Re: [PATCH 1/2] mm: hotplug: implement non-movable version ofget_user_pages() called get_user_pages_non_movable()

[Mel Gorman]

On Tue, Feb 19, 2013 at 05:55:30PM +0800, Lin Feng wrote:
> Hi Mel,
> 
> On 02/18/2013 11:17 PM, Mel Gorman wrote:
> >>> > > <SNIP>
> >>> > >
> >>> > > result. It's a little clumsy but the memory hot-remove failure message
> >>> > > could list what applications have pinned the pages that cannot be removed
> >>> > > so the administrator has the option of force-killing the application. It
> >>> > > is possible to discover what application is pinning a page from userspace
> >>> > > but it would involve an expensive search with /proc/kpagemap
> >>> > > 
> >>>>> > >>> +	if (migrate_pre_flag && !isolate_err) {
> >>>>> > >>> +		ret = migrate_pages(&pagelist, alloc_migrate_target, 1,
> >>>>> > >>> +					false, MIGRATE_SYNC, MR_SYSCALL);
> >>> > > 
> >>> > > The conversion of alloc_migrate_target is a bit problematic. It strips
> >>> > > the __GFP_MOVABLE flag and the consequence of this is that it converts
> >>> > > those allocation requests to MIGRATE_UNMOVABLE. This potentially is a large
> >>> > > number of pages, particularly if the number of get_user_pages_non_movable()
> >>> > > increases for short-lived pins like direct IO.
> >> >
> >> > Sorry, I don't quite understand here neither. If we use the following new 
> >> > migration allocation function as you said, the increasing number of 
> >> > get_user_pages_non_movable() will also lead to large numbers of MIGRATE_UNMOVABLE
> >> > pages. What's the difference, do I miss something?
> >> > 
> > The replacement function preserves the __GFP_MOVABLE flag. It cannot use
> > ZONE_MOVABLE but otherwise the newly allocated page will be grouped with
> > other movable pages.
> 
> Ah, got it " But GFP_MOVABLE is not only a zone specifier but also an allocation policy.".
> 

Update the comment and describe the exception then.

> Could I clear __GFP_HIGHMEM flag in alloc_migrate_target depending on private parameter so
> that we can keep MIGRATE_UNMOVABLE policy also allocate page none movable zones with little
> change?
> 

It should work (double check gfp_zone) but then the allocation cannot
use the highmem zone. If you can be 100% certain that zone will not exist
be populated then it will work as expected but it's a hack and should be
commented clearly. You could do a BUILD_BUG_ON if CONFIG_HIGHMEM is set
to enforce it.

-- 
Mel Gorman
SUSE Labs
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