Re: [PATCH v6 5/9] mm: thp: Extend THP to allocate anonymous large folios

[John Hubbard <jhubbard@nvidia.com>]

On 10/30/23 04:43, Ryan Roberts wrote:
> On 28/10/2023 00:04, John Hubbard wrote:
>> On 9/29/23 04:44, Ryan Roberts wrote:
...
>>>    +static bool vmf_pte_range_changed(struct vm_fault *vmf, int nr_pages)
>>> +{
>>> +    int i;
>>> +
>>> +    if (nr_pages == 1)
>>> +        return vmf_pte_changed(vmf);
>>> +
>>> +    for (i = 0; i < nr_pages; i++) {
>>> +        if (!pte_none(ptep_get_lockless(vmf->pte + i)))
>>> +            return true;
>>
>> This seems like something different than the function name implies.
>> It's really confusing: for a single page case, return true if the
>> pte in the page tables has changed, yes that is very clear.
>>
>> But then for multiple page cases, which is really the main
>> focus here--for that, claim that the range has changed if any
>> pte is present (!pte_none). Can you please help me understand
>> what this means?
> 
> Yes I understand your confusion. Although I'm confident that the code is
> correct, its a bad name - I'll make the excuse that this has evolved through
> rebasing to cope with additions to UFFD. Perhaps something like
> vmf_is_large_folio_suitable() is a better name.
> 
> It used to be that we would only take the do_anonymous_page() path if the pte
> was none; i.e. this is the first time we are faulting on an address covered by
> an anon VMA and we need to allocate some memory. But more recently we also end
> up here if the pte is a uffd_wp marker. So for a single pte, instead of checking
> none, we can check if the pte has changed from our original check (where we
> determined it was a uffd_wp marker or none). But for multiple ptes, we don't
> have storage to store all the original ptes from the first check.
> 
> Fortunately, if uffd is in use for a vma, then we don't want to use a large
> folio anyway (this would break uffd semantics because we would no longer get a
> fault for every page). So we only care about the "same but not none" case for
> nr_pages=1.
> 
> Would changing the name to vmf_is_large_folio_suitable() help here?

Yes it would! And adding in a sentence or two from above about the uffd, as
a function-level comment might be just the right of demystification for
the code.

...
pte_offset_map() can only fail due to:
>>
>>      a) Wrong pmd type. These include:
>>          pmd_none
>>          pmd_bad
>>          pmd migration entry
>>          pmd_trans_huge
>>          pmd_devmap
>>
>>      b) __pte_map() failure
>>
>> For (a), why is it that -EAGAIN is used here? I see that that
>> will lead to a re-fault, I got that far, but am missing something
>> still.
>>
>> For (b), same question, actually. I'm not completely sure why
>> why a retry is going to fix a __pte_map() failure?
> 
> I'm not going to claim to understand all the details of this. But this is due to
> a change that Hugh introduced and we concluded at [1] that its always correct to
> return EAGAIN here to rerun the fault. In fact, with the current implementation
> pte_offset_map() should never fail for anon IIUC, but the view was that EAGAIN
> makes it safe for tomorrow, and because this would only fail due to a race,
> retrying is correct.
> 
> [1] https://lore.kernel.org/linux-mm/8bdfd8d8-5662-4615-86dc-d60259bd16d@google.com/
> 

OK, got it.

...
>> And finally: is it accurate to say that there are *no* special
>> page flags being set, for PTE-mapped THPs? I don't see any here,
>> but want to confirm.
> 
> The page flags are coming from 'gfp = vma_thp_gfp_mask(vma)', which pulls in the
> correct flags based on transparent_hugepage/defrag file.
> 

OK that all is pretty clear now, thanks for the answers!


thanks,
-- 
John Hubbard
NVIDIA