Re: [PATCH v17 7/7] mm: Don't split THP page when syscall is called

[Minchan Kim]

On Thu, Nov 27, 2014 at 04:49:21PM +0100, Michal Hocko wrote:
> On Mon 20-10-14 19:12:04, Minchan Kim wrote:
> > We don't need to split THP page when MADV_FREE syscall is
> > called. It could be done when VM decide really frees it so
> > we could avoid unnecessary THP split.
> > 
> > Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> > Acked-by: Rik van Riel <riel@xxxxxxxxxx>
> > Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> > Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx>
> 
> Other than a minor comment below
> Reviewed-by: Michal Hocko <mhocko@xxxxxxx>

Thanks!

> 
> > ---
> >  include/linux/huge_mm.h |  4 ++++
> >  mm/huge_memory.c        | 35 +++++++++++++++++++++++++++++++++++
> >  mm/madvise.c            | 21 ++++++++++++++++++++-
> >  mm/rmap.c               |  8 ++++++--
> >  mm/vmscan.c             | 28 ++++++++++++++++++----------
> >  5 files changed, 83 insertions(+), 13 deletions(-)
> > 
> [...]
> > diff --git a/mm/madvise.c b/mm/madvise.c
> > index a21584235bb6..84badee5f46d 100644
> > --- a/mm/madvise.c
> > +++ b/mm/madvise.c
> > @@ -271,8 +271,26 @@ static int madvise_free_pte_range(pmd_t *pmd, unsigned long addr,
> >  	spinlock_t *ptl;
> >  	pte_t *pte, ptent;
> >  	struct page *page;
> > +	unsigned long next;
> > +
> > +	next = pmd_addr_end(addr, end);
> > +	if (pmd_trans_huge(*pmd)) {
> > +		if (next - addr != HPAGE_PMD_SIZE) {
> > +#ifdef CONFIG_DEBUG_VM
> > +			if (!rwsem_is_locked(&mm->mmap_sem)) {
> > +				pr_err("%s: mmap_sem is unlocked! addr=0x%lx end=0x%lx vma->vm_start=0x%lx vma->vm_end=0x%lx\n",
> > +					__func__, addr, end,
> > +					vma->vm_start,
> > +					vma->vm_end);
> > +				BUG();
> > +			}
> > +#endif
> 
> Why is this code here? madvise_free_pte_range is called only from the
> madvise path and we are holding mmap_sem and relying on that for regular
> pages as well.

Make sense.