On Fri 16-09-16 15:13:56, Hugh Dickins wrote: > On Wed, 14 Sep 2016, Michal Hocko wrote: > > On Wed 14-09-16 10:42:19, Michal Hocko wrote: > > > [Let's CC Hugh] > > > > now for real... > > > > > > > > On Wed 14-09-16 15:13:50, zhong jiang wrote: > > > [...] > > > > hi, Michal > > > > > > > > Recently, I hit the same issue when run a OOM case of the LTP and ksm enable. > > > > > > > > [ 601.937145] Call trace: > > > > [ 601.939600] [<ffffffc000086a88>] __switch_to+0x74/0x8c > > > > [ 601.944760] [<ffffffc000a1bae0>] __schedule+0x23c/0x7bc > > > > [ 601.950007] [<ffffffc000a1c09c>] schedule+0x3c/0x94 > > > > [ 601.954907] [<ffffffc000a1eb84>] rwsem_down_write_failed+0x214/0x350 > > > > [ 601.961289] [<ffffffc000a1e32c>] down_write+0x64/0x80 > > > > [ 601.966363] [<ffffffc00021f794>] __ksm_exit+0x90/0x19c > > > > [ 601.971523] [<ffffffc0000be650>] mmput+0x118/0x11c > > > > [ 601.976335] [<ffffffc0000c3ec4>] do_exit+0x2dc/0xa74 > > > > [ 601.981321] [<ffffffc0000c46f8>] do_group_exit+0x4c/0xe4 > > > > [ 601.986656] [<ffffffc0000d0f34>] get_signal+0x444/0x5e0 > > > > [ 601.991904] [<ffffffc000089fcc>] do_signal+0x1d8/0x450 > > > > [ 601.997065] [<ffffffc00008a35c>] do_notify_resume+0x70/0x78 > > > > > > So this is a hung task triggering because the exiting task cannot get > > > the mmap sem for write because the ksmd holds it for read while > > > allocating memory which just takes ages to complete, right? > > > > > > > > > > > The root case is that ksmd hold the read lock. and the lock is not released. > > > > scan_get_next_rmap_item > > > > down_read > > > > get_next_rmap_item > > > > alloc_rmap_item #ksmd will loop permanently. > > > > > > > > How do you see this kind of situation ? or let the issue alone. > > > > > > I am not familiar with the ksmd code so it is hard for me to judge but > > > one thing to do would be __GFP_NORETRY which would force a bail out from > > > the allocation rather than looping for ever. A quick look tells me that > > > the allocation failure here is quite easy to handle. There might be > > > others... > > Yes, very good suggestion in this case: the ksmd code does exactly the > right thing when that allocation fails, but was too stupid to use an > allocation mode which might fail - and it can allocate rather a lot of > slots along that path, so it will be good to let it break out there. > > Thank you, Zhongjiang, please send akpm a fully signed-off patch, tagged > for stable, with your explanation above (which was a lot more helpful > to me than what you wrote in your other mail of Sept 13th). But please > make it GFP_KERNEL | __GFP_NORETRY | __GFP_NOWARN (and break that line agreed > before 80 cols): the allocation will sometimes fail, and we're not at > all interested in hearing about that. > > Michal, how would you feel about this or a separate patch adding > __GFP_HIGH to the allocation in ksm's alloc_stable_node()? That > allocation could cause the same problem, but it is much less common > (so less important to do anything about it), and differs from the > rmap_item case in that if it succeeds, it will usually free a page; > whereas if it fails, the fallback (two break_cow()s) may want to > allocate a couple of pages. So __GFP_HIGH makes more sense for it > than __GFP_NORETRY: but perhaps we prefer not to add __GFP_HIGHs? I am not familiar with the ksmd code enough to have a strong opinion here. __GFP_HIGH should be imho used only when really necessary but as you point out and comment in cmp_and_merge_page explain /* * If we fail to insert the page into the stable tree, * we will have 2 virtual addresses that are pointing * to a ksm page left outside the stable tree, * in which case we need to break_cow on both. */ this can actually save some memory if succeed. So I will leave the decision to you. I have no experience in how much this path can actually eat and whether the flag actually makes much difference. Thanks! -- Michal Hocko SUSE Labs -- 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>