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

[David Hildenbrand]

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