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

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

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
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.

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.