Re: [PATCH v6 03/11] mm, x86: Add support for eXclusive Page Frame Ownership (XPFO)

Matthew Wilcox
2017-11-15T03:44:40Z

On Mon, Nov 13, 2017 at 02:46:25PM -0800, Dave Hansen wrote:
> On 11/13/2017 02:20 PM, Dave Hansen wrote:
> > On 11/09/2017 05:09 PM, Tycho Andersen wrote:
> >> which I guess is from the additional flags in grow_dev_page() somewhere down
> >> the stack. Anyway... it seems this is a kernel allocation that's using
> >> MIGRATE_MOVABLE, so perhaps we need some more fine tuned heuristic than just
> >> all MOVABLE allocations are un-mapped via xpfo, and all the others are mapped.
> >>
> >> Do you have any ideas?
> >
> > It still has to do a kmap() or kmap_atomic() to be able to access it. I
> > thought you hooked into that. Why isn't that path getting hit for these?
>
> Oh, this looks to be accessing data mapped by a buffer_head. It
> (rudely) accesses data via:
>
> void set_bh_page(struct buffer_head *bh,
> ...
> bh->b_data = page_address(page) + offset;

We don't need to kmap in order to access MOVABLE allocations. kmap is
only needed for HIGHMEM allocations. So there's nothing wrong with ext4
or set_bh_page().