On 2/12/21 3:09 PM, David Hildenbrand wrote: > On 12.02.21 08:02, Anshuman Khandual wrote: >> >> On 2/11/21 2:07 PM, David Hildenbrand wrote: >>> On 11.02.21 07:22, Anshuman Khandual wrote: >>>> The following warning gets triggered while trying to boot a 64K page size >>>> without THP config kernel on arm64 platform. >>>> >>>> WARNING: CPU: 5 PID: 124 at mm/vmstat.c:1080 >>>> __fragmentation_index+0xa4/0xc0 >>>> Modules linked in: >>>> CPU: 5 PID: 124 Comm: kswapd0 Not tainted 5.11.0-rc6-00004-ga0ea7d62002 >>>> #159 >>>> Hardware name: linux,dummy-virt (DT) >>>> [ 8.810673] pstate: 20400005 (nzCv daif +PAN -UAO -TCO BTYPE=--) >>>> [ 8.811732] pc : __fragmentation_index+0xa4/0xc0 >>>> [ 8.812555] lr : fragmentation_index+0xf8/0x138 >>>> [ 8.813360] sp : ffff0000864079b0 >>>> [ 8.813958] x29: ffff0000864079b0 x28: 0000000000000372 >>>> [ 8.814901] x27: 0000000000007682 x26: ffff8000135b3948 >>>> [ 8.815847] x25: 1fffe00010c80f48 x24: 0000000000000000 >>>> [ 8.816805] x23: 0000000000000000 x22: 000000000000000d >>>> [ 8.817764] x21: 0000000000000030 x20: ffff0005ffcb4d58 >>>> [ 8.818712] x19: 000000000000000b x18: 0000000000000000 >>>> [ 8.819656] x17: 0000000000000000 x16: 0000000000000000 >>>> [ 8.820613] x15: 0000000000000000 x14: ffff8000114c6258 >>>> [ 8.821560] x13: ffff6000bff969ba x12: 1fffe000bff969b9 >>>> [ 8.822514] x11: 1fffe000bff969b9 x10: ffff6000bff969b9 >>>> [ 8.823461] x9 : dfff800000000000 x8 : ffff0005ffcb4dcf >>>> [ 8.824415] x7 : 0000000000000001 x6 : 0000000041b58ab3 >>>> [ 8.825359] x5 : ffff600010c80f48 x4 : dfff800000000000 >>>> [ 8.826313] x3 : ffff8000102be670 x2 : 0000000000000007 >>>> [ 8.827259] x1 : ffff000086407a60 x0 : 000000000000000d >>>> [ 8.828218] Call trace: >>>> [ 8.828667] __fragmentation_index+0xa4/0xc0 >>>> [ 8.829436] fragmentation_index+0xf8/0x138 >>>> [ 8.830194] compaction_suitable+0x98/0xb8 >>>> [ 8.830934] wakeup_kcompactd+0xdc/0x128 >>>> [ 8.831640] balance_pgdat+0x71c/0x7a0 >>>> [ 8.832327] kswapd+0x31c/0x520 >>>> [ 8.832902] kthread+0x224/0x230 >>>> [ 8.833491] ret_from_fork+0x10/0x30 >>>> [ 8.834150] ---[ end trace 472836f79c15516b ]--- >>>> >>>> This warning comes from __fragmentation_index() when the requested order >>>> is greater than MAX_ORDER. >>>> >>>> static int __fragmentation_index(unsigned int order, >>>> struct contig_page_info *info) >>>> { >>>> unsigned long requested = 1UL << order; >>>> >>>> if (WARN_ON_ONCE(order >= MAX_ORDER)) <===== Triggered here >>>> return 0; >>>> >>>> Digging it further reveals that pageblock_order has been assigned a value >>>> which is greater than MAX_ORDER failing the above check. But why this >>>> happened ? Because HUGETLB_PAGE_ORDER for the given config on arm64 is >>>> greater than MAX_ORDER. >>>> >>>> The solution involves enabling HUGETLB_PAGE_SIZE_VARIABLE which would make >>>> pageblock_order a variable instead of constant HUGETLB_PAGE_ORDER. But that >>>> change alone also did not really work as pageblock_order still got assigned >>>> as HUGETLB_PAGE_ORDER in set_pageblock_order(). HUGETLB_PAGE_ORDER needs to >>>> be less than MAX_ORDER for its appropriateness as pageblock_order otherwise >>>> just fallback to MAX_ORDER - 1 as before. While here it also fixes a build >>>> problem via type casting MAX_ORDER in rmem_cma_setup(). >>> >>> I'm wondering, is there any real value in allowing FORCE_MAX_ZONEORDER to >>> be "11" with ARM64_64K_PAGES/ARM64_16K_PAGES? >> >> MAX_ORDER should be as high as would be required for the current config. >> Unless