Re: [PATCH v2 3/7] mm/sparse.c: introduce a new function clear_subsection_map()

From: Baoquan He <bhe@redhat.com>
To: David Hildenbrand <david@redhat.com>
Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org,
	akpm@linux-foundation.org, richardw.yang@linux.intel.com,
	osalvador@suse.de, dan.j.williams@intel.com, mhocko@suse.com,
	rppt@linux.ibm.com, robin.murphy@arm.com
Subject: Re: [PATCH v2 3/7] mm/sparse.c: introduce a new function clear_subsection_map()
Date: Tue, 3 Mar 2020 16:22:04 +0800	[thread overview]
Message-ID: <20200303082204.GA4433@MiWiFi-R3L-srv> (raw)
In-Reply-To: <1346f0c2-7b1f-6feb-5e9b-2854fd0022ba@redhat.com>

On 03/02/20 at 04:43pm, David Hildenbrand wrote:
> On 01.03.20 06:20, Baoquan He wrote:
> > On 02/28/20 at 03:36pm, David Hildenbrand wrote:
> >> On 20.02.20 05:33, Baoquan He wrote:
> >>> Wrap the codes which clear subsection map of one memory region from
> >>> section_deactivate() into clear_subsection_map().
> >>>
> >>> Signed-off-by: Baoquan He <bhe@redhat.com>
> >>> ---
> >>>  mm/sparse.c | 46 ++++++++++++++++++++++++++++++++++++++--------
> >>>  1 file changed, 38 insertions(+), 8 deletions(-)
> >>>
> >>> diff --git a/mm/sparse.c b/mm/sparse.c
> >>> index 977b47acd38d..df857ee9330c 100644
> >>> --- a/mm/sparse.c
> >>> +++ b/mm/sparse.c
> >>> @@ -726,14 +726,25 @@ static void free_map_bootmem(struct page *memmap)
> >>>  }
> >>>  #endif /* CONFIG_SPARSEMEM_VMEMMAP */
> >>>  
> >>> -static void section_deactivate(unsigned long pfn, unsigned long nr_pages,
> >>> -		struct vmem_altmap *altmap)
> >>> +/**
> >>> + * clear_subsection_map - Clear subsection map of one memory region
> >>> + *
> >>> + * @pfn - start pfn of the memory range
> >>> + * @nr_pages - number of pfns to add in the region
> >>> + *
> >>> + * This is only intended for hotplug, and clear the related subsection
> >>> + * map inside one section.
> >>> + *
> >>> + * Return:
> >>> + * * -EINVAL	- Section already deactived.
> >>> + * * 0		- Subsection map is emptied.
> >>> + * * 1		- Subsection map is not empty.
> >>> + */
> >>
> >> Less verbose please (in my preference: none and simplify return handling)
> >>
> >>> +static int clear_subsection_map(unsigned long pfn, unsigned long nr_pages)
> >>>  {
> >>>  	DECLARE_BITMAP(map, SUBSECTIONS_PER_SECTION) = { 0 };
> >>>  	DECLARE_BITMAP(tmp, SUBSECTIONS_PER_SECTION) = { 0 };
> >>>  	struct mem_section *ms = __pfn_to_section(pfn);
> >>> -	bool section_is_early = early_section(ms);
> >>> -	struct page *memmap = NULL;
> >>>  	unsigned long *subsection_map = ms->usage
> >>>  		? &ms->usage->subsection_map[0] : NULL;
> >>>  
> >>> @@ -744,8 +755,28 @@ static void section_deactivate(unsigned long pfn, unsigned long nr_pages,
> >>>  	if (WARN(!subsection_map || !bitmap_equal(tmp, map, SUBSECTIONS_PER_SECTION),
> >>>  				"section already deactivated (%#lx + %ld)\n",
> >>>  				pfn, nr_pages))
> >>> -		return;
> >>> +		return -EINVAL;
> >>> +
> >>> +	bitmap_xor(subsection_map, map, subsection_map, SUBSECTIONS_PER_SECTION);
> >>>  
> >>> +	if (bitmap_empty(subsection_map, SUBSECTIONS_PER_SECTION))
> >>> +		return 0;
> >>> +
> >>
> >> Can we please just have a
> >>
> >> subsection_map_empty() instead and handle that in the caller?
> >> (you can then always return true in the !VMEMMAP variant)
> > 
> > I don't follow. Could you be more specific? or pseudo code please?
> > 
> > The old code has to handle below case in which subsection_map has been
> > cleared. And I introduce clear_subsection_map() to encapsulate all
> > subsection map realted code so that !VMEMMAP won't have to see it any
> > more.
> > 
> 
> Something like this on top would be easier to understand IMHO
> 
> 
> diff --git a/mm/sparse.c b/mm/sparse.c
> index dc79b00ddaaa..be5c80e9cfee 100644
> --- a/mm/sparse.c
> +++ b/mm/sparse.c
> @@ -726,20 +726,6 @@ static void free_map_bootmem(struct page *memmap)
>  }
>  #endif /* CONFIG_SPARSEMEM_VMEMMAP */
>  
> -/**
> - * clear_subsection_map - Clear subsection map of one memory region
> - *
> - * @pfn - start pfn of the memory range
> - * @nr_pages - number of pfns to add in the region
> - *
> - * This is only intended for hotplug, and clear the related subsection
> - * map inside one section.
> - *
> - * Return:
> - * * -EINVAL	- Section already deactived.
> - * * 0		- Subsection map is emptied.
> - * * 1		- Subsection map is not empty.
> - */
>  static int clear_subsection_map(unsigned long pfn, unsigned long nr_pages)
>  {
>  	DECLARE_BITMAP(map, SUBSECTIONS_PER_SECTION) = { 0 };
> @@ -758,11 +744,12 @@ static int clear_subsection_map(unsigned long pfn, unsigned long nr_pages)
>  		return -EINVAL;
>  
>  	bitmap_xor(subsection_map, map, subsection_map, SUBSECTIONS_PER_SECTION);
> +	return 0;
> +}
>  
> -	if (bitmap_empty(subsection_map, SUBSECTIONS_PER_SECTION))
> -		return 0;
> -
> -	return 1;
> +static bool is_subsection_map_empty(unsigned long pfn, unsigned long nr_pages)
> +{
> +	return bitmap_empty(subsection_map, SUBSECTIONS_PER_SECTION);
>  }
>  
>  static void section_deactivate(unsigned long pfn, unsigned long nr_pages,
> @@ -771,11 +758,8 @@ static void section_deactivate(unsigned long pfn, unsigned long nr_pages,
>  	struct mem_section *ms = __pfn_to_section(pfn);
>  	bool section_is_early = early_section(ms);
>  	struct page *memmap = NULL;
> -	int rc;
> -
>  
> -	rc = clear_subsection_map(pfn, nr_pages);
> -	if (IS_ERR_VALUE((unsigned long)rc))
> +	if (unlikely(clear_subsection_map(pfn, nr_pages)))
>  		return;
>  	/*
>  	 * There are 3 cases to handle across two configurations
> @@ -794,7 +778,7 @@ static void section_deactivate(unsigned long pfn, unsigned long nr_pages,
>  	 *
>  	 * For 2/ and 3/ the SPARSEMEM_VMEMMAP={y,n} cases are unified
>  	 */
> -	if (!rc) {
> +	if (is_subsection_map_empty(pfn, nr_pages)) {
>  		unsigned long section_nr = pfn_to_section_nr(pfn);

Tried this way, it's not good in this patch. Since ms->usage might be
freed in this place.

                if (!PageReserved(virt_to_page(ms->usage))) {
                        kfree(ms->usage);
                        ms->usage = NULL;
                }

If have to introduce is_subsection_map_empty(), the code need be
adjusted a little bit. It may need be done in another separate patch to
adjust it. If you agree, I would like to keep this patch as is, later
I can refactor this on top of this patchset, or if anyone else post
patch to adjust it, I can help review. 

>  
>  		/*
> @@ -816,7 +800,7 @@ static void section_deactivate(unsigned long pfn, unsigned long nr_pages,
>  	else
>  		depopulate_section_memmap(pfn, nr_pages, altmap);
>  
> -	if (!rc)
> +	if (is_subsection_map_empty(pfn, nr_pages))
>  		ms->section_mem_map = (unsigned long)NULL;
>  }
>  
> 
> -- 
> Thanks,
> 
> David / dhildenb