linux-kernel - Re: [PATCH v4 03/11] mm/gup: Applies counting method to monitor gup_pgd

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Date:   Tue, 01 Oct 2019 14:56:19 -0300
From:   Leonardo Bras <leonardo@...ux.ibm.com>
To:     John Hubbard <jhubbard@...dia.com>, linuxppc-dev@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org, kvm-ppc@...r.kernel.org,
        linux-arch@...r.kernel.org, linux-mm@...ck.org
Cc:     Keith Busch <keith.busch@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        YueHaibing <yuehaibing@...wei.com>,
        Nicholas Piggin <npiggin@...il.com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Mahesh Salgaonkar <mahesh@...ux.vnet.ibm.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Paul Mackerras <paulus@...ba.org>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>,
        Ganesh Goudar <ganeshgr@...ux.ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Ira Weiny <ira.weiny@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Allison Randal <allison@...utok.net>
Subject: Re: [PATCH v4 03/11] mm/gup: Applies counting method to monitor
 gup_pgd_range

On Mon, 2019-09-30 at 14:51 -0700, John Hubbard wrote:
> On 9/27/19 4:40 PM, Leonardo Bras wrote:
> > As decribed, gup_pgd_range is a lockless pagetable walk. So, in order to
> > monitor against THP split/collapse with the couting method, it's necessary
> 
> s/couting/counting/
> 

Thanks, fixed for v5.

> > to bound it with {start,end}_lockless_pgtbl_walk.
> > 
> > There are dummy functions, so it is not going to add any overhead on archs
> > that don't use this method.
> > 
> > Signed-off-by: Leonardo Bras <leonardo@...ux.ibm.com>
> > ---
> >  mm/gup.c | 8 ++++++++
> >  1 file changed, 8 insertions(+)
> > 
> > diff --git a/mm/gup.c b/mm/gup.c
> > index 98f13ab37bac..7105c829cf44 100644
> > --- a/mm/gup.c
> > +++ b/mm/gup.c
> > @@ -2325,6 +2325,7 @@ static bool gup_fast_permitted(unsigned long start, unsigned long end)
> >  int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
> >  			  struct page **pages)
> >  {
> > +	struct mm_struct *mm;
> 
> I don't think that this local variable adds any value, so let's not use it.
> Similar point in a few other patches too.

It avoids 1 deference of current->mm, it's a little performance gain.

> 
> >  	unsigned long len, end;
> >  	unsigned long flags;
> >  	int nr = 0;
> > @@ -2352,9 +2353,12 @@ int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
> >  
> >  	if (IS_ENABLED(CONFIG_HAVE_FAST_GUP) &&
> >  	    gup_fast_permitted(start, end)) {
> > +		mm = current->mm;
> > +		start_lockless_pgtbl_walk(mm);
> >  		local_irq_save(flags);
> >  		gup_pgd_range(start, end, write ? FOLL_WRITE : 0, pages, &nr);
> >  		local_irq_restore(flags);
> > +		end_lockless_pgtbl_walk(mm);
> >  	}
> >  
> >  	return nr;
> > @@ -2404,6 +2408,7 @@ int get_user_pages_fast(unsigned long start, int nr_pages,
> >  			unsigned int gup_flags, struct page **pages)
> >  {
> >  	unsigned long addr, len, end;
> > +	struct mm_struct *mm;
> 
> Same here.
> 
> >  	int nr = 0, ret = 0;
> >  
> >  	if (WARN_ON_ONCE(gup_flags & ~(FOLL_WRITE | FOLL_LONGTERM)))
> > @@ -2421,9 +2426,12 @@ int get_user_pages_fast(unsigned long start, int nr_pages,
> >  
> >  	if (IS_ENABLED(CONFIG_HAVE_FAST_GUP) &&
> >  	    gup_fast_permitted(start, end)) {
> > +		mm = current->mm;
> > +		start_lockless_pgtbl_walk(mm);
> 
> Minor: I'd like to rename this register_lockless_pgtable_walker().
> 
> >  		local_irq_disable();
> >  		gup_pgd_range(addr, end, gup_flags, pages, &nr);
> >  		local_irq_enable();
> > +		end_lockless_pgtbl_walk(mm);
> 
> ...and deregister_lockless_pgtable_walker().
> 

I have no problem changing the name, but I don't register/deregister
are good terms for this. 

I would rather use start/finish, begin/end, and so on. Register sounds
like something more complicated than what we are trying to achieve
here. 

> 
> thanks,

Thank you!

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