Re: [HELP-NEEDED, PATCH 0/3] Do not loose dirty bit on THP pages

[Vlastimil Babka]

On 06/14/2017 06:55 PM, Will Deacon wrote:
>>
>> May be we should relook at pmd PTE udpate interface. We really need an
>> interface that can update pmd entries such that we don't clear it in
>> between. IMHO, we can avoid the pmdp_invalidate() completely, if we can
>> switch from a pmd PTE entry to a pointer to PTE page (pgtable_t). We also
>> need this interface to avoid the madvise race fixed by
> 
> There's a good chance I'm not following your suggestion here, but it's
> probably worth me pointing out that swizzling a page table entry from a
> block mapping (e.g. a huge page mapped at the PMD level) to a table entry
> (e.g. a pointer to a page of PTEs) can lead to all sorts of horrible
> problems on ARM, including amalgamation of TLB entries and fatal aborts.

AFAIK some AMD x86_64 CPU's had the same problem and generated MCE's,
and on Intel there are some restrictions when you can do that. See the
large comment in __split_huge_pmd_locked().

> So we really need to go via an invalid entry, with appropriate TLB
> invalidation before installing the new entry.
> 
> Will
>