Re: [RFC][PATCH 0/5] VM_PINNED

[Peter Zijlstra]

On Tue, May 27, 2014 at 11:56:44AM -0500, Christoph Lameter wrote:
> On Tue, 27 May 2014, Peter Zijlstra wrote:
> 
> > > Code could be easily added to alloc_pages_vma() to consider the pinned
> > > status on allocation. Remove GFP_MOVABLE if the vma is pinned.
> >
> > Yes, but alloc_pages_vma() isn't used for shared pages (with exception
> > of shmem and hugetlbfs).
> 
> alloc_pages_vma() is used for all paths where we populate address ranges
> with pages. This is what we are doing when pinning. Pages are not
> allocated outside of a vma context.
> 
> What do you mean by shared pages that are not shmem pages? AnonPages that
> are referenced from multiple processes?

Regular files.. they get allocated through __page_cache_alloc(). AFAIK
there is nothing stopping people from pinning file pages for RDMA or
other purposes. Unusual maybe, but certainly not impossible, and
therefore we must be able to handle it.

> > So whichever way around we have to do the mm_populate() + eviction hook
> > + migration code, and since that equally covers the anon case, why
> > bother?
> 
> Migration is expensive and the memory registration overhead already
> causes lots of complaints.

Sure, but first to the simple thing, then if its a problem do something
else.
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