Re: [Bug report] hash_name() may cross page boundary and trigger sleep in RCU context

[Zizhi Wo <wozizhi@huaweicloud.com>]

在 2025/11/29 11:37, Al Viro 写道:
> On Thu, Nov 27, 2025 at 10:24:19AM +0800, Zizhi Wo wrote:
> 
>> Why does x86 have special handling in do_kern_addr_fault(), including
>> logic for vmalloc faults? For example, on CONFIG_X86_32, it still takes
>> the vmalloc_fault path. As noted in the x86 comments, "We can fault-in
>> kernel-space virtual memory on-demand"...
>>
>> But on arm64, I don’t see similar logic — is there a specific reason
>> for this difference? Maybe x86's vmalloc area is mapped lazily, while
>> ARM maps it fully during early boot?
> 
> x86 MMU uses the same register for kernel and userland top-level page
> tables; arm64 MMU has separate page tables for those - TTBR0 and TTBR1
> point to the table to be used for translation, depending upon the bit
> 55 of virtual address.
> 
> vmalloc works with page table of init_mm (see pgd_offset_k() uses in
> there).  On arm64 that's it - TTBR1 is set to that and it stays that way,
> so access to vmalloc'ed area will do the right thing.
> 
> On 32bit x86 you need to propagate the change into top-level page tables
> of every thread.  That's what arch_sync_kernel_mappings() is for; look for
> the calls in mm/vmalloc.c and see the discussion of race in the comment in
> front of x86 vmalloc_fault().  Nothing of that sort is needed of arm64,
> since all threads are using the same page table for kernel part of the
> address space.
> 
> The reason why 64bit x86 doesn't need to bother is different - there we
> fill all relevant top-level page table slots in preallocate_vmalloc_pages()
> before any additional threads could be created.  The pointers in those
> slots are not going to change and they will be propagated to all subsequent
> threads by pgd_alloc(), so the page tables actually modified by vmalloc()
> are shared by all threads.

Thank you very much for your answer. This well explains my confusion!

Thanks,
Zizhi Wo

> 
> AFAICS, 32bit arm is similar to 32bit x86 in that respect; propagation
> is lazier, though - there arch_sync_kernel_mappings() bumps a counter
> in init_mm and context switches use that to check if propagation needs
> to be done.  No idea how well does that work on vfree() side of things -
> hadn't looked into that rabbit hole...
>