Re: mm/hugetlb: GFP_KERNEL allocation under spinlock?

[Hugh Dickins <hugh.dickins@tiscali.co.uk>]

On Sun, 2 Aug 2009, Jiri Slaby wrote:
> 
> could anybody please confirm this cannot happen?

I'm no authority on hugetlb.c nowadays: you'll have studied this
in more detail than I have, so please don't believe me.  (And I'm
no longer at my old address, but lkml's enjoying a quiet Sunday.)

> 
> hugetlb_fault()
> -> spin_lock()
> -> hugetlb_cow()
>    -> alloc_huge_page()
>       -> vma_needs_reservation()
>          -> region_chg() (either of the 2)
>             -> kmalloc(*, GFP_KERNEL)
> 
> Thanks.

That should be taken care of by the successful vma_needs_reservation()
on the same address in hugetlb_fault(), before taking page_table_lock,
shouldn't it?

It is possible that a hugetlb_vmtruncate() comes in between that
vma_needs_reservation() and taking the page_table_lock, which could
remove the region (or "nrg") needed.

But if that's the case then the pte_same test immediately after taking
page_table_lock should catch it: we're in the part of hugetlb_fault()
dealing with !huge_pte_none, whereas truncation would have made it
huge_pte_none (and it won't get to freeing the reservations before
it's nullified the page tables, holding page_table_lock).

Hugh

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