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

[Ralph Campbell]

On 10/9/20 9:53 AM, Ira Weiny wrote:
> 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?

Thanks for spotting this.

Actually, this whole bit of code is never reached because the
  if (non_swap_entry(ent))
    return NULL;
covers device private pages and returns.

The device private pages are accounted for in mem_cgroup so this
probably needs fixing. I'll probably move the code before the
non_swap_entry() check and change the refcount increment to
page_ref_add_unless(page, 1, 0).

> > 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?

Device private struct pages are created with memmap_pages() and destroyed with
memunmap_pages(). They are never put on the LRU and never freed to the page
allocator. The refcount is set to zero by put_page() which triggers
the call to pgmap->page_free() instead. So only the driver handles the free pages
it creates.


> >   		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