THP is enabled, there is no need for it to be any higher than 11. >> But I might be missing historical reasons around this as well. Probably >> others from arm64 could help here. > > Theoretically yes, practically no. If nobody cares about a configuration, no > need to make the code more complicated for that configuration. > >> >>> >>> Meaning: are there any real use cases that actually build a kernel without >>> TRANSPARENT_HUGEPAGE and with ARM64_64K_PAGES/ARM64_16K_PAGES? >> >> THP is always optional. Besides kernel builds without THP should always >> be supported. Assuming that all builds will have THP enabled, might not >> be accurate. >> >>> >>> As builds are essentially broken, I assume this is not that relevant? Or >>> how long has it been broken? >> >> Git blame shows that it's been there for some time now. But how does >> that make this irrelevant ? A problem should be fixed nonetheless. > > When exactly did I say not to fix it? I'm saying if nobody uses it, we might > be able to simplify.
I might have misunderstood the relevance of a problematic configuration wrt the need for it to be fixed. My bad, never mind :) > >> >>> >>> It might be easier to just drop the "TRANSPARENT_HUGEPAGE" part from the >>> FORCE_MAX_ZONEORDER config. >>> >> >> Not sure if it would be a good idea to unnecessarily have larger MAX_ORDER >> value for a given config. But I might be missing other contexts here. > > My point is: keep it simple if there is no need to make it complicated. If > these arm64 variants are the only cases where we run into that issue and > nobody uses them ("hat it's been there for some time now"), why make stuff > complicated? > > The current code seems to assume that HUGETLB_PAGE_ORDER <= MAX_ORDER. > Instead of changing that for optimizing an unused use case (it is broken), > just simplify the arm64 conditions. I'd even say add a Will try and change MAX_ORDER on arm64. > > /* > * Some code assumes that HUGETLB_PAGE_ORDER <= MAX_ORDER. For now, only > * !TRANSPARENT_HUGEPAGE could lead in archs creating such setups. > * Forbid it for now; anybody that has a valid use case has to sanitize > * the code. > */ > BUILD_BUG_ON(HUGETLB_PAGE_ORDER <= MAX_ORDER); Also add another, might have to find a proper place to put these. BUILD_BUG_ON(HPAGE_PMD_ORDER <= MAX_ORDER) > > > But again, if there are valid use cases then sure, let's make the code fully > compatible with HUGETLB_PAGE_ORDER > MAX_ORDER. Given that gigantic HugeTLB allocation can fallback on alloc_contig_pages() or CMA if/when available, is there a real need for HUGETLB_PAGE_ORDER to be upto MAX_ORDER, used as pageblock_order or as COMPACTION_HPAGE_ORDER ? With gigantic HugeTLB pages being available, HUGETLB_PAGE_ORDER seems to be just detached from the buddy allocator. But I am not sure, will keep looking. > > > BTW: can we be sure that you caught all issues? For example: > > #define COMPACTION_HPAGE_ORDER HUGETLB_PAGE_ORDER > > I'm not sure if it will work as expected if HUGETLB_PAGE_ORDER > MAX_ORDER > ... fill_contig_page_info() will never find any suitable free blocks ... > Right, there is an assumption that COMPACTION_HPAGE_ORDER < MAX_ORDER in fill_contig_page_info() and will never find any suitable free blocks. This along with pageblock_order needs to change when HUGETLB_PAGE_ORDER can be greater than equal to MAX_ORDER. - Anshuman _______________________________________________ iommu mailing list iommu@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/iommu