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

Tycho Andersen
2017-09-21T01:37:17Z

On Wed, Sep 20, 2017 at 05:27:02PM -0700, Dave Hansen wrote:
> On 09/20/2017 05:09 PM, Tycho Andersen wrote:
> >> I think the only thing that will really help here is if you batch the
> >> allocations. For instance, you could make sure that the per-cpu-pageset
> >> lists always contain either all kernel or all user data. Then remap the
> >> entire list at once and do a single flush after the entire list is consumed.
> > Just so I understand, the idea would be that we only flush when the
> > type of allocation alternates, so:
> >
> > kmalloc(..., GFP_KERNEL);
> > kmalloc(..., GFP_KERNEL);
> > /* remap+flush here */
> > kmalloc(..., GFP_HIGHUSER);
> > /* remap+flush here */
> > kmalloc(..., GFP_KERNEL);
>
> Not really. We keep a free list per migrate type, and a per_cpu_pages
> (pcp) list per migratetype:
>
> > struct per_cpu_pages {
> > int count; /* number of pages in the list */
> > int high; /* high watermark, emptying needed */
> > int batch; /* chunk size for buddy add/remove */
> >
> > /* Lists of pages, one per migrate type stored on the pcp-lists */
> > struct list_head lists[MIGRATE_PCPTYPES];
> > };
>
> The migratetype is derived from the GFP flags in
> gfpflags_to_migratetype(). In general, GFP_HIGHUSER and GFP_KERNEL come
> from different migratetypes, so they come from different free lists.
>
> In your case above, the GFP_HIGHUSER allocation come through the
> MIGRATE_MOVABLE pcp list while the GFP_KERNEL ones come from the
> MIGRATE_UNMOVABLE one. Since we add a bunch of pages to those lists at
> once, you could do all the mapping/unmapping/flushing on a bunch of
> pages at once
>
> Or, you could hook your code into the places where the migratetype of
> memory is changed (set_pageblock_migratetype(), plus where we fall
> back). Those changes are much more rare than page allocation.

I see, thanks for all this discussion. It has been very helpful!

Tycho