Re: [v2 PATCH] mm: thp: fix false negative of shmem vma's THP eligibility

[Yang Shi <yang.shi@linux.alibaba.com>]

On 6/12/19 11:44 AM, Hugh Dickins wrote:
> On Mon, 10 Jun 2019, Yang Shi wrote:
>> On 6/7/19 8:58 PM, Hugh Dickins wrote:
>>> Yes, that is correct; and correctly placed. But a little more is needed:
>>> see how mm/memory.c's transhuge_vma_suitable() will only allow a pmd to
>>> be used instead of a pte if the vma offset and size permit. smaps should
>>> not report a shmem vma as THPeligible if its offset or size prevent it.
>>>
>>> And I see that should also be fixed on anon vmas: at present smaps
>>> reports even a 4kB anon vma as THPeligible, which is not right.
>>> Maybe a test like transhuge_vma_suitable() can be added into
>>> transparent_hugepage_enabled(), to handle anon and shmem together.
>>> I say "like transhuge_vma_suitable()", because that function needs
>>> an address, which here you don't have.
>> Thanks for the remind. Since we don't have an address I'm supposed we just
>> need check if the vma's size is big enough or not other than other alignment
>> check.
>>
>> And, I'm wondering whether we could reuse transhuge_vma_suitable() by passing
>> in an impossible address, i.e. -1 since it is not a valid userspace address.
>> It can be used as and indicator that this call is from THPeligible context.
> Perhaps, but sounds like it will abuse and uglify transhuge_vma_suitable()
> just for smaps. Would passing transhuge_vma_suitable() the address
>      ((vma->vm_end & HPAGE_PMD_MASK) - HPAGE_PMD_SIZE)
> give the the correct answer in all cases?

Yes, it looks better.

>
>>> The anon offset situation is interesting: usually anon vm_pgoff is
>>> initialized to fit with its vm_start, so the anon offset check passes;
>>> but I wonder what happens after mremap to a different address - does
>>> transhuge_vma_suitable() then prevent the use of pmds where they could
>>> actually be used? Not a Number#1 priority to investigate or fix here!
>>> but a curiosity someone might want to look into.
>> Will mark on my TODO list.
>>
>>> Even with your changes
>>> ShmemPmdMapped:     4096 kB
>>> THPeligible:    0
>>> will easily be seen: THPeligible reflects whether a huge page can be
>>> allocated and mapped by pmd in that vma; but if something else already
>>> allocated the huge page earlier, it will be mapped by pmd in this vma
>>> if offset and size allow, whatever THPeligible says. We could change
>>> transhuge_vma_suitable() to force ptes in that case, but it would be
>>> a silly change, just to make what smaps shows easier to explain.
>> Where did this come from? From the commit log? If so it is the example for
>> the wrong smap output. If that case really happens, I think we could document
>> it since THPeligible should just show the current status.
> Please read again what I explained there: it's not necessarily an example
> of wrong smaps output, it's reasonable smaps output for a reasonable case.
>
> Yes, maybe Documentation/filesystems/proc.txt should explain "THPeligble"
> a little better - "eligible for allocating THP pages" rather than just
> "eligible for THP pages" would be good enough? we don't want to write
> a book about the various cases.

Yes, I agree.

>
> Oh, and the "THPeligible" output lines up very nicely there in proc.txt:
> could the actual alignment of that 0 or 1 be fixed in smaps itself too?

Sure.

Thanks,
Yang

>
> Thanks,
> Hugh