Re: [PATCH] mm: thp: use down_read_trylock in khugepaged to avoid long block

[Anshuman Khandual]

On 12/15/2017 01:23 AM, Yang Shi wrote:
> In the current design, khugepaged need acquire mmap_sem before scanning
> mm, but in some corner case, khugepaged may scan the current running
> process which might be modifying memory mapping, so khugepaged might
> block in uninterruptible state. But, the process might hold the mmap_sem
> for long time when modifying a huge memory space, then it may trigger
> the below khugepaged hung issue:
> 
> INFO: task khugepaged:270 blocked for more than 120 seconds. 
> Tainted: G E 4.9.65-006.ali3000.alios7.x86_64 #1
> "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message. 
> khugepaged D 0 270 2 0x00000000 
> ffff883f3deae4c0 0000000000000000 ffff883f610596c0 ffff883f7d359440
> ffff883f63818000 ffffc90019adfc78 ffffffff817079a5 d67e5aa8c1860a64
> 0000000000000246 ffff883f7d359440 ffffc90019adfc88 ffff883f610596c0
> Call Trace: 
> [<ffffffff817079a5>] ? __schedule+0x235/0x6e0 
> [<ffffffff81707e86>] schedule+0x36/0x80
> [<ffffffff8170a970>] rwsem_down_read_failed+0xf0/0x150
> [<ffffffff81384998>] call_rwsem_down_read_failed+0x18/0x30
> [<ffffffff8170a1c0>] down_read+0x20/0x40
> [<ffffffff81226836>] khugepaged+0x476/0x11d0
> [<ffffffff810c9d0e>] ? idle_balance+0x1ce/0x300
> [<ffffffff810d0850>] ? prepare_to_wait_event+0x100/0x100
> [<ffffffff812263c0>] ? collapse_shmem+0xbf0/0xbf0
> [<ffffffff810a8d46>] kthread+0xe6/0x100
> [<ffffffff810a8c60>] ? kthread_park+0x60/0x60
> [<ffffffff8170cd15>] ret_from_fork+0x25/0x30
> 
> So, it sounds pointless to just block for waiting for the semaphore for
> khugepaged, here replace down_read() to down_read_trylock() to move to
> scan next mm quickly instead of just blocking on the semaphore so that
> other processes can get more chances to install THP.
> Then khugepaged can come back to scan the skipped mm when finish the
> current round full_scan.

That may be too harsh on the process which now has to wait for a complete
round of full scan before the khugepaged comes back. What if the mmap_sem
contention because of VMA changes in the process was just temporary ?