Re: pipe/page fault oddness.

[Hugh Dickins]

On Wed, 1 Oct 2014, Linus Torvalds wrote:
> On Wed, Oct 1, 2014 at 1:19 AM, Hugh Dickins <hughd@xxxxxxxxxx> wrote:
> 
> Can we please just get rid of _PAGE_NUMA. There is no excuse for it.

I'm no lover of _PAGE_NUMA, and hope that it can be simplified away
as you outline.  What we have in 3.16+3.17 is already an attempt to
improve on what you hated before, but not obviously an improvement.

Mel is the one who knows these issues inside out: maybe he's been
blinkered, but I wouldn't dare to pull it apart without his input.
Myself, I'm not looking beyond fixing whatever is the bug for 3.17.

> > However, that would still not explain Dave's endless refaulting;
> 
> Why not? You start out with a PROTNONE, trigger shrink_page_list() on
> a hugepage,.which calls add_to_swap(), which does
> split_huge_page_to_list(), which in turn calls __split_huge_page(),
> and that turns (_PAGE_PROTNONE) into (_PAGE_PROTNONE|_PAGE_NUMA),
> which you will then fault on forever, because the kernel thinks the
> page is present, but not a NUMA page.

I hesitate to admit, I still don't see it: please illuminate further.

We're talking about the loop in __split_huge_page_map(), where it does

			entry = mk_pte(page + i, vma->vm_page_prot);
			entry = maybe_mkwrite(pte_mkdirty(entry), vma);
			if (!pmd_write(*pmd))
				entry = pte_wrprotect(entry);
			if (!pmd_young(*pmd))
				entry = pte_mkold(entry);
			if (pmd_numa(*pmd))
				entry = pte_mknuma(entry);

, right?  I only see that adding _PAGE_NUMA to _PAGE_PROTNONE if
pmd_numa(*pmd): but that would mean we had already gone wrong, setting
pmd_numa in a PROT_NONE vma, which task_numa_work takes care not to do;
or have mprotected an area to PROT_NONE without doing the pmd_mknonnuma.

Ah, we won't have mmap_sem in the add_to_swap case; so we could be
racing with an mprotect which already updated vm_flags and vm_page_prot,
but has not yet reached this pmd: is that a part of how you see it?

Or are you noticing a deficiency in the pmd locking?  I have not
worked my way through that, so cannot guarantee it, but please
point me to the weakness where you see it.

But when you convince me on that, then I still don't see how we get to
doing the repeated write fault, instead of hitting access_error(), as
you pointed out originally.  That still seems to require a PROTNONE
pte to be left behind in a VM_WRITE vma, which I do not see happening
here.  pte_modify leaves _PAGE_NUMA alone, but updates _PAGE_PROTNONE.
The pte_numa<->pte_special confusion is messy, but I don't yet get
how it would manifest in the manner observed.

But certainly, a bug in the THP splitting feels just right to
match the frequency of the sightings: I hope you've got it.

Hugh
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