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Subject: Re: [PATCH v4 03/11] mm/gup: Applies counting method to monitor
 gup_pgd_range
To: Leonardo Bras <leonardo@linux.ibm.com>, <linuxppc-dev@lists.ozlabs.org>,
	<linux-kernel@vger.kernel.org>, <kvm-ppc@vger.kernel.org>,
	<linux-arch@vger.kernel.org>, <linux-mm@kvack.org>
CC: Benjamin Herrenschmidt <benh@kernel.crashing.org>, Paul Mackerras
	<paulus@samba.org>, Michael Ellerman <mpe@ellerman.id.au>, Arnd Bergmann
	<arnd@arndb.de>, Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>, "Christophe
 Leroy" <christophe.leroy@c-s.fr>, Andrew Morton <akpm@linux-foundation.org>,
	Dan Williams <dan.j.williams@intel.com>, Nicholas Piggin <npiggin@gmail.com>,
	Mahesh Salgaonkar <mahesh@linux.vnet.ibm.com>, Allison Randal
	<allison@lohutok.net>, Thomas Gleixner <tglx@linutronix.de>, Ganesh Goudar
	<ganeshgr@linux.ibm.com>, Mike Rapoport <rppt@linux.ibm.com>, YueHaibing
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	"Ira Weiny" <ira.weiny@intel.com>, Jason Gunthorpe <jgg@ziepe.ca>, Keith
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References: <20190927234008.11513-1-leonardo@linux.ibm.com>
 <20190927234008.11513-4-leonardo@linux.ibm.com>
From: John Hubbard <jhubbard@nvidia.com>
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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/

> 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@linux.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.

>  	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().


thanks,
-- 
John Hubbard
NVIDIA