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

[Mel Gorman]

On Thu, 30 Nov 2006, Andrew Morton wrote:

> On Thu, 30 Nov 2006 17:07:46 +0000
> mel@xxxxxxxxx (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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