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Date: Thu, 14 Feb 2019 11:56:31 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Khalid Aziz <khalid.aziz@...cle.com>
Cc: juergh@...il.com, tycho@...ho.ws, jsteckli@...zon.de,
ak@...ux.intel.com, torvalds@...ux-foundation.org,
liran.alon@...cle.com, keescook@...gle.com,
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will.deacon@....com, jmorris@...ei.org, konrad.wilk@...cle.com,
Juerg Haefliger <juerg.haefliger@...onical.com>,
deepa.srinivasan@...cle.com, chris.hyser@...cle.com,
tyhicks@...onical.com, dwmw@...zon.co.uk, andrew.cooper3@...rix.com,
jcm@...hat.com, boris.ostrovsky@...cle.com,
kanth.ghatraju@...cle.com, oao.m.martins@...cle.com,
jmattson@...gle.com, pradeep.vincent@...cle.com,
john.haxby@...cle.com, tglx@...utronix.de,
kirill.shutemov@...ux.intel.com, hch@....de,
steven.sistare@...cle.com, labbott@...hat.com, luto@...nel.org,
dave.hansen@...el.com, kernel-hardening@...ts.openwall.com,
linux-mm@...ck.org, x86@...nel.org,
linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
Tycho Andersen <tycho@...ker.com>,
Marco Benatto <marco.antonio.780@...il.com>
Subject: Re: [RFC PATCH v8 03/14] mm, x86: Add support for eXclusive Page
Frame Ownership (XPFO)
On Wed, Feb 13, 2019 at 05:01:26PM -0700, Khalid Aziz wrote:
> static inline void *kmap_atomic(struct page *page)
> {
> + void *kaddr;
> +
> preempt_disable();
> pagefault_disable();
> + kaddr = page_address(page);
> + xpfo_kmap(kaddr, page);
> + return kaddr;
> }
> #define kmap_atomic_prot(page, prot) kmap_atomic(page)
>
> static inline void __kunmap_atomic(void *addr)
> {
> + xpfo_kunmap(addr, virt_to_page(addr));
> pagefault_enable();
> preempt_enable();
> }
How is that supposed to work; IIRC kmap_atomic was supposed to be
IRQ-safe.
> +/* Per-page XPFO house-keeping data */
> +struct