[Libhugetlbfs-devel] Query about __unmap_hugepage_range

[David Gibson]

On Fri, Apr 08, 2011 at 04:06:09PM +0800, bill wrote:
> Hey, MM developers:)
> 
> I don't know if this posting is proper at here, so sorry for disturbing if it does. 
> 
> for normal 4K page: in unmap_page_range 
> 1: tlb_start_vma(tlb, vma); <------ call  flush_cache_range to invalidate icache if vma is VM_EXEC
> 2: clear pagetable mapping
> 3: tlb_end_vma(tlb, vma); <-------- call flush_tlb_range to invalidate unmapped vma tlb entry
> 
> for hugepage: in __unmap_hugepage_range
> 1: clear pagetable mapping
>  2: call flush_tlb_range(vma, start, end); to invalidate unmapped vma tlb entry
> 
> I really don't understand about two things:
> A: why there is no  flush_cache_range for hugepage when we do the unmapping?
> B: How does kernel take care of such case for both normal 4K page and hugepage:
>     a: mmap a page with PROT_EXEC at location p;
>     b: copy bunch instruction into p ,call cacheflush to make ICACHE see the new instruction; 
>     c: run instruction at location p, then unmap it;
>     d: mmap a new page with MAP_FIXED/PROT_EXEC at location p, and run unexpected instruction at p;
>         there is a great chance we got the same page at step_a;
>         user space should see a clean icache, not a stale one;
>      
> I am really puzzled for a long time.

> I am porting hugepage for ARM ,and one testcase in libhugetlbfs
> called icache-hygiene failed, test rationale is described in above
> B.

Yes, that testcase is designed to check exactly this.

This is a bit of a hack.  On x86 machines, nothing special is required
here, because the dcache and icache are coherent in hardware.  This is
also true on many power machines, including all moderm POWER
hardware.  However, this is not true on old POWER4 hardware, and that
testcase was designed to detect this bug which we once had on that
hardware.

For powerpc, the cache flush is handled in the arch specific code:
flush_dcache_icache_page() is called from set_pte_filter() and
set_access_flags_filter().  Those I believe are called from set_pte()
and set_ptep_access_flags().

There is some extra code here to only lazily flush the icache if the
page is not immediately executed from.  That is, we keep track of
whether the page is icache clean, and if we receive a read or write
fault on the page we don't clean it but map it without execute
permission.  We only perform the icache flush when we get an actual
execute fault on the page.

You will either need to implement similar hacks in ARM, or move the
flushing logic into the generic code.

-- 
David Gibson			| I'll have my music baroque, and my code
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				| _way_ _around_!
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