Re: [PATCH] mm: make device private reference counts zero based

[Ira Weiny]

On Thu, Oct 08, 2020 at 10:25:44AM -0700, Ralph Campbell wrote:
> ZONE_DEVICE struct pages have an extra reference count that complicates the
> code for put_page() and several places in the kernel that need to check the
> reference count to see that a page is not being used (gup, compaction,
> migration, etc.). Clean up the code so the reference count doesn't need to
> be treated specially for device private pages, leaving DAX as still being
> a special case.

What about the check in mc_handle_swap_pte()?

mm/memcontrol.c:

5513                 /*
5514                  * MEMORY_DEVICE_PRIVATE means ZONE_DEVICE page and which have
5515                  * a refcount of 1 when free (unlike normal page)
5516                  */
5517                 if (!page_ref_add_unless(page, 1, 1))
5518                         return NULL;

... does that need to change?  Perhaps just the comment?

> 
> Signed-off-by: Ralph Campbell <rcampbell@xxxxxxxxxx>
> ---
> 

[snip]

>  
>  void put_devmap_managed_page(struct page *page);
> diff --git a/lib/test_hmm.c b/lib/test_hmm.c
> index e151a7f10519..bf92a261fa6f 100644
> --- a/lib/test_hmm.c
> +++ b/lib/test_hmm.c
> @@ -509,10 +509,15 @@ static bool dmirror_allocate_chunk(struct dmirror_device *mdevice,
>  		mdevice->devmem_count * (DEVMEM_CHUNK_SIZE / (1024 * 1024)),
>  		pfn_first, pfn_last);
>  
> +	/*
> +	 * Pages are created with an initial reference count of one but should
> +	 * have a reference count of zero while in the free state.
> +	 */
>  	spin_lock(&mdevice->lock);
>  	for (pfn = pfn_first; pfn < pfn_last; pfn++) {
>  		struct page *page = pfn_to_page(pfn);
>  
> +		set_page_count(page, 0);

This confuses me.  How does this and init_page_count() not confuse the buddy
allocator?  Don't you have to reset the refcount somewhere after the test?

>  		page->zone_device_data = mdevice->free_pages;
>  		mdevice->free_pages = page;
>  	}
> @@ -561,7 +566,7 @@ static struct page *dmirror_devmem_alloc_page(struct dmirror_device *mdevice)
>  	}
>  
>  	dpage->zone_device_data = rpage;
> -	get_page(dpage);
> +	init_page_count(dpage);
>  	lock_page(dpage);
>  	return dpage;
>  
> diff --git a/mm/internal.h b/mm/internal.h
> index c43ccdddb0f6..e1443b73aa9b 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
>  

[snip]

> diff --git a/mm/swap.c b/mm/swap.c
> index 0eb057141a04..93d880c6f73c 100644
> --- a/mm/swap.c
> +++ b/mm/swap.c
> @@ -116,12 +116,11 @@ static void __put_compound_page(struct page *page)
>  void __put_page(struct page *page)
>  {
>  	if (is_zone_device_page(page)) {
> -		put_dev_pagemap(page->pgmap);
> -
>  		/*
>  		 * The page belongs to the device that created pgmap. Do
>  		 * not return it to page allocator.
>  		 */
> +		free_zone_device_page(page);

I really like this.

Ira