On Sun, 18 Sep 2016 10:26:10 +0800 zhongjiang <zhongjiang@xxxxxxxxxx> wrote: > I hit the following issue when run a OOM case of the LTP and > ksm enable. > > Call trace: > [<ffffffc000086a88>] __switch_to+0x74/0x8c > [<ffffffc000a1bae0>] __schedule+0x23c/0x7bc > [<ffffffc000a1c09c>] schedule+0x3c/0x94 > [<ffffffc000a1eb84>] rwsem_down_write_failed+0x214/0x350 > [<ffffffc000a1e32c>] down_write+0x64/0x80 > [<ffffffc00021f794>] __ksm_exit+0x90/0x19c > [<ffffffc0000be650>] mmput+0x118/0x11c > [<ffffffc0000c3ec4>] do_exit+0x2dc/0xa74 > [<ffffffc0000c46f8>] do_group_exit+0x4c/0xe4 > [<ffffffc0000d0f34>] get_signal+0x444/0x5e0 > [<ffffffc000089fcc>] do_signal+0x1d8/0x450 > [<ffffffc00008a35c>] do_notify_resume+0x70/0x78 > > it will leads to a hung task because the exiting task cannot get the > mmap sem for write. but the root cause is that the ksmd holds it for > read while allocateing memory which just takes ages to complete. > and ksmd will loop in the following path. > > scan_get_next_rmap_item > down_read > get_next_rmap_item > alloc_rmap_item #ksmd will loop permanently. > > we fix it by changing the GFP to allow the allocation sometimes fail, and > we're not at all interested in hearing abot that. It would be better if the changelog were to describe *why* this is harmless. I assume that if the allocation fails, scan_get_next_rmap_item() will bale out and ksmd just gives up and takes a sleep? Also, did you instead consider changing scan_get_next_rmap_item() to simply not hold mmap_sem for so long? Scan a megabyte or so then drop mmap_sem for a while, then scan some more? The whole thing is driven by ksm.scan_address so handling the races should be simple. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>