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SubjectRe: [PATCH] mm/sparsemem: fix race in accessing memory_section->usage
On 15.10.23 00:25, Andrew Morton wrote:
> On Fri, 13 Oct 2023 18:34:27 +0530 Charan Teja Kalla <quic_charante@quicinc.com> wrote:
>
>> The below race is observed on a PFN which falls into the device memory
>> region with the system memory configuration where PFN's are such that
>> [ZONE_NORMAL ZONE_DEVICE ZONE_NORMAL]. Since normal zone start and
>> end pfn contains the device memory PFN's as well, the compaction
>> triggered will try on the device memory PFN's too though they end up in
>> NOP(because pfn_to_online_page() returns NULL for ZONE_DEVICE memory
>> sections). When from other core, the section mappings are being removed
>> for the ZONE_DEVICE region, that the PFN in question belongs to,
>> on which compaction is currently being operated is resulting into the
>> kernel crash with CONFIG_SPASEMEM_VMEMAP enabled.
>
> Seems this bug is four years old, yes? It must be quite hard to hit.

From the description, it's not quite clear to me if this was actually
hit -- usually people include the dmesg bug/crash info.

>
> When people review this, please offer opinions on whether a fix should
> be backported into -stable kernels, thanks.
>
>> compact_zone() memunmap_page
>> ------------- ---------------
>> __pageblock_pfn_to_page
>> ......
>> (a)pfn_valid():
>> valid_section()//return true
>> (b)__remove_pages()->
>> sparse_remove_section()->
>> section_deactivate():
>> [Free the array ms->usage and set
>> ms->usage = NULL]
>> pfn_section_valid()
>> [Access ms->usage which
>> is NULL]
>>
>> NOTE: From the above it can be said that the race is reduced to between
>> the pfn_valid()/pfn_section_valid() and the section deactivate with
>> SPASEMEM_VMEMAP enabled.
>>
>> The commit b943f045a9af("mm/sparse: fix kernel crash with
>> pfn_section_valid check") tried to address the same problem by clearing
>> the SECTION_HAS_MEM_MAP with the expectation of valid_section() returns
>> false thus ms->usage is not accessed.
>>
>> 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.


This affects any kind of memory hotunplug. When hotunplugging memory we
will end up calling synchronize_rcu() for each and every memory section,
which sounds extremely wasteful.

Can't we find a way to kfree_rcu() that thing and read/write the pointer
using READ?ONCE?WRITE_ONCE instead?

--
Cheers,

David / dhildenb

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