Re: [PATCH] Add __GFP_MOVABLE for callers to flag allocations that may be migrated

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On Thu, 30 Nov 2006, Andrew Morton wrote:

On Thu, 30 Nov 2006 17:07:46 +0000
[email protected] (Mel Gorman) wrote:

Am reporting this patch after there were no further comments on the last
version.
Am not sure what to do with it - nothing actually uses __GFP_MOVABLE.

Nothing yet. To begin with, this is just a documentation mechanism. I'll
be trying to push page clustering one piece at a time which will need
this. The markings may also be of interest to containers and to pagesets
because it will clearly flag what are allocations in use by userspace.
It is often known at allocation time when a page may be migrated or not.
"often", yes.

This
page adds a flag called __GFP_MOVABLE and GFP_HIGH_MOVABLE. Allocations using
the __GFP_MOVABLE can be either migrated using the page migration mechanism
or reclaimed by syncing with backing storage and discarding.

Additional credit goes to Christoph Lameter and Linus Torvalds for shaping
the concept. Credit to Hugh Dickens for catching issues with shmem swap
vector and ramfs allocations.

...

@@ -65,7 +65,7 @@ static inline void clear_user_highpage(s
 static inline struct page *
 alloc_zeroed_user_highpage(struct vm_area_struct *vma, unsigned long vaddr)
 {
-	struct page *page = alloc_page_vma(GFP_HIGHUSER, vma, vaddr);
+	struct page *page = alloc_page_vma(GFP_HIGH_MOVABLE, vma, vaddr);

 	if (page)
 		clear_user_highpage(page, vaddr);
But this change is presumptuous.  alloc_zeroed_user_highpage() doesn't know
that its caller is going to use the page for moveable purposes.  (Ditto lots
of other places in this patch).

according to grep -r, alloc_zeroed_user_highpage() is only used in two
places, do_wp_page() (when write faulting the zero page)[1] and
do_anonymous_page() (when mapping the zero page for the first time and
writing). In these cases, they are known to be movable. What am I missing?
[1] I missed a call to GFP_HIGHUSER in do_wp_page() that should have been
GFP_HIGH_MOVABLE.
--
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab
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