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From:   Zi Yan <ziy@nvidia.com>
To:     Matthew Wilcox <willy@infradead.org>
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Subject: Re: [RFC PATCH 01/31] mm: migrate: Add exchange_pages to exchange two
 lists of pages.
Date:   Mon, 18 Feb 2019 09:31:24 -0800
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On 17 Feb 2019, at 3:29, Matthew Wilcox wrote:

> On Fri, Feb 15, 2019 at 02:08:26PM -0800, Zi Yan wrote:
>> +struct page_flags {
>> +	unsigned int page_error :1;
>> +	unsigned int page_referenced:1;
>> +	unsigned int page_uptodate:1;
>> +	unsigned int page_active:1;
>> +	unsigned int page_unevictable:1;
>> +	unsigned int page_checked:1;
>> +	unsigned int page_mappedtodisk:1;
>> +	unsigned int page_dirty:1;
>> +	unsigned int page_is_young:1;
>> +	unsigned int page_is_idle:1;
>> +	unsigned int page_swapcache:1;
>> +	unsigned int page_writeback:1;
>> +	unsigned int page_private:1;
>> +	unsigned int __pad:3;
>> +};
>
> I'm not sure how to feel about this.  It's a bit fragile versus 
> somebody adding
> new page flags.  I don't know whether it's needed or whether you can 
> just
> copy page->flags directly because you're holding PageLock.

I agree with you that current way of copying page flags individually 
could miss
new page flags. I will try to come up with something better. Copying 
page->flags as a whole
might not simply work, since the upper part of page->flags has the page 
node information,
which should not be changed. I think I need to add a helper function to 
just copy/exchange
all page flags, like calling migrate_page_stats() twice.

>> +static void exchange_page(char *to, char *from)
>> +{
>> +	u64 tmp;
>> +	int i;
>> +
>> +	for (i = 0; i < PAGE_SIZE; i += sizeof(tmp)) {
>> +		tmp = *((u64 *)(from + i));
>> +		*((u64 *)(from + i)) = *((u64 *)(to + i));
>> +		*((u64 *)(to + i)) = tmp;
>> +	}
>> +}
>
> I have a suspicion you'd be better off allocating a temporary page and
> using copy_page().  Some architectures have put a lot of effort into
> making copy_page() run faster.

When I am doing exchange_pages() between two NUMA nodes on a x86_64 
machine,
I actually can saturate the QPI bandwidth with this operation. I think 
cache
prefetching was doing its job.

The purpose of proposing exchange_pages() is to avoid allocating any new 
page,
so that we would not trigger any potential page reclaim or memory 
compaction.
Allocating a temporary page defeats the purpose.

>
>> +		xa_lock_irq(&to_mapping->i_pages);
>> +
>> +		to_pslot = radix_tree_lookup_slot(&to_mapping->i_pages,
>> +			page_index(to_page));
>
> This needs to be converted to the XArray.  radix_tree_lookup_slot() is
> going away soon.  You probably need:
>
> 	XA_STATE(to_xas, &to_mapping->i_pages, page_index(to_page));

Thank you for pointing this out. I will do the change.

>
> This is a lot of code and I'm still trying to get my head aroud it 
> all.
> Thanks for putting in this work; it's good to see this approach being
> explored.

Thank you for taking a look at the code.

--
Best Regards,
Yan Zi