RE: [PATCH v2] mm/hugetlb: split hugetlb_cma in nodes with memory

[Song Bao Hua (Barry Song)]

> -----Original Message-----
> From: Anshuman Khandual [mailto:anshuman.khandual@xxxxxxx]
> Sent: Friday, July 10, 2020 8:59 PM
> To: Song Bao Hua (Barry Song) <song.bao.hua@xxxxxxxxxxxxx>;
> akpm@xxxxxxxxxxxxxxxxxxxx
> Cc: x86@xxxxxxxxxx; linux-mm@xxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx;
> Linuxarm <linuxarm@xxxxxxxxxx>; linux-arm-kernel@xxxxxxxxxxxxxxxxxxx;
> Roman Gushchin <guro@xxxxxx>; Catalin Marinas
> <catalin.marinas@xxxxxxx>; Will Deacon <will@xxxxxxxxxx>; Thomas Gleixner
> <tglx@xxxxxxxxxxxxx>; Ingo Molnar <mingo@xxxxxxxxxx>; Borislav Petkov
> <bp@xxxxxxxxx>; H . Peter Anvin <hpa@xxxxxxxxx>; Mike Kravetz
> <mike.kravetz@xxxxxxxxxx>; Mike Rapoport <rppt@xxxxxxxxxxxxx>; Jonathan
> Cameron <jonathan.cameron@xxxxxxxxxx>
> Subject: Re: [PATCH v2] mm/hugetlb: split hugetlb_cma in nodes with memory
> 
> 
> 
> On 07/10/2020 09:20 AM, Barry Song wrote:
> > Rather than splitting huge_cma in online nodes, it is better to do it in
> > nodes with memory.
> > Without this patch, for an ARM64 server with four numa nodes and only
> > node0 has memory. If I set hugetlb_cma=4G in bootargs,
> >
> > without this patch, I got the below printk:
> > hugetlb_cma: reserve 4096 MiB, up to 1024 MiB per node
> > hugetlb_cma: reserved 1024 MiB on node 0
> > hugetlb_cma: reservation failed: err -12, node 1
> > hugetlb_cma: reservation failed: err -12, node 2
> > hugetlb_cma: reservation failed: err -12, node 3
> 
> Seems like you have not changed the commit message from the last time.
> This does not explain the problem as we had discussed before. This just
> states the effects of the problem not the cause which is really required
> when there is a proposal for change in behavior.

OK, will think about some better words. 
But, it seems the changelog is clearly showing we get only 1GB CMA even though
we set 4GB size. And you have understood that from the 1st beginning as you
commented, which probably proves it is valid enough to let people understand
what's going on :-)

> 
> >
> > With this patch, I got the below printk:
> >
> > hugetlb_cma: reserve 4096 MiB, up to 4096 MiB per node
> > hugetlb_cma: reserved 4096 MiB on node 0
> 
> Before and after the patch behavior should be described after the problem
> and it's solution have been articulated.

Ok, can add some summary.

> 
> >
> > So this patch makes the hugetlb_cma size consistent with users' setting
> > on ARM64 platforms.
> 
> As mentioned before, there is nothing arm64 specific here. The problem just
> manifests on arm64 platform because of it's memory init process.
> 

Apology if the words make you misunderstand.

Here it is just using ARM64 as an example that problem gets fixed. It has never said this
problem is specific to ARM64. Before the change, we get 1GB CMA; after the patch, we
get 4GB CMA which is same with users' setting. The size issue is not specific to ARM64.

And the changelog also said what you wanted to say:
 The problem is always there if N_ONLINE != N_MEMORY. In x86 case, it
 is just hidden because N_ONLINE happen to match N_MEMORY during the
 boot process when hugetlb_cma_reserve() gets called.

Will think about some better words.

> >
> > Jonathan Cameron tested this patch on x86 platform. Jonathan figured out
> > the boot code of x86 is much different with arm64. On arm64 all nodes are
> > marked online at the same time. On x86, only nodes with memory are
> > initially marked as online:
> > initmem_init()->x86_numa_init()->numa_init()->
> > numa_register_memblks()->alloc_node_data()->node_set_online()
> 
> Function call sequence should be described (if at all) cleanly with proper
> indentation.

Agreed

> 
> > So at time of the existing cma setup call only the memory containing nodes
> > are online. The other nodes are brought up much later.
> > Therefore, on x86 platform, hugetlb_cma size is actually consistent with
> > users' setting even though system has nodes without memory.
> >
> > The problem is always there if N_ONLINE != N_MEMORY. In x86 case, it
> > is just hidden because N_ONLINE happen to match N_MEMORY during the
> boot
> > process when hugetlb_cma_reserve() gets called.
> >
> > This patch documents this problem in the comment of
> hugetlb_cma_reserve()
> > and makes hugetlb_cma size optimal.
> 
> The commit message here is not proper for the change it's proposing. It needs
> to describe the problem, how it manifests on arm64 but remains hidden on
> x86,
> proposed solution i.e changes in generic and platform code, also if required
> before and after patch behavior on arm64 which had triggered this patch. Also
> please take care of the indentation, paragraph formatting and avoid writing
> in first person singular format.

Ok. will try one more time.

> 
> >
> > Cc: Roman Gushchin <guro@xxxxxx>
> > Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> > Cc: Will Deacon <will@xxxxxxxxxx>
> > Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> > Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> > Cc: Borislav Petkov <bp@xxxxxxxxx>
> > Cc: H. Peter Anvin <hpa@xxxxxxxxx>
> > Cc: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
> > Cc: Mike Rapoport <rppt@xxxxxxxxxxxxx>
> > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> > Cc: Anshuman Khandual <anshuman.khandual@xxxxxxx>
> > Cc: Jonathan Cameron <jonathan.cameron@xxxxxxxxxx>
> > Signed-off-by: Barry Song <song.bao.hua@xxxxxxxxxxxxx>
> > ---
> >  -v2: document better according to Anshuman Khandual's comment
> >
> >  arch/arm64/mm/init.c    | 19 ++++++++++---------
> >  arch/x86/kernel/setup.c | 12 +++++++++---
> >  include/linux/hugetlb.h |  7 +++++++
> >  mm/hugetlb.c            |  4 ++--
> >  4 files changed, 28 insertions(+), 14 deletions(-)
> >
> > diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
> > index 1e93cfc7c47a..420f5e55615c 100644
> > --- a/arch/arm64/mm/init.c
> > +++ b/arch/arm64/mm/init.c
> > @@ -420,15 +420,6 @@ void __init bootmem_init(void)
> >
> >  	arm64_numa_init();
> >
> > -	/*
> > -	 * must be done after arm64_numa_init() which calls numa_init() to
> > -	 * initialize node_online_map that gets used in hugetlb_cma_reserve()
> > -	 * while allocating required CMA size across online nodes.
> > -	 */
> > -#ifdef CONFIG_ARM64_4K_PAGES
> > -	hugetlb_cma_reserve(PUD_SHIFT - PAGE_SHIFT);
> > -#endif
> > -
> >  	/*
> >  	 * Sparsemem tries to allocate bootmem in memory_present(), so must
> be
> >  	 * done after the fixed reservations.
> > @@ -438,6 +429,16 @@ void __init bootmem_init(void)
> >  	sparse_init();
> >  	zone_sizes_init(min, max);
> >
> > +	/*
> > +	 * must be done after zone_sizes_init() which calls free_area_init()
> > +	 * that calls node_set_state() to initialize node_states[N_MEMORY]
> > +	 * because hugetlb_cma_reserve() will scan over nodes with N_MEMORY
> > +	 * state
> > +	 */
> > +#ifdef CONFIG_ARM64_4K_PAGES
> > +	hugetlb_cma_reserve(PUD_SHIFT - PAGE_SHIFT);
> > +#endif
> > +
> >  	memblock_dump_all();
> >  }
> >
> > diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c
> > index a3767e74c758..a1a9712090ae 100644
> > --- a/arch/x86/kernel/setup.c
> > +++ b/arch/x86/kernel/setup.c
> > @@ -1164,9 +1164,6 @@ void __init setup_arch(char **cmdline_p)
> >  	initmem_init();
> >  	dma_contiguous_reserve(max_pfn_mapped << PAGE_SHIFT);
> >
> > -	if (boot_cpu_has(X86_FEATURE_GBPAGES))
> > -		hugetlb_cma_reserve(PUD_SHIFT - PAGE_SHIFT);
> > -
> >  	/*
> >  	 * Reserve memory for crash kernel after SRAT is parsed so that it
> >  	 * won't consume hotpluggable memory.
> > @@ -1180,6 +1177,15 @@ void __init setup_arch(char **cmdline_p)
> >
> >  	x86_init.paging.pagetable_init();
> >
> > +	/*
> > +	 * must be done after zone_sizes_init() which calls free_area_init()
> > +	 * that calls node_set_state() to initialize node_states[N_MEMORY]
> > +	 * because hugetlb_cma_reserve() will scan over nodes with N_MEMORY
> > +	 * state
> > +	 */
> > +	if (boot_cpu_has(X86_FEATURE_GBPAGES))
> > +		hugetlb_cma_reserve(PUD_SHIFT - PAGE_SHIFT);
> > +
> >  	kasan_init();
> >
> >  	/*
> > diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
> > index 50650d0d01b9..6df411d91040 100644
> > --- a/include/linux/hugetlb.h
> > +++ b/include/linux/hugetlb.h
> > @@ -909,6 +909,13 @@ static inline spinlock_t *huge_pte_lock(struct
> hstate *h,
> >  }
> >
> >  #if defined(CONFIG_HUGETLB_PAGE) && defined(CONFIG_CMA)
> > +/**
> > + * hugetlb_cma_reserve() -- reserve CMA for gigantic pages on nodes with
> memory
> > + *
> > + * must be called after free_area_init() that updates N_MEMORY via
> node_set_state().
> > + * hugetlb_cma_reserve() scans over N_MEMORY nodemask and hence
> expects the platforms
> > + * to have initialized N_MEMORY state.
> > + */
> 
> The in-code documentation should be where it is defined i.s mm/hugetlb.c and
> also in existing format e.g see unmap_kernel_range_noflush() in mm/vmalloc.c

Just switched from another subsystem in which in-code documentation is put in head
files. Will move it to c.

Thanks
Barry