Re: RFC: backport of commit a32c1c61212d

[Doug Berger]

On 9/1/2020 7:00 AM, Greg Kroah-Hartman wrote:
> On Tue, Aug 25, 2020 at 03:58:27PM -0700, Doug Berger wrote:
>> I recently tracked down a problem I observed when booting a v5.4 kernel
>> on a sparsemem UMA arm platform which includes a no-map reserved-memory
>> region in the middle of its HighMem zone.
>>
>> When memmap_init_zone() is invoked the pfn's that correspond to the
>> no-map region fail the early_pfn_valid() check and the struct page
>> structures are not initialized creating a "hole" in the memmap. Later in
>> my boot sequence the sock_init() initcall leads to a bpf_prog_alloc()
>> which ends up stealing a page from the block containing the no-map
>> region which then leads to a call of move_freepages_block() to
>> reclassify the migratetype of the entire block.
>>
>> The function move_freepages() includes a check of pfn_valid_within for
>> each page in the range, but since the arm architecture doesn't include
>> HOLES_IN_ZONE this check is optimized out and the uninitialized struct
>> page is accessed. Specifically, PageLRU() calls compound_head() on the
>> page and if the page->compound_head value is odd the value is used as a
>> pointer to the head struct page. For uninitialized memory there is a
>> high chance that a random value of compound head will be odd and contain
>> an invalid pointer value that causes the kernel to abort and panic.
>>
>> As you might imagine specifying HOLES_IN_ZONE for the arm build allows
>> pfn_valid_within to protect against accessing the uninitialized struct
>> page. However, the performance penalty this incurs seems unnecessary.
>>
>> Commit 35fd1eb1e821 ("mm/sparse: abstract sparse buffer allocations") as
>> part of the "sparse_init rewrite" series introduced in v4.19 changed the
>> way sparsemem memmaps are initialized. Prior to this patch the sparsemem
>> memmaps are initialized to all 0's. I observed that on older kernels the
>> "uninitialized" struct page access also occurs, but the 0
>> page->compound_head indicates no compound head and the page pointer is
>> therefore not corrupted. The other logic ends up causing the page to be
>> skipped and everything "happens to work".
>>
>> While considering solutions to this issue I observed that the problem
>> does not occur in the current upstream as a result of a combination of
>> other commits. The following commits provided functionality to
>> initialize struct page structures for pages that are unavailable like
>> the no-map region in my system:
>> commit a4a3ede2132a ("mm: zero reserved and unavailable struct pages")
>> commit 907ec5fca3dc ("mm: zero remaining unavailable struct pages")
>> commit ec393a0f014e ("mm: return zero_resv_unavail optimization")
>> commit e822969cab48 ("mm/page_alloc.c: fix uninitialized memmaps on a
>> partially populated last section")
>> commit 4b094b7851bf ("mm/page_alloc.c: initialize memmap of unavailable
>> memory directly")
>>
>> However, those commits added the functionality to the free_area_init()
>> and free_area_init_nodes() functions and the non-NUMA arm architecture
>> did not begin calling free_area_init() until the following commit in v5.8:
>> commit a32c1c61212d ("arm: simplify detection of memory zone boundaries")
>>
>> Prior to that commit the non-NUMA arm architecture called
>> free_area_init_node() directly at the end of zone_sizes_init().
>>
>> So while the problem appears to be fixed upstream by commit a32c1c61212d
>> ("arm: simplify detection of memory zone boundaries") it is still
>> present in stable branches between v4.19.y and v5.7.y inclusive and
>> probably for architectures other than arm as well that didn't call
>> free_area_init(). This upstream commit is not easily/safely backportable
>> to stable branches, but if we focus on the sliver of functionality that
>> adds the initialization code from free_area_init() to the
>> zones_sizes_init() function used by non-NUMA arm kernels I believe a
>> simple patch could be developed for each relevant stable branch to
>> resolve the issue I am observing. Similar patches could also be applied
>> for other architectures that now call free_area_init() upstream but not
>> in one of these stable branches, but I am not in a position to test
>> those architectures.
>>
>> For the linux-5.4.y branch such a patch might look like this:
>> >From 671c341b5cdb8360349c33ade43115e28ca56a8a Mon Sep 17 00:00:00 2001
>> From: Doug Berger <opendmb@xxxxxxxxx>
>> Date: Tue, 25 Aug 2020 14:39:43 -0700
>> Subject: [PATCH] ARM: mm: sync zone_sizes_init with free_area_init
>>
>> The arm architecture does not invoke the common function
>> free_area_init(). Instead for non-NUMA builds it invokes
>> free_area_init_node() directly from zone_sizes_init().
>>
>> As a result recent changes in free_area_init() are not
>> picked up by arm architecture builds.
>>
>> This commit adds the updates to the zone_sizes_init()
>> function to achieve parity with the free_area_init()
>> functionality.
>>
>> Fixes: 35fd1eb1e821 ("mm/sparse: abstract sparse buffer allocations")
>> Signed-off-by: Doug Berger <opendmb@xxxxxxxxx>
>> Cc: stable@xxxxxxxxxxxxxxx
>> ---
>>  arch/arm/mm/init.c | 1 +
>>  1 file changed, 1 insertion(+)
>>
>> diff --git a/arch/arm/mm/init.c b/arch/arm/mm/init.c
>> index 6f19ba53fd1f..4f171d834c60 100644
>> --- a/arch/arm/mm/init.c
>> +++ b/arch/arm/mm/init.c
>> @@ -169,6 +169,7 @@ static void __init zone_sizes_init(unsigned long
>> min, unsigned long max_low,
>>                         arm_dma_zone_size >> PAGE_SHIFT);
>>  #endif
>>
>> +       zero_resv_unavail();
>>         free_area_init_node(0, zone_size, min, zhole_size);
>>  }
>>
>> -- 
>> 2.7.4
>>
>> I am unclear of the mechanics for submitting such a stable patch when it
>> represents a perhaps less than obvious sliver of the upstream commit
>> that fixes the issue, so I am soliciting guidance with this email.
>>
>> Thank you for taking the time to read this far, and please let me know
>> how I can improve the situation,
> 
> I'm confused, what exactly are you asking for here?  Is there a specific
> commit you wish to see backported to the 5.4 kernel, or are you trying
> to say you want something that is not in Linus's tree, backported
> instead?
> 
> thanks,
> 
> greg k-h
> 
Sorry for the confusion, but thanks for the reply.

There is functionality that exists in Linus' tree, but it is not the
result of a single commit that can be easily backported. I have been
unable to find anything in the documentation for submitting a patch to a
stable branch that covers this type of submission so I have sent this as
an RFC about process rather than a patch.

The upstream commit that ultimately results in the functional change is:
commit a32c1c61212d ("arm: simplify detection of memory zone boundaries")

That commit is dependent on other commits that aren't necessary for the
stable branches.

In my downstream kernel I would apply the single line patch included in
my original email, but it is not appropriate to apply that patch to
Linus' tree since the problem does not exist there.

This creates the situation where a simple patch could be applied to a
stable branch to improve its stability, but there is not a clear
upstream commit to reference.

My best guess at this point is to submit patches to the affected stable
branches like the one in my RFC and reference a32c1c61212d as the
upstream commit. This would be confusing to anyone that tried to compare
the submitted patch with the upstream patch since they
wouldn't look at all alike, but the fixes and upstream tags would define
the affected range in Linus' tree.

I would appreciate any guidance on how best to handle this kind of
situation.

Thanks again,
    Doug