On 14.11.18 23:23, Matthew Wilcox wrote:

On Wed, Nov 14, 2018 at 10:17:00PM +0100, David Hildenbrand wrote:

quoted

Rename PG_balloon to PG_offline. This is an indicator that the page is
logically offline, the content stale and that it should not be touched
(e.g. a hypervisor would have to allocate backing storage in order for the
guest to dump an unused page).  We can then e.g. exclude such pages from
dumps.

In following patches, we will make use of this bit also in other balloon
drivers.  While at it, document PGTABLE.

Thank you for documenting PGTABLE.  I didn't realise I also had this
document to update when I added PGTABLE.

Thank you for looking into this :)

quoted

+++ b/Documentation/admin-guide/mm/pagemap.rst

@@ -78,6 +78,8 @@ number of times a page is mapped.
     23. BALLOON
     24. ZERO_PAGE
     25. IDLE
+    26. PGTABLE
+    27. OFFLINE

So the offline *user* bit is new ... even though the *kernel* bit
just renames the balloon bit.  I'm not sure how I feel about this.
I'm going to think about it some more.  Could you share your decision
process with us?

BALLOON was/is documented as

"23 - BALLOON
    balloon compaction page
"

and only includes all virtio-ballon pages after the non-lru migration
feature has been implemented for ballooned pages. Since then, this flag
does basically no longer stands for what it actually was supposed to do.

To not break uapi I decided to not rename it but instead to add a new flag.

I have no objection to renaming the balloon bit inside the kernel; I
think that's a wise idea.  I'm just not sure whether we should rename
the user balloon bit rather than adding a new bit.

Can we rename without breaking uapi?

-- 

Thanks,

David / dhildenb