Re: [PATCH 00 of 30] Transparent Hugepage support #3

[Andrea Arcangeli <aarcange@redhat.com>]

On Fri, Jan 22, 2010 at 10:51:35AM -0600, Christoph Lameter wrote:
> On Fri, 22 Jan 2010, Andrea Arcangeli wrote:
> 
> > On Fri, Jan 22, 2010 at 08:46:50AM -0600, Christoph Lameter wrote:
> > > Jus thinking about yesterdays fix to page migration:
> > >
> > > This means that huge pages are unstable right? Kernel code cannot
> > > establish a reference to a 2M/4M page and be sure that the page is not
> > > broken up due to something in the VM that cannot handle huge pages?
> >
> > Physically speaking DMA-wise they cannot be broken up, only thing that
> > gets broken up is the pmd that instead of mapping the page directly
> > starts to map the pte. Nothing changes on the physical side of
> > hugepages. khugepaged only collapse pages into hugepages if there are
> > no references at all (no gup no nothing) so again no issue DMA-wise.
> 
> Reclaim cannot kick out page size pieces of the huge page?

Before the VM can kick out any hugepage it has to split it, then each
page-sized-piece will be considered individually, so reclaim only
kicks out page-sized-pieces of the hugepage.

> > have irq disabled so the ipi of collapse_huge_page will wait. It's all
> > handled transparently by the patch, you won't notice you're dealing
> > with hugepage if you're gup user (unless you use gup to migrate pages
> > in which case calling split_huge_page is enough like in patch ;).
> 
> What if I want to use hugepages for some purpose and I dont want to use
> 512 pointers to keep track of the individual pieces?

If you use hugepages and there's no VM activity or other activity that
triggers split_huge_page, there are no 512 pointers, but just 1
pointer in the pmd to the hugepage, and no other link at all. There is
also one preallocated uninitialized all-zero pte queued in the mm in
case we have to split the hugepage later but it has no pointers to the
hugepage at all (it will have those only if the page is splitted later
for some reason, and then the pmd will point the preallocated pte
instead of the hugepage directly).

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