Messages in this thread | | | Date | Thu, 2 Oct 2014 08:57:32 -0700 | Subject | Re: pipe/page fault oddness. | From | Linus Torvalds <> |
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On Thu, Oct 2, 2014 at 1:47 AM, Hugh Dickins <hughd@google.com> wrote: > > I hesitate to admit, I still don't see it: please illuminate further.
No, your'e looking at what I was looking.
> We're talking about the loop in __split_huge_page_map(), where it does
Yes.
> 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.
Fair enough. Except this code has no locking that I see, so if we *ever* see that numa entry in the pmd while walking the page tables in vmscan, we're basically screwed.
> 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.
So I don't see any locking at all wrt mprotect (or new mmap). That's kind of the whole point for page-out - it bypasses all the normal VM locks, and only uses the last pte locking.
So the whole use of vma->vm_page_prot here is a bit scary. That gets modified outside of the page table locks. So how do you know it's not already PROT_NONE, but mprotect just hasn't gotten to actually take the page table locks yet?
I dunno. It all makes me just very nervous. The whole "numa bit is separate from the protections, has different locking, and is just oddly and subtly different" is really what I fundamentally object to. And it seems so _unnecessary_. All this odd complexity for no actual gain - just extra code, and extra room for subtle bugs. Which is exactly why I hate that magic NUMA bit so much.
Linus
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