Re: [PATCH v8 04/10] mm: thp: Support allocation of anonymous multi-size THP

[Ryan Roberts <ryan.roberts@arm.com>]

On 07/12/2023 15:01, David Hildenbrand wrote:
> On 07.12.23 15:45, Ryan Roberts wrote:
>> On 07/12/2023 13:28, David Hildenbrand wrote:
>>>>>
>>>>> Right, but you know from the first loop which order is applicable (and will be
>>>>> fed to the second loop) and could just pte_unmap(pte) + tryalloc. If that
>>>>> fails,
>>>>> remap and try with the next orders.
>>>>
>>>> You mean something like this?
>>>>
>>>>      pte = pte_offset_map(vmf->pmd, vmf->address & PMD_MASK);
>>>>      if (!pte)
>>>>          return ERR_PTR(-EAGAIN);
>>>>
>>>>      order = highest_order(orders);
>>>>      while (orders) {
>>>>          addr = ALIGN_DOWN(vmf->address, PAGE_SIZE << order);
>>>>          if (!pte_range_none(pte + pte_index(addr), 1 << order)) {
>>>>              order = next_order(&orders, order);
>>>>              continue;
>>>>          }
>>>>
>>>>          pte_unmap(pte);
>>>>                  folio = vma_alloc_folio(gfp, order, vma, addr, true);
>>>>          if (folio) {
>>>>              clear_huge_page(&folio->page, vmf->address, 1 << order);
>>>>              return folio;
>>>>          }
>>>>
>>>>          pte = pte_offset_map(vmf->pmd, vmf->address & PMD_MASK);
>>>>          if (!pte)
>>>>              return ERR_PTR(-EAGAIN);
>>>>
>>>>          order = next_order(&orders, order);
>>>>      }
>>>>
>>>>      pte_unmap(pte);
>>>>
>>>> I don't really like that because if high order folio allocations fail, then you
>>>> are calling pte_range_none() again for the next lower order; once that check
>>>> has
>>>> succeeded for an order it shouldn't be required for any lower orders. In this
>>>> case you also have lots of pte map/unmap.
>>>
>>> I see what you mean.
>>>
>>>>
>>>> The original version feels more efficient to me.
>>> Yes it is. Adding in some comments might help, like
>>>
>>> /*
>>>   * Find the largest order where the aligned range is completely prot_none().
>>> Note
>>>   * that all remaining orders will be completely prot_none().
>>>   */
>>> ...
>>>
>>> /* Try allocating the largest of the remaining orders. */
>>
>> OK added.
>>
>>>
>>>>
>>>>>
>>>>> That would make the code certainly easier to understand. That "orders"
>>>>> magic of
>>>>> constructing, filtering, walking is confusing :)
>>>>>
>>>>>
>>>>> I might find some time today to see if there is an easy way to cleanup all
>>>>> what
>>>>> I spelled out above. It really is a mess. But likely that cleanup could be
>>>>> deferred (but you're touching it, so ... :) ).
>>>>
>>>> I'm going to ignore the last 5 words. I heard the "that cleanup could be
>>>> deferred" part loud and clear though :)
>>>
>>> :)
>>>
>>> If we could stop passing orders into thp_vma_allowable_orders(), that would
>>> probably
>>> be the biggest win. It's just all a confusing mess.
>>
>>
>>
>> I tried an approach like you suggested in the other thread originally, but I
>> struggled to define exactly what "thp_vma_configured_orders()" should mean;
>> Ideally, I just want "all the THP orders that are currently enabled for this
>> VMA+flags". But some callers want to enforce_sysfs and others don't, so you
>> probably have to at least pass that flag. Then you have DAX which explicitly
> 
> Yes, the flags would still be passed. It's kind of the "context".
> 
>> ignores enforce_sysfs, but only in a page fault. And shmem, which ignores
>> enforce_sysfs, but only outside of a page fault. So it quickly becomes pretty
>> complex. It is basically thp_vma_allowable_orders() as currently defined.
> 
> Yeah, but moving the "can we actually fit a THP in there" check out of the picture.
> 
>>
>> If this could be a simple function then it could be inline and as you say, we
>> can do the masking in the caller and exit early for the order-0 case. But it is
>> very complex (at least if you want to retain the equivalent logic to what
>> thp_vma_allowable_orders() has) so I'm not sure how to do the order-0 early exit
>> without passing in the orders bitfield. And we are unlikely to exit early
>> because PMD-sized THP is likely enabled and because we didn't pass in a orders
>> bitfield, that wasn't filtered out.
>>
>> In short, I can't see a solution that's better than the one I have. But if you
>> have something in mind, if you can spell it out, then I'll have a go at tidying
>> it up and integrating it into the series. Otherwise I really would prefer to
>> leave it for a separate series.
> 
> I'm playing with some cleanups, but they can all be built on top if they
> materialize.

OK, I'm going to post a v9 then. And cross my fingers and hope that's the final
version.