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

Juerg Haefliger
2017-09-11T16:03:59Z

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
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