Re: [PATCH] THP: need is_vma_temporary_stack() when reference transparent_hugepage_enabled

[Andrea Arcangeli <aarcange@redhat.com>]

Hi,

On Thu, May 03, 2012 at 08:56:57AM +0800, Alex Shi wrote:
> 
> > My preference would still be to remove the is_vma_temporary_stack and
> > use two vmas during mremap of execve, that would remove the "vma"
> > parameter from transparent_hugepage_enabled() but others prefers to
> > skip a vma allocation in execve and stick to is_vma_temporary_stack,
> > which is fair enough argument.
> 
> 
> Actually, current transparent_hugepage_enabled just means the vma is in
> THP enable ENV, the vma is just possibly has some large page, no grantee
> really has.  But in lots situations, user wants to know if a vma or a
> part of memory really include a large page. not the possibility.
> 
> So, it will be great to see a real large page checking function appearing.

Well, to know if a VMA (or a memory range) really includes a THP, it'd
require to hold the page_table_lock and a loop on all pmds in the
range, but by the time you relase the lock things may have already
changed as split_huge_page can run at any time, madvise(MADV_DONTNEED)
too if you only hold the mmap_sem in read mode and the THP page
fault. In fact while holding the mmap_sem in read mode (the usual read
lock you need to take to lookup and stabilize the vma) a THP can be
freed and reallocated under it, and that's what pmd_trans_unstable is
about.