linux-kernel - Re: [PATCH 1/8] mm: Add optional support for PUD-sized transparent hugepages

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Date:	Sat, 2 Jan 2016 12:06:38 -0500
From:	Matthew Wilcox <willy@...ux.intel.com>
To:	"Kirill A. Shutemov" <kirill@...temov.name>
Cc:	Matthew Wilcox <matthew.r.wilcox@...el.com>, linux-mm@...ck.org,
	linux-nvdimm@...ts.01.org, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, x86@...nel.org
Subject: Re: [PATCH 1/8] mm: Add optional support for PUD-sized transparent
 hugepages

On Mon, Dec 28, 2015 at 12:05:51PM +0200, Kirill A. Shutemov wrote:
> On Thu, Dec 24, 2015 at 11:20:30AM -0500, Matthew Wilcox wrote:
> > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > index 4bf3811..e14634f 100644
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -1958,6 +1977,17 @@ static inline spinlock_t *pmd_lock(struct mm_struct *mm, pmd_t *pmd)
> >  	return ptl;
> >  }
> >  
> > +/*
> > + * No scalability reason to split PUD locks yet, but follow the same pattern
> > + * as the PMD locks to make it easier if we have to.
> > + */
> 
> I don't think it makes any good unless you convert all other places where
> we use page_table_lock to protect pud table (like __pud_alloc()) to the
> same API.
> I think this would deserve separate patch.

Sure, a separate patch to convert existing users of the PTL.  But I
don't think it does any harm to introduce the PUD version of the PMD API.
Maybe with a comment indicating that tere is significant work to be done
in converting existing users to this API?

> > diff --git a/mm/memory.c b/mm/memory.c
> > index 416b129..7328df0 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -1220,9 +1220,27 @@ static inline unsigned long zap_pud_range(struct mmu_gather *tlb,
> >  	pud = pud_offset(pgd, addr);
> >  	do {
> >  		next = pud_addr_end(addr, end);
> > +		if (pud_trans_huge(*pud) || pud_devmap(*pud)) {
> > +			if (next - addr != HPAGE_PUD_SIZE) {
> > +#ifdef CONFIG_DEBUG_VM
> 
> IS_ENABLED(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);
> 
> dump_vma(), I guess.

These two issues are copy-and-paste from the existing PMD code.  I'm happy
to update the PMD code to the new-and-improved way of doing things;
I'm just not keen to have the PMD and PUD code diverge unnecessarily.
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