Re: [PATCH] mm/sparsemem: fix race in accessing memory_section->usage

From: David Hildenbrand <david@redhat.com>
To: Charan Teja Kalla <quic_charante@quicinc.com>,
	Pavan Kondeti <quic_pkondeti@quicinc.com>
Cc: akpm@linux-foundation.org, osalvador@suse.de,
	dan.j.williams@intel.com, vbabka@suse.cz,
	mgorman@techsingularity.net, aneesh.kumar@linux.ibm.com,
	linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm/sparsemem: fix race in accessing memory_section->usage
Date: Tue, 17 Oct 2023 16:53:46 +0200	[thread overview]
Message-ID: <17a890e2-a87b-446e-9088-bd608504378b@redhat.com> (raw)
In-Reply-To: <b4a2bbe8-4181-68ad-1a6c-463fbe64d8f8@quicinc.com>

On 17.10.23 16:10, Charan Teja Kalla wrote:
> Thanks Pavan!!
> 
> On 10/16/2023 4:03 PM, Pavan Kondeti wrote:
>>> Fix this issue by the below steps:
>>> a) Clear SECTION_HAS_MEM_MAP before freeing the ->usage.
>>> b) RCU protected read side critical section will either return NULL when
>>> SECTION_HAS_MEM_MAP is cleared or can successfully access ->usage.
>>> c) Synchronize the rcu on the write side and free the ->usage. No
>>> attempt will be made to access ->usage after this as the
>>> SECTION_HAS_MEM_MAP is cleared thus valid_section() return false.
>>>
>>> Since the section_deactivate() is a rare operation and will come in the
>>> hot remove path, impact of synchronize_rcu() should be negligble.
>> struct mem_section_usage has other field like pageblock_flags. Do we
>> need to protect its readers with RCU? Also can we annotate usage field
>> in struct mem_section with __rcu and use RCU accessors like
>> rcu_dereference() while using memsection::usage field?
> 
> Good question about the pageblock_flags!! I think we rely on the
> get_pageblock_bitmap() to read the ms->usage->pageblock_flags by passing
> struct page*.
> 
> 1) All the functions that I have come across calling
> get_pageblock_bitmap()/get_pfnblock_flags_mask() passing the page* which
> it get from buddy. I think we are safe here as the device pages(from
> which the problem is coming will never be onlined/added to buddy)
> 
> 2) There are functions in compaction which directly operate on the pfn's
> through pfn_to_online_page(). As for device pages, it is going to return
> NULL, I think we are safe here too.
> 
> 3) alloc_contig_range() which also operate on the pfn's directly, and
> TMK, we will not pass the [start , end) values of this function to
> contains the hole/offlined pages. I think we are safe here too.
> 
> May be David/other reviewers can help in commenting if there are some
> mistakes above.

Sound reasonable to me; most PFN walkers shouldn't deal with pageblock 
flags. alloc_contig_range() is certainly interesting, I suspect it's 
fine but we better double-check.

-- 
Cheers,

David / dhildenb