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

["Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>]

On Wednesday 14 June 2017 10:30 PM, Vlastimil Babka wrote:
> 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().
> 

I was wondering whether we can do pmdp_establish(pgtable); and document 
all quirks needed for that in the per arch implementation of 
pmdp_establish(). We could also then switch all the 
pmdp_clear/set_pmd_at() usage to pmdp_establish().

-aneesh