linux-kernel - Re: [PATCH 7/7] mm/hotplug: fix hot remove failure in SPARSEMEM|!VMEMMAP case

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Date:   Mon, 10 Feb 2020 15:54:06 +0800
From:   Baoquan He <bhe@...hat.com>
To:     Wei Yang <richardw.yang@...ux.intel.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        akpm@...ux-foundation.org, dan.j.williams@...el.com,
        david@...hat.com
Subject: Re: [PATCH 7/7] mm/hotplug: fix hot remove failure in
 SPARSEMEM|!VMEMMAP case

On 02/10/20 at 02:08pm, Wei Yang wrote:
> On Mon, Feb 10, 2020 at 11:41:05AM +0800, Baoquan He wrote:
> >On 02/10/20 at 07:52am, Wei Yang wrote:
> >> >---
> >> > mm/sparse.c | 4 +++-
> >> > 1 file changed, 3 insertions(+), 1 deletion(-)
> >> >
> >> >diff --git a/mm/sparse.c b/mm/sparse.c
> >> >index 623755e88255..345d065ef6ce 100644
> >> >--- a/mm/sparse.c
> >> >+++ b/mm/sparse.c
> >> >@@ -854,13 +854,15 @@ static void section_deactivate(unsigned long pfn, unsigned long nr_pages,
> >> > 			ms->usage = NULL;
> >> > 		}
> >> > 		memmap = sparse_decode_mem_map(ms->section_mem_map, section_nr);
> >> >-		ms->section_mem_map = (unsigned long)NULL;
> >> > 	}
> >> > 
> >> > 	if (section_is_early && memmap)
> >> > 		free_map_bootmem(memmap);
> >> > 	else
> >> > 		depopulate_section_memmap(pfn, nr_pages, altmap);
> >> 
> >> The crash happens in depopulate_section_memmap() when trying to get memmap by
> >> pfn_to_page(). Can we pass memmap directly?
> >
> >Yes, that's also a good idea. While it needs to add a parameter for
> >depopulate_section_memmap(), the parameter is useless for VMEMMAP
> >though, I personally prefer the current fix which is a little simpler.
> >
> 
> Not a new parameter, but replace pfn with memmap.
> 
> Not sure why the parameter is useless for VMEMMAP? memmap will be assigned to
> start and finally pass to vmemmap_free().

In section_deactivate(), per the code comments from Dan, we can know
that:

	/*
	 * section which only contains bootmem will be handled by
	 * free_map_bootmem(), including a complete section, or partial
	 * section which only has memory starting from the begining.
	 */
        if (section_is_early && memmap)
                free_map_bootmem(memmap);                                                                                                         
        else
	/* 
	 * section which contains region mixing bootmem with hot added
	 * sub-section region, only sub-section region, complete
	 * section. And in the mxied case, if hot remove the hot added
	 * sub-section aligned part, no memmap is got in the current
	 * code. So we still need pfn to calculate it for vmemmap case.
	 * To me, whenever we need, it looks better that we always use
	 * pfn to get its own memmap. 
	 */
                depopulate_section_memmap(pfn, nr_pages, altmap);

This is why I would like to keep the current logic as is,only one line
of code adjusting can fix the issue. Please let me know if I miss
anything.


> 
> >Anyway, both is fine to me, I can update if you think passing memmap is
> >better.
> >
> >> 
> >> >+
> >> >+	if(!rc)
> >> >+		ms->section_mem_map = (unsigned long)NULL;
> >> > }
> >> > 
> >> > static struct page * __meminit section_activate(int nid, unsigned long pfn,
> >> >-- 
> >> >2.17.2
> >> 
> >> -- 
> >> Wei Yang
> >> Help you, Help me
> >> 
> 
> -- 
> Wei Yang
> Help you, Help me
>