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

[David Hildenbrand]

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.

-- 
Cheers,

David / dhildenb