Re: [PATCH v2 1/3] mm: add vm_normal_lru_pages for LRU handled pages only

[Christoph Hellwig <hch@lst.de>]

> -	page = vm_normal_page(vma, addr, pte);
> +	page = vm_normal_lru_page(vma, addr, pte);

Why can't this deal with ZONE_DEVICE pages?  It certainly has
nothing do with a LRU I think.  In fact being able to have
stats that count say the number of device pages here would
probably be useful at some point.

In general I find the vm_normal_lru_page vs vm_normal_page
API highly confusing.  An explicit check for zone device pages
in the dozen or so spots that care has a much better documentation
value, especially if accompanied by comments where it isn't entirely
obvious.

>  		page = follow_page(vma, addr,
> -				FOLL_GET | FOLL_MIGRATION | FOLL_REMOTE);
> +				FOLL_GET | FOLL_MIGRATION | FOLL_REMOTE | FOLL_LRU);

Overly long line here.

> +/*
> + * NOTE: Technically this should goto check_pfn label. However, page->_mapcount
> + * is never incremented for device pages that are mmap through DAX mechanism
> + * using pmem driver mounted into ext4 filesystem. When these pages are unmap,
> + * zap_pte_range is called and vm_normal_page return a valid page with
> + * page_mapcount() = 0, before page_remove_rmap is called.
> + */

Please properly indent comments.

> + * zone, as long as the pte's are present and vm_normal_lru_page() succeeds. These
>   * pages also get pinned.

Another overly long line here.