On Mon, 19 Sep 2016, Andrew Morton wrote: > 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? Exactly. (If that sleep time has been configured to 0, so be it.) Michal asked for the same reassurance, I expect a new version will be coming. > > 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. It already does that, configurable intervals: the "endless looping in allocating memory" is not at the ksm.c level, but inside page_alloc.c: the __GFP_NORETRY being to get it out of there and back to ksm.c, which then does the right thing on failure. Hugh -- 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>