Re: [PATCH v10 7/7] mm: Don't split THP page when syscall is called

[Minchan Kim]

On Mon, Jul 07, 2014 at 02:13:03PM +0300, Kirill A. Shutemov wrote:
> On Mon, Jul 07, 2014 at 09:53:58AM +0900, 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 reduce the number of THP split.
> > 
> > Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx>
> > ---
> >  include/linux/huge_mm.h |  3 +++
> >  mm/huge_memory.c        | 25 +++++++++++++++++++++++++
> >  mm/madvise.c            | 19 +++++++++++++++++--
> >  mm/rmap.c               |  4 ++++
> >  mm/vmscan.c             | 24 ++++++++++++++++--------
> >  5 files changed, 65 insertions(+), 10 deletions(-)
> > 
> > diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> > index 63579cb8d3dc..f0d37238cf8f 100644
> > --- a/include/linux/huge_mm.h
> > +++ b/include/linux/huge_mm.h
> > @@ -19,6 +19,9 @@ extern struct page *follow_trans_huge_pmd(struct vm_area_struct *vma,
> >  					  unsigned long addr,
> >  					  pmd_t *pmd,
> >  					  unsigned int flags);
> > +extern int madvise_free_pmd(struct mmu_gather *tlb,
> > +			struct vm_area_struct *vma,
> > +			pmd_t *pmd, unsigned long addr);
> >  extern int zap_huge_pmd(struct mmu_gather *tlb,
> >  			struct vm_area_struct *vma,
> >  			pmd_t *pmd, unsigned long addr);
> > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > index 5d562a9fe931..2a70069dcfc0 100644
> > --- a/mm/huge_memory.c
> > +++ b/mm/huge_memory.c
> > @@ -1384,6 +1384,31 @@ out:
> >  	return 0;
> >  }
> >  
> > +int madvise_free_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
> > +		 pmd_t *pmd, unsigned long addr)
> > +{
> > +	spinlock_t *ptl;
> > +	int ret = 0;
> > +
> > +	if (__pmd_trans_huge_lock(pmd, vma, &ptl) == 1) {
> > +		pmd_t orig_pmd;
> > +		struct mm_struct *mm = vma->vm_mm;
> > +
> > +		/* No hugepage in swapcache */
> > +		VM_BUG_ON(PageSwapCache(pmd_page(orig_pmd)));
> 
> VM_BUG_ON_PAGE() ?

NP.

> 
> > +
> > +		orig_pmd = pmdp_get_and_clear(tlb->mm, addr, pmd);
> > +		orig_pmd = pmd_mkold(orig_pmd);
> > +		orig_pmd = pmd_mkclean(orig_pmd);
> > +
> > +		set_pmd_at(mm, addr, pmd, orig_pmd);
> > +		tlb_remove_pmd_tlb_entry(tlb, pmd, addr);
> > +		spin_unlock(ptl);
> > +		ret = 1;
> > +	}
> > +	return ret;
> > +}
> > +
> >  int zap_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
> >  		 pmd_t *pmd, unsigned long addr)
> >  {
> > diff --git a/mm/madvise.c b/mm/madvise.c
> > index 372a25a8ea82..3c99919ee094 100644
> > --- a/mm/madvise.c
> > +++ b/mm/madvise.c
> > @@ -320,8 +320,23 @@ static inline unsigned long madvise_free_pmd_range(struct mmu_gather *tlb,
> >  		 * if the range covers.
> >  		 */
> >  		next = pmd_addr_end(addr, end);
> > -		if (pmd_trans_huge(*pmd))
> > -			split_huge_page_pmd(vma, addr, pmd);
> > +		if (pmd_trans_huge(*pmd)) {
> > +			if (next - addr != HPAGE_PMD_SIZE) {
> > +#ifdef CONFIG_DEBUG_VM
> > +				if (!rwsem_is_locked(&tlb->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
> > +				split_huge_page_pmd(vma, addr, pmd);
> > +			} else if (madvise_free_pmd(tlb, vma, pmd, addr))
> > +				goto next;
> > +			/* fall through */
> > +		}
> > +
> >  		/*
> >  		 * Here there can be other concurrent MADV_DONTNEED or
> >  		 * trans huge page faults running, and if the pmd is
> > diff --git a/mm/rmap.c b/mm/rmap.c
> > index ee495d84c8b3..3c415eb8b6f0 100644
> > --- a/mm/rmap.c
> > +++ b/mm/rmap.c
> > @@ -702,6 +702,10 @@ static int page_referenced_one(struct page *page, struct vm_area_struct *vma,
> >  		/* go ahead even if the pmd is pmd_trans_splitting() */
> >  		if (pmdp_clear_flush_young_notify(vma, address, pmd))
> >  			referenced++;
> > +
> > +		if (pmd_dirty(*pmd))
> > +			dirty++;
> > +
> >  		spin_unlock(ptl);
> >  	} else {
> >  		pte_t *pte;
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index f7a45600846f..4e15babf4414 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -971,15 +971,23 @@ static unsigned long shrink_page_list(struct list_head *page_list,
> >  		 * Anonymous process memory has backing store?
> >  		 * Try to allocate it some swap space here.
> >  		 */
> > -		if (PageAnon(page) && !PageSwapCache(page) && !freeable) {
> > -			if (!(sc->gfp_mask & __GFP_IO))
> > -				goto keep_locked;
> > -			if (!add_to_swap(page, page_list))
> > -				goto activate_locked;
> > -			may_enter_fs = 1;
> > +		if (PageAnon(page) && !PageSwapCache(page)) {
> > +			if (!freeable) {
> > +				if (!(sc->gfp_mask & __GFP_IO))
> > +					goto keep_locked;
> > +				if (!add_to_swap(page, page_list))
> > +					goto activate_locked;
> > +				may_enter_fs = 1;
> >  
> > -			/* Adding to swap updated mapping */
> > -			mapping = page_mapping(page);
> > +				/* Adding to swap updated mapping */
> > +				mapping = page_mapping(page);
> > +			} else {
> > +				if (unlikely(PageTransHuge(page))) {
> > +					if (unlikely(split_huge_page_to_list(
> > +						page, page_list)))
> > +						goto keep_locked;
> 
> Hm. It would be better to free the huge page without splitting. 
> It shouldn't be a big deal: walk over rmap and zap all pmds.
> Or I miss something?

Actually, I did but found no problem except CONFIG_DEBUG_VM but rollback
after peeking [1].
When I read the description in detail by your review, I think we can remove
BUG_ON(PageTransHuge(page)) in try_to_unmap and go with no split for lazyfree
page because they are not in swapcache any more so the assumption of [1] is
not valid. Will do it in next revision.

Thanks for the review, Kirill!

[1] thp: split_huge_page paging, 3f04f62f9


> 
> > +				}
> > +			}
> >  		}
> >  
> >  		/*
> > -- 
> > 2.0.0
> > 
> > --
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> 
> -- 
>  Kirill A. Shutemov
> 
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-- 
Kind regards,
Minchan Kim
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