Hi Dan,
We've been doing some experimenting and testing with this patchset.
Specifically, we are trying to use you're ZONE_DEVICE work to enable
peer to peer PCIe transfers. This is actually working pretty well
(though we're still testing and working through some things).
However, we've found a couple of issues:
On Wed, Sep 23, 2015 at 12:42:27AM -0400, Dan Williams wrote:
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 3d6baa7d4534..20097e7b679a 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -49,12 +49,16 @@ struct page {
> * updated asynchronously */
> union {
> struct address_space *mapping; /* If low bit clear, points to
> - * inode address_space, or NULL.
> + * inode address_space, unless
> + * the page is in ZONE_DEVICE
> + * then it points to its parent
> + * dev_pagemap, otherwise NULL.
> * If page mapped as anonymous
> * memory, low bit is set, and
> * it points to anon_vma object:
> * see PAGE_MAPPING_ANON below.
> */
> + struct dev_pagemap *pgmap;
> void *s_mem; /* slab first object */
> };
When you add to this union and overide the mapping value, we see bugs
in calls to set_page_dirty when it tries to dereference mapping. I believe
a change to page_mapping is required such as the patch that's at the end of
this email.
> diff --git a/mm/gup.c b/mm/gup.c
> index a798293fc648..1064e9a489a4 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -98,7 +98,16 @@ retry:
> }
>
> page = vm_normal_page(vma, address, pte);
> - if (unlikely(!page)) {
> + if (!page && pte_devmap(pte) && (flags & FOLL_GET)) {
> + /*
> + * Only return device mapping pages in the FOLL_GET case since
> + * they are only valid while holding the pgmap reference.
> + */
> + if (get_dev_pagemap(pte_pfn(pte), NULL))
> + page = pte_page(pte);
> + else
> + goto no_page;
> + } else if (unlikely(!page)) {
I've found that if a driver creates a ZONE_DEVICE mapping but doesn't
create the pagemap (using devm_register_pagemap) then the get_user_pages code
will go into an infinite loop. I'm not really sure if this as an issue or
not but it seems a bit undesirable for a buggy driver to be able to cause this.
My thoughts are that either devm_register_pagemap needs to be done by
devm_memremap_pages so a driver cannot use one without the other,
or the GUP code needs to return EFAULT if no pagemap was registered so
it doesn't loop forever.
Thanks!
Logan
diff --git a/mm/util.c b/mm/util.c
index 68ff8a5..19af683 100644
--- a/mm/util.c
+++ b/mm/util.c
@@ -368,6 +368,9 @@ struct address_space *page_mapping(struct page *page)
return swap_address_space(entry);
}
+ if (unlikely(is_zone_device_page(page)))
+ return NULL;
+
mapping = (unsigned long)page->mapping;
if (mapping & PAGE_MAPPING_FLAGS)
return NULL;
--
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