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Date:   Tue, 25 Feb 2020 10:57:13 +0100
From:   Michal Hocko <mhocko@kernel.org>
To:     Baoquan He <bhe@redhat.com>
Cc:     linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        akpm@linux-foundation.org, richardw.yang@linux.intel.com,
        david@redhat.com, osalvador@suse.de, dan.j.williams@intel.com,
        rppt@linux.ibm.com, robin.murphy@arm.com
Subject: Re: [PATCH v2 4/7] mm/sparse.c: only use subsection map in VMEMMAP
 case
Message-ID: <20200225095713.GL22443@dhcp22.suse.cz>
References: <20200220043316.19668-1-bhe@redhat.com>
 <20200220043316.19668-5-bhe@redhat.com>
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On Thu 20-02-20 12:33:13, Baoquan He wrote:
> Currently, subsection map is used when SPARSEMEM is enabled, including
> VMEMMAP case and !VMEMMAP case. However, subsection hotplug is not
> supported at all in SPARSEMEM|!VMEMMAP case, subsection map is unnecessary
> and misleading. Let's adjust code to only allow subsection map being
> used in VMEMMAP case.

This really needs more explanation I believe. What exactly happens if
somebody tries to hotremove a part of the section with !VMEMMAP? I can
see that clear_subsection_map returns 0 but that is not an error code.
Besides that section_deactivate doesn't propagate the error upwards.
/me stares into the code

OK, I can see it now. It is relying on check_pfn_span to use the proper
subsection granularity. This really begs for a comment in the code
somewhere.

> 
> Signed-off-by: Baoquan He <bhe@redhat.com>
> ---
>  include/linux/mmzone.h |  2 ++
>  mm/sparse.c            | 20 ++++++++++++++++++++
>  2 files changed, 22 insertions(+)
> 
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 462f6873905a..fc0de3a9a51e 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -1185,7 +1185,9 @@ static inline unsigned long section_nr_to_pfn(unsigned long sec)
>  #define SUBSECTION_ALIGN_DOWN(pfn) ((pfn) & PAGE_SUBSECTION_MASK)
>  
>  struct mem_section_usage {
> +#ifdef CONFIG_SPARSEMEM_VMEMMAP
>  	DECLARE_BITMAP(subsection_map, SUBSECTIONS_PER_SECTION);
> +#endif
>  	/* See declaration of similar field in struct zone */
>  	unsigned long pageblock_flags[0];
>  };
> diff --git a/mm/sparse.c b/mm/sparse.c
> index df857ee9330c..66c497d6a229 100644
> --- a/mm/sparse.c
> +++ b/mm/sparse.c
> @@ -209,6 +209,7 @@ static inline unsigned long first_present_section_nr(void)
>  	return next_present_section_nr(-1);
>  }
>  
> +#ifdef CONFIG_SPARSEMEM_VMEMMAP
>  static void subsection_mask_set(unsigned long *map, unsigned long pfn,
>  		unsigned long nr_pages)
>  {
> @@ -243,6 +244,11 @@ void __init subsection_map_init(unsigned long pfn, unsigned long nr_pages)
>  		nr_pages -= pfns;
>  	}
>  }
> +#else
> +void __init subsection_map_init(unsigned long pfn, unsigned long nr_pages)
> +{
> +}
> +#endif
>  
>  /* Record a memory area against a node. */
>  void __init memory_present(int nid, unsigned long start, unsigned long end)
> @@ -726,6 +732,7 @@ static void free_map_bootmem(struct page *memmap)
>  }
>  #endif /* CONFIG_SPARSEMEM_VMEMMAP */
>  
> +#ifdef CONFIG_SPARSEMEM_VMEMMAP
>  /**
>   * clear_subsection_map - Clear subsection map of one memory region
>   *
> @@ -764,6 +771,12 @@ static int clear_subsection_map(unsigned long pfn, unsigned long nr_pages)
>  
>  	return 1;
>  }
> +#else
> +static int clear_subsection_map(unsigned long pfn, unsigned long nr_pages)
> +{
> +	return 0;
> +}
> +#endif
>  
>  static void section_deactivate(unsigned long pfn, unsigned long nr_pages,
>  		struct vmem_altmap *altmap)
> @@ -820,6 +833,7 @@ static void section_deactivate(unsigned long pfn, unsigned long nr_pages,
>  		ms->section_mem_map = (unsigned long)NULL;
>  }
>  
> +#ifdef CONFIG_SPARSEMEM_VMEMMAP
>  /**
>   * fill_subsection_map - fill subsection map of a memory region
>   * @pfn - start pfn of the memory range
> @@ -854,6 +868,12 @@ static int fill_subsection_map(unsigned long pfn, unsigned long nr_pages)
>  
>  	return rc;
>  }
> +#else
> +static int fill_subsection_map(unsigned long pfn, unsigned long nr_pages)
> +{
> +	return 0;
> +}
> +#endif
>  
>  static struct page * __meminit section_activate(int nid, unsigned long pfn,
>  		unsigned long nr_pages, struct vmem_altmap *altmap)
> -- 
> 2.17.2
> 

-- 
Michal Hocko
SUSE Labs