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

[Hugh Dickins <hughd@google.com>]

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?

> > 
> > 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.

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?

Thanks,
Hugh