Re: [PATCH 1/2] mm,thp: refactor generic deposit/withdraw routines for wider usage

[Vineet Gupta <Vineet.Gupta1@synopsys.com>]

On Thursday 11 February 2016 04:50 PM, Martin Schwidefsky wrote:
> On Thu, 11 Feb 2016 16:23:33 +0530
> Vineet Gupta <Vineet.Gupta1@synopsys.com> wrote:
> 
>> On Thursday 11 February 2016 03:52 PM, Martin Schwidefsky wrote:
>>> On Thu, 11 Feb 2016 14:58:26 +0530
>>> Vineet Gupta <Vineet.Gupta1@synopsys.com> wrote:
>>>
>>>> Generic pgtable_trans_huge_deposit()/pgtable_trans_huge_withdraw()
>>>> assume pgtable_t to be struct page * which is not true for all arches.
>>>> Thus arc, s390, sparch end up with their own copies despite no special
>>>> hardware requirements (unlike powerpc).
>>>
>>> s390 does have a special hardware requirement. pgtable_t is an address
>>> for a 2K block of memory. It is *not* equivalent to a struct page *
>>> which refers to a 4K block of memory. That has been the whole point
>>> to introduce pgtable_t.
>>
>> Actually my reference to hardware requirement was more like powerpc style save a
>> hash value some where etc.
>>
>> Now pgtable_t need not be struct page * even if the actual sizes are same - e.g.
>> in ARC port I kept pgtable_t as pte_t * simply to avoid a few page_address() calls
>> in mm code (you could argue that is was a micro-optimization, anyways..)
>>
>> So given I know nothing about s390 MMU internals, I still think you can switch to
>> the update generic version despite 2K vs. 4K. Agree ?
> 
> No, we can not. For s390 a page table is aligned on a 2K boundary and is
> only half the size of a page (except for KVM but that is another story).
> For s390 a pgtable_t is a pointer to the memory location with the 256 ptes
> and not a struct page *.
> 
> The cast "struct page *new = (struct page*)pgtable;" in your first patch
> is already broken, "new" points to the memory of the page table and
> the list_head operations will clobber that memory.

The current s390 code does something similar using a different struct cast. It is
still writing in pgtable_t - although at a different location.

> You try to fix it up
> with the memset to zero in pgtable_trans_huge_withdraw but that does not
> correct the pte entries for s390 as an invalid page-table entry is *not*
> all zeros.

Right so that is the problem - just trying to understand.

> In short, please let s390 keep its own copy of deposit/withdraw.

You got it - I'm out of the way :-)

Thx,
-Vineet

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