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

[David Hildenbrand]

On 16.10.23 15:38, Charan Teja Kalla wrote:
> Thanks Andrew/David,
>
> On 10/16/2023 1:53 PM, David Hildenbrand 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.
>
> On Snapdragon SoC,  with the mentioned memory configuration of PFN's as
> [ZONE_NORMAL ZONE_DEVICE  ZONE_NORMAL],  we are able to see bunch of
> issues daily while testing on a device farm.
>
> I note that from next time on wards will send the demsg bug/crash info
> for these type of issues. For this particular issue below is the log.
> Though the below log is not directly pointing to the
> pfn_section_valid(){ ms->usage;}, when we loaded this dump on T32
> lauterbach tool, it is pointing.
>
> [  540.578056] Unable to handle kernel NULL pointer dereference at
> virtual address 0000000000000000
> [  540.578068] Mem abort info:
> [  540.578070]   ESR = 0x0000000096000005
> [  540.578073]   EC = 0x25: DABT (current EL), IL = 32 bits
> [  540.578077]   SET = 0, FnV = 0
> [  540.578080]   EA = 0, S1PTW = 0
> [  540.578082]   FSC = 0x05: level 1 translation fault
> [  540.578085] Data abort info:
> [  540.578086]   ISV = 0, ISS = 0x00000005
> [  540.578088]   CM = 0, WnR = 0
> [  540.579431] pstate: 82400005 (Nzcv daif +PAN -UAO +TCO -DIT -SSBS
> BTYPE=--)
> [  540.579436] pc : __pageblock_pfn_to_page+0x6c/0x14c
> [  540.579454] lr : compact_zone+0x994/0x1058
> [  540.579460] sp : ffffffc03579b510
> [  540.579463] x29: ffffffc03579b510 x28: 0000000000235800 x27:
> 000000000000000c
> [  540.579470] x26: 0000000000235c00 x25: 0000000000000068 x24:
> ffffffc03579b640
> [  540.579477] x23: 0000000000000001 x22: ffffffc03579b660 x21:
> 0000000000000000
> [  540.579483] x20: 0000000000235bff x19: ffffffdebf7e3940 x18:
> ffffffdebf66d140
> [  540.579489] x17: 00000000739ba063 x16: 00000000739ba063 x15:
> 00000000009f4bff
> [  540.579495] x14: 0000008000000000 x13: 0000000000000000 x12:
> 0000000000000001
> [  540.579501] x11: 0000000000000000 x10: 0000000000000000 x9 :
> ffffff897d2cd440
> [  540.579507] x8 : 0000000000000000 x7 : 0000000000000000 x6 :
> ffffffc03579b5b4
> [  540.579512] x5 : 0000000000027f25 x4 : ffffffc03579b5b8 x3 :
> 0000000000000001
> [  540.579518] x2 : ffffffdebf7e3940 x1 : 0000000000235c00 x0 :
> 0000000000235800
> [  540.579524] Call trace:
> [  540.579527]  __pageblock_pfn_to_page+0x6c/0x14c
> [  540.579533]  compact_zone+0x994/0x1058
> [  540.579536]  try_to_compact_pages+0x128/0x378
> [  540.579540]  __alloc_pages_direct_compact+0x80/0x2b0
> [  540.579544]  __alloc_pages_slowpath+0x5c0/0xe10
> [  540.579547]  __alloc_pages+0x250/0x2d0
> [  540.579550]  __iommu_dma_alloc_noncontiguous+0x13c/0x3fc
> [  540.579561]  iommu_dma_alloc+0xa0/0x320
> [  540.579565]  dma_alloc_attrs+0xd4/0x108
>
> > > > 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?
>
> I am inspired to use the synchronize_rcu() because of [1] where we did
> use it in offline_page_ext(). And my limited understanding is that, a
> user can trigger the offline operation more often than the unplug operation.

In theory yes. In practice there are not many use cases where we do that.

Further, page_ext is already not used that frequently (page owner, 
young, idle tracking only), especially in most production (!debug) 
environments.

> I agree here that there is a scope to use kfree_rcu() unlike in [1]. Let
> me check for a way to use it.

Exactly, if we could use that, it would be ideal.

--
Cheers,

David / dhildenb