Ccing Steven. Hello, On Wed, Nov 18, 2015 at 04:34:30PM +0100, Vlastimil Babka wrote: > On 11/09/2015 08:23 AM, Joonsoo Kim wrote: > > CMA allocation should be guaranteed to succeed by definition, but, > > unfortunately, it would be failed sometimes. It is hard to track down > > the problem, because it is related to page reference manipulation and > > we don't have any facility to analyze it. > > Reminds me of the PeterZ's VM_PINNED patchset. What happened to it? > https://lwn.net/Articles/600502/ > > > This patch adds tracepoints to track down page reference manipulation. > > With it, we can find exact reason of failure and can fix the problem. > > Following is an example of tracepoint output. > > > > <...>-9018 [004] 92.678375: page_ref_set: pfn=0x17ac9 flags=0x0 count=1 mapcount=0 mapping=(nil) mt=4 val=1 > > <...>-9018 [004] 92.678378: kernel_stack: > > => get_page_from_freelist (ffffffff81176659) > > => __alloc_pages_nodemask (ffffffff81176d22) > > => alloc_pages_vma (ffffffff811bf675) > > => handle_mm_fault (ffffffff8119e693) > > => __do_page_fault (ffffffff810631ea) > > => trace_do_page_fault (ffffffff81063543) > > => do_async_page_fault (ffffffff8105c40a) > > => async_page_fault (ffffffff817581d8) > > [snip] > > <...>-9018 [004] 92.678379: page_ref_mod: pfn=0x17ac9 flags=0x40048 count=2 mapcount=1 mapping=0xffff880015a78dc1 mt=4 val=1 > > [snip] > > ... > > ... > > <...>-9131 [001] 93.174468: test_pages_isolated: start_pfn=0x17800 end_pfn=0x17c00 fin_pfn=0x17ac9 ret=fail > > [snip] > > <...>-9018 [004] 93.174843: page_ref_mod_and_test: pfn=0x17ac9 flags=0x40068 count=0 mapcount=0 mapping=0xffff880015a78dc1 mt=4 val=-1 ret=1 > > => release_pages (ffffffff8117c9e4) > > => free_pages_and_swap_cache (ffffffff811b0697) > > => tlb_flush_mmu_free (ffffffff81199616) > > => tlb_finish_mmu (ffffffff8119a62c) > > => exit_mmap (ffffffff811a53f7) > > => mmput (ffffffff81073f47) > > => do_exit (ffffffff810794e9) > > => do_group_exit (ffffffff81079def) > > => SyS_exit_group (ffffffff81079e74) > > => entry_SYSCALL_64_fastpath (ffffffff817560b6) > > > > This output shows that problem comes from exit path. In exit path, > > to improve performance, pages are not freed immediately. They are gathered > > and processed by batch. During this process, migration cannot be possible > > and CMA allocation is failed. This problem is hard to find without this > > page reference tracepoint facility. > > Yeah but when you realized it was this problem, what was the fix? Probably not > remove batching from exit path? Shouldn't CMA in this case just try waiting for > the pins to go away, which would eventually happen? And for long-term pins, > VM_PINNED would make sure the pages are migrated away from CMA pageblocks first? > > So I'm worried that this is quite nontrivial change for a very specific usecase. Minchan already answered it. Thanks, Minchan. This also can be used for memory-offlining and compaction. > > Enabling this feature bloat kernel text 20 KB in my configuration. > > It's not just that, see below. > > [...] > > > > static inline int page_ref_freeze(struct page *page, int count) > > { > > - return likely(atomic_cmpxchg(&page->_count, count, 0) == count); > > + int ret = likely(atomic_cmpxchg(&page->_count, count, 0) == count); > > The "likely" mean makes no sense anymore, doe it? I don't know how compiler uses this hint exactly but it will have same effect as before. Why do you think it makes no sense now? > > > diff --git a/mm/Kconfig.debug b/mm/Kconfig.debug > > index 957d3da..71d2399 100644 > > --- a/mm/Kconfig.debug > > +++ b/mm/Kconfig.debug > > @@ -28,3 +28,7 @@ config DEBUG_PAGEALLOC > > > > config PAGE_POISONING > > bool > > + > > +config DEBUG_PAGE_REF > > + bool "Enable tracepoint to track down page reference manipulation" > > So you should probably state the costs. Which is the extra memory, and also that > all the page ref manipulations are now turned to function calls, even if the > tracepoints are disabled. Yes, will do. > Patch 1 didn't change that many callsites, so maybe it > would be feasible to have the tracepoints inline, where being disabled has > near-zero overhead? Hmm...Although I changed just one get_page() in previous patch, it would be used in many places. So, it's not feasible to inline tracepoints to each callsites directly. In fact, I tried to inline tracepoint into wrapper function directly, but, it fails due to (maybe) header file dependency. Steven, is it possible to add tracepoint to inlined fucntion such as get_page() in include/linux/mm.h? Thanks. -- 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>