Re: riscv32 EXT4 splat, 6.8 regression?

["Björn Töpel" <bjorn@kernel.org>]

Christian Brauner <brauner@kernel.org> writes:

> [Adding Mike who's knowledgeable in this area]

>> > Further, it seems like riscv32 indeed inserts a page like that to the
>> > buddy allocator, when the memblock is free'd:
>> > 
>> >   | [<c024961c>] __free_one_page+0x2a4/0x3ea
>> >   | [<c024a448>] __free_pages_ok+0x158/0x3cc
>> >   | [<c024b1a4>] __free_pages_core+0xe8/0x12c
>> >   | [<c0c1435a>] memblock_free_pages+0x1a/0x22
>> >   | [<c0c17676>] memblock_free_all+0x1ee/0x278
>> >   | [<c0c050b0>] mem_init+0x10/0xa4
>> >   | [<c0c1447c>] mm_core_init+0x11a/0x2da
>> >   | [<c0c00bb6>] start_kernel+0x3c4/0x6de
>> > 
>> > Here, a page with VA 0xfffff000 is a added to the freelist. We were just
>> > lucky (unlucky?) that page was used for the page cache.
>> 
>> I just educated myself about memory mapping last night, so the below
>> may be complete nonsense. Take it with a grain of salt.
>> 
>> In riscv's setup_bootmem(), we have this line:
>> 	max_low_pfn = max_pfn = PFN_DOWN(phys_ram_end);
>> 
>> I think this is the root cause: max_low_pfn indicates the last page
>> to be mapped. Problem is: nothing prevents PFN_DOWN(phys_ram_end) from
>> getting mapped to the last page (0xfffff000). If max_low_pfn is mapped
>> to the last page, we get the reported problem.
>> 
>> There seems to be some code to make sure the last page is not used
>> (the call to memblock_set_current_limit() right above this line). It is
>> unclear to me why this still lets the problem slip through.
>> 
>> The fix is simple: never let max_low_pfn gets mapped to the last page.
>> The below patch fixes the problem for me. But I am not entirely sure if
>> this is the correct fix, further investigation needed.
>> 
>> Best regards,
>> Nam
>> 
>> diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
>> index fa34cf55037b..17cab0a52726 100644
>> --- a/arch/riscv/mm/init.c
>> +++ b/arch/riscv/mm/init.c
>> @@ -251,7 +251,8 @@ static void __init setup_bootmem(void)
>>  	}
>>  
>>  	min_low_pfn = PFN_UP(phys_ram_base);
>> -	max_low_pfn = max_pfn = PFN_DOWN(phys_ram_end);
>> +	max_low_pfn = PFN_DOWN(memblock_get_current_limit());
>> +	max_pfn = PFN_DOWN(phys_ram_end);
>>  	high_memory = (void *)(__va(PFN_PHYS(max_low_pfn)));
>>  
>>  	dma32_phys_limit = min(4UL * SZ_1G, (unsigned long)PFN_PHYS(max_low_pfn));

Yeah, AFAIU memblock_set_current_limit() only limits the allocation from
memblock. The "forbidden" page (PA 0xc03ff000 VA 0xfffff000) will still
be allowed in the zone.

I think your patch requires memblock_set_current_limit() is
unconditionally called, which currently is not done.

The hack I tried was (which seems to work):

--
diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
index fe8e159394d8..3a1f25d41794 100644
--- a/arch/riscv/mm/init.c
+++ b/arch/riscv/mm/init.c
@@ -245,8 +245,10 @@ static void __init setup_bootmem(void)
         */
        if (!IS_ENABLED(CONFIG_64BIT)) {
                max_mapped_addr = __pa(~(ulong)0);
-               if (max_mapped_addr == (phys_ram_end - 1))
+               if (max_mapped_addr == (phys_ram_end - 1)) {
                        memblock_set_current_limit(max_mapped_addr - 4096);
+                       phys_ram_end -= 4096;
+               }
        }
 
        min_low_pfn = PFN_UP(phys_ram_base);
--

I'd really like to see an actual MM person (Mike or Alex?) have some
input here, and not simply my pasta-on-wall approach. ;-)


Björn