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

Tycho Andersen
2017-09-11T16:59:41Z

On Mon, Sep 11, 2017 at 06:03:55PM +0200, Juerg Haefliger wrote:
>
>
> On 09/11/2017 04:50 PM, Tycho Andersen wrote:
> > Hi Yisheng,
> >
> > On Mon, Sep 11, 2017 at 03:24:09PM +0800, Yisheng Xie wrote:
> >>> +void xpfo_alloc_pages(struct page *page, int order, gfp_t gfp)
> >>> +{
> >>> + int i, flush_tlb = 0;
> >>> + struct xpfo *xpfo;
> >>> +
> >>> + if (!static_branch_unlikely(&xpfo_inited))
> >>> + return;
> >>> +
> >>> + for (i = 0; i < (1 << order); i++) {
> >>> + xpfo = lookup_xpfo(page + i);
> >>> + if (!xpfo)
> >>> + continue;
> >>> +
> >>> + WARN(test_bit(XPFO_PAGE_UNMAPPED, &xpfo->flags),
> >>> + "xpfo: unmapped page being allocated\n");
> >>> +
> >>> + /* Initialize the map lock and map counter */
> >>> + if (unlikely(!xpfo->inited)) {
> >>> + spin_lock_init(&xpfo->maplock);
> >>> + atomic_set(&xpfo->mapcount, 0);
> >>> + xpfo->inited = true;
> >>> + }
> >>> + WARN(atomic_read(&xpfo->mapcount),
> >>> + "xpfo: already mapped page being allocated\n");
> >>> +
> >>> + if ((gfp & GFP_HIGHUSER) == GFP_HIGHUSER) {
> >>> + /*
> >>> + * Tag the page as a user page and flush the TLB if it
> >>> + * was previously allocated to the kernel.
> >>> + */
> >>> + if (!test_and_set_bit(XPFO_PAGE_USER, &xpfo->flags))
> >>> + flush_tlb = 1;
> >>
> >> I'm not sure whether I am miss anything, however, when the page was previously allocated
> >> to kernel, should we unmap the physmap (the kernel's page table) here? For we allocate
> >> the page to user now
> >>
> > Yes, I think you're right. Oddly, the XPFO_READ_USER test works
> > correctly for me, but I think (?) should not because of this bug...
>
> IIRC, this is an optimization carried forward from the initial
> implementation. The assumption is that the kernel will map the user
> buffer so it's not unmapped on allocation but only on the first (and

Does the kernel always map it, though? e.g. in the case of
XPFO_READ_USER, I'm not sure where the kernel would do a kmap() of the
test's user buffer.

Tycho

> subsequent) call of kunmap. I.e.:
> - alloc -> noop
> - kmap -> noop
> - kunmap -> unmapped from the kernel
> - kmap -> mapped into the kernel
> - kunmap -> unmapped from the kernel
> and so on until:
> - free -> mapped back into the kernel
>
> I'm not sure if that make sense though since it leaves a window.
>
> ...Juerg
>
>
>
> > Tycho
> >