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

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