76/0xa0
[1935109.907884]  [<ffffffffa570a475>] release_pages+0x4d5/0x9f0
[1935109.913027]  [<ffffffffa5769bea>] tlb_flush_mmu_free+0x8a/0x120
[1935109.913957]  [<ffffffffa576f993>] unmap_page_range+0xe73/0x1460
[1935109.915737]  [<ffffffffa57700a6>] unmap_single_vma+0x126/0x2f0
[1935109.916646]  [<ffffffffa577270d>] unmap_vmas+0xdd/0x190
[1935109.917454]  [<ffffffffa5790361>] exit_mmap+0x221/0x430
[1935109.921176]  [<ffffffffa5366da1>] mmput+0xb1/0x240
[1935109.921919]  [<ffffffffa537b3b2>] do_exit+0x732/0x27c0
[1935109.928561]  [<ffffffffa537d599>] do_group_exit+0xf9/0x300
[1935109.929786]  [<ffffffffa537d7bd>] SyS_exit_group+0x1d/0x20
[1935109.930617]  [<ffffffffaf59fbf6>] entry_SYSCALL_64_fastpath+0x16/0x7a


Thanks,
Sasha
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