linux-kernel - Re: [PATCH v2] mm: hugetlb: optionally allocate gigantic hugepages using cma

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Date:   Tue, 10 Mar 2020 12:19:06 -0700
From:   Roman Gushchin <guro@...com>
To:     Mike Kravetz <mike.kravetz@...cle.com>
CC:     Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>, <linux-mm@...ck.org>,
        <kernel-team@...com>, <linux-kernel@...r.kernel.org>,
        Rik van Riel <riel@...riel.com>
Subject: Re: [PATCH v2] mm: hugetlb: optionally allocate gigantic hugepages
 using cma

On Tue, Mar 10, 2020 at 11:33:47AM -0700, Mike Kravetz wrote:
> On 3/10/20 11:05 AM, Roman Gushchin wrote:
> > On Tue, Mar 10, 2020 at 10:27:01AM -0700, Mike Kravetz wrote:
> >> On 3/9/20 5:25 PM, Roman Gushchin wrote:
> >>> diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c
> >>> index a74262c71484..ceeb06ddfd41 100644
> >>> --- a/arch/x86/kernel/setup.c
> >>> +++ b/arch/x86/kernel/setup.c
> >>> @@ -16,6 +16,7 @@
> >>>  #include <linux/pci.h>
> >>>  #include <linux/root_dev.h>
> >>>  #include <linux/sfi.h>
> >>> +#include <linux/hugetlb.h>
> >>>  #include <linux/tboot.h>
> >>>  #include <linux/usb/xhci-dbgp.h>
> >>>  
> >>> @@ -1158,6 +1159,8 @@ void __init setup_arch(char **cmdline_p)
> >>>  	initmem_init();
> >>>  	dma_contiguous_reserve(max_pfn_mapped << PAGE_SHIFT);
> >>>  
> >>> +	hugetlb_cma_reserve();
> >>> +
> >>
> >> I know this is called from arch specific code here to fit in with the timing
> >> of CMA setup/reservation calls.  However, there really is nothing architecture
> >> specific about this functionality.  It would be great IMO if we could make
> >> this architecture independent.  However, I can not think of a straight forward
> >> way to do this.
> > 
> > I agree. Unfortunately I have no better idea than having an arch-dependent hook.
> > 
> >>
> >>>  	/*
> >>>  	 * Reserve memory for crash kernel after SRAT is parsed so that it
> >>>  	 * won't consume hotpluggable memory.
> >> <snip>
> >>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> >> <snip>
> >>> +void __init hugetlb_cma_reserve(void)
> >>> +{
> >>> +	unsigned long totalpages = 0;
> >>> +	unsigned long start_pfn, end_pfn;
> >>> +	phys_addr_t size;
> >>> +	int nid, i, res;
> >>> +
> >>> +	if (!hugetlb_cma_size && !hugetlb_cma_percent)
> >>> +		return;
> >>> +
> >>> +	if (hugetlb_cma_percent) {
> >>> +		for_each_mem_pfn_range(i, MAX_NUMNODES, &start_pfn, &end_pfn,
> >>> +				       NULL)
> >>> +			totalpages += end_pfn - start_pfn;
> >>> +
> >>> +		size = PAGE_SIZE * (hugetlb_cma_percent * 100 * totalpages) /
> >>> +			10000UL;
> >>> +	} else {
> >>> +		size = hugetlb_cma_size;
> >>> +	}
> >>> +
> >>> +	pr_info("hugetlb_cma: reserve %llu, %llu per node\n", size,
> >>> +		size / nr_online_nodes);
> >>> +
> >>> +	size /= nr_online_nodes;
> >>> +
> >>> +	for_each_node_state(nid, N_ONLINE) {
> >>> +		unsigned long min_pfn = 0, max_pfn = 0;
> >>> +
> >>> +		for_each_mem_pfn_range(i, nid, &start_pfn, &end_pfn, NULL) {
> >>> +			if (!min_pfn)
> >>> +				min_pfn = start_pfn;
> >>> +			max_pfn = end_pfn;
> >>> +		}
> >>> +
> >>> +		res = cma_declare_contiguous(PFN_PHYS(min_pfn), size,
> >>> +					     PFN_PHYS(max_pfn), (1UL << 30),
> >>
> >> The alignment is hard coded for x86 gigantic page size.  If this supports
> >> more architectures or becomes arch independent we will need to determine
> >> what this alignment should be.  Perhaps an arch specific call back to get
> >> the alignment for gigantic pages.  That will require a little thought as
> >> some arch's support multiple gigantic page sizes.
> > 
> > Good point!
> > Should we take the biggest possible size as a reference?
> > Or the smallest (larger than MAX_ORDER)?
> 
> As mentioned, it is pretty simple for architectures like x86 that only
> have one gigantic page size.  Just a random thought, but since
> hugetlb_cma_reserve is called from arch specific code perhaps the arch
> code could pass in at least alignment as an argument to this function?
> That way we can somewhat push the issue to the architectures.  For example,
> power supports 16GB gigantic page size but I believe they are allocated
> via firmware somehow.  So, they would not pass 16G as alignment.  In this
> case setup of the CMA area is somewhat architecture dependent.  So, perhaps
> the call from arch specific code is not such a bad idea.
> 
> With that in mind, we may want some type of coordination between arch
> specific and independent code.  Right now, cmdline_parse_hugetlb_cma
> is will accept a hugetlb_cma command line option without complaint even
> if the architecture does not call hugetlb_cma_reserve.
> 
> Just a nit, but cma_declare_contiguous if going to round up size by alignment.  So, the actual reserved size may not match what is printed with,
> +		pr_info("hugetlb_cma: successfully reserved %llu on node %d\n",
> +			size, nid);
> 
> I found this out by testing code and specifying hugetlb_cma=2M.  Messages
> in log were:
> 	kernel: hugetlb_cma: reserve 2097152, 1048576 per node
> 	kernel: hugetlb_cma: successfully reserved 1048576 on node 0
> 	kernel: hugetlb_cma: successfully reserved 1048576 on node 1
> But, it really reserved 1GB per node.

Good point! In the passed size is too small to cover a single huge page,
we should probably print a warning and bail out.

Will fix in the next version.

Thanks!