Re: [PATCH v7 03/14] mm: handling Non-LRU pages returned by vm_normal_pages

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 29 Jun 2022 11:59:26 +0200 David Hildenbrand <david@redhat.com> wrote:

> On 29.06.22 05:54, Alex Sierra wrote:
> > With DEVICE_COHERENT, we'll soon have vm_normal_pages() return
> > device-managed anonymous pages that are not LRU pages. Although they
> > behave like normal pages for purposes of mapping in CPU page, and for
> > COW. They do not support LRU lists, NUMA migration or THP.
> > 
> > Callers to follow_page that expect LRU pages, are also checked for
> > device zone pages due to DEVICE_COHERENT type.
> 
> Can we rephrase that to (because zeropage)
> 
> "Callers to follow_page() currently don't expect ZONE_DEVICE pages,
> however, with DEVICE_COHERENT we might now return ZONE_DEVICE. Check for
> ZONE_DEVICE pages in applicable users of follow_page() as well."

I made that change to my copy.

> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -624,6 +624,13 @@ struct page *vm_normal_page(struct vm_area_struct *vma, unsigned long addr,
> >  		if (is_zero_pfn(pfn))
> >  			return NULL;
> >  		if (pte_devmap(pte))
> > +/*
> > + * NOTE: New uers of ZONE_DEVICE will not set pte_devmap() and will have
> 
> s/uers/users/
> 
> > + * refcounts incremented on their struct pages when they are inserted into
> > + * PTEs, thus they are safe to return here. Legacy ZONE_DEVICE pages that set
> > + * pte_devmap() do not have refcounts. Example of legacy ZONE_DEVICE is
> > + * MEMORY_DEVICE_FS_DAX type in pmem or virtio_fs drivers.
> > + */

And let's regularize that comment placement?

--- a/mm/memory.c~mm-handling-non-lru-pages-returned-by-vm_normal_pages-fix
+++ a/mm/memory.c
@@ -632,16 +632,16 @@ struct page *vm_normal_page(struct vm_ar
 			return NULL;
 		if (is_zero_pfn(pfn))
 			return NULL;
+		/*
+		 * NOTE: New users of ZONE_DEVICE will not set pte_devmap()
+		 * and will have refcounts incremented on their struct pages
+		 * when they are inserted into PTEs, thus they are safe to
+		 * return here. Legacy ZONE_DEVICE pages that set pte_devmap()
+		 * do not have refcounts. Example of legacy ZONE_DEVICE is
+		 * MEMORY_DEVICE_FS_DAX type in pmem or virtio_fs drivers.
+		 */
 		if (pte_devmap(pte))
-/*
- * NOTE: New uers of ZONE_DEVICE will not set pte_devmap() and will have
- * refcounts incremented on their struct pages when they are inserted into
- * PTEs, thus they are safe to return here. Legacy ZONE_DEVICE pages that set
- * pte_devmap() do not have refcounts. Example of legacy ZONE_DEVICE is
- * MEMORY_DEVICE_FS_DAX type in pmem or virtio_fs drivers.
- */
 			return NULL;
-
 		print_bad_pte(vma, addr, pte, NULL);
 		return NULL;
 	}
_