Re: [PATCH 10/12] hugetlb: batch PMD split for bulk vmemmap dedup

[Joao Martins <joao.m.martins@oracle.com>]

On 26/08/2023 06:56, kernel test robot wrote:
> Hi Mike,
> 
> kernel test robot noticed the following build errors:
> 
> [auto build test ERROR on next-20230825]
> [cannot apply to akpm-mm/mm-everything v6.5-rc7 v6.5-rc6 v6.5-rc5 linus/master v6.5-rc7]
> [If your patch is applied to the wrong git tree, kindly drop us a note.
> And when submitting patch, we suggest to use '--base' as documented in
> https://git-scm.com/docs/git-format-patch#_base_tree_information]
> 
> url:    https://github.com/intel-lab-lkp/linux/commits/Mike-Kravetz/hugetlb-clear-flags-in-tail-pages-that-will-be-freed-individually/20230826-030805
> base:   next-20230825
> patch link:    https://lore.kernel.org/r/20230825190436.55045-11-mike.kravetz%40oracle.com
> patch subject: [PATCH 10/12] hugetlb: batch PMD split for bulk vmemmap dedup
> config: s390-randconfig-001-20230826 (https://download.01.org/0day-ci/archive/20230826/202308261325.ipTttZHZ-lkp@intel.com/config)
> compiler: clang version 17.0.0 (https://github.com/llvm/llvm-project.git 4a5ac14ee968ff0ad5d2cc1ffa0299048db4c88a)
> reproduce: (https://download.01.org/0day-ci/archive/20230826/202308261325.ipTttZHZ-lkp@intel.com/reproduce)
> 
> If you fix the issue in a separate patch/commit (i.e. not just a new version of
> the same patch/commit), kindly add following tags
> | Reported-by: kernel test robot <lkp@intel.com>
> | Closes: https://lore.kernel.org/oe-kbuild-all/202308261325.ipTttZHZ-lkp@intel.com/
> 
> All error/warnings (new ones prefixed by >>):
> 

[...]

>>> mm/hugetlb_vmemmap.c:698:28: error: use of undeclared identifier 'TLB_FLUSH_ALL'
>      698 |         flush_tlb_kernel_range(0, TLB_FLUSH_ALL);
>          |                                   ^
>    2 warnings and 1 error generated.
> 
> 

TLB_FLUSH_ALL is x86 only so what I wrote above is wrong in what should be
architecture independent. The way I should have written the global TLB flush is
to use flush_tlb_all(), which is what is implemented by the arch.

The alternative is to compose a start/end tuple in the top-level optimize-folios
function as we iterate over folios to remap, and flush via
flush_tlb_kernel_range(). But this would likely only be relevant on x86 only,
that is to optimize the flushing of 3 contiguous 2M hugetlb pages (~24 vmemmap
pages) as that's where the TLB flush ceiling is put (31 pages) for per-page VA
flush, before falling back to a global TLB flush. Weren't sure of the added
complexity for dubious benefit thus kept it in global TLB flush.

> vim +/TLB_FLUSH_ALL +698 mm/hugetlb_vmemmap.c
> 
>    666	
>  > 667	void hugetlb_vmemmap_split(const struct hstate *h, struct page *head)
>    668	{
>    669		unsigned long vmemmap_start = (unsigned long)head, vmemmap_end;
>    670		unsigned long vmemmap_reuse;
>    671	
>    672		if (!vmemmap_should_optimize(h, head))
>    673			return;
>    674	
>    675		static_branch_inc(&hugetlb_optimize_vmemmap_key);
>    676	
>    677		vmemmap_end     = vmemmap_start + hugetlb_vmemmap_size(h);
>    678		vmemmap_reuse   = vmemmap_start;
>    679		vmemmap_start   += HUGETLB_VMEMMAP_RESERVE_SIZE;
>    680	
>    681		/*
>    682		 * Remap the vmemmap virtual address range [@vmemmap_start, @vmemmap_end)
>    683		 * to the page which @vmemmap_reuse is mapped to, then free the pages
>    684		 * which the range [@vmemmap_start, @vmemmap_end] is mapped to.
>    685		 */
>    686		if (vmemmap_remap_split(vmemmap_start, vmemmap_end, vmemmap_reuse))
>    687			static_branch_dec(&hugetlb_optimize_vmemmap_key);
>    688	}
>    689	
>    690	void hugetlb_vmemmap_optimize_folios(struct hstate *h, struct list_head *folio_list)
>    691	{
>    692		struct folio *folio;
>    693		LIST_HEAD(vmemmap_pages);
>    694	
>    695		list_for_each_entry(folio, folio_list, lru)
>    696			hugetlb_vmemmap_split(h, &folio->page);
>    697	
>  > 698		flush_tlb_kernel_range(0, TLB_FLUSH_ALL);
>    699	
>    700		list_for_each_entry(folio, folio_list, lru)
>    701			hugetlb_vmemmap_optimize_bulk(h, &folio->page, &vmemmap_pages);
>    702	
>    703		free_vmemmap_page_list(&vmemmap_pages);
>    704	}
>    705	
>