Messages in this thread | | | From | Zi Yan <> | 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
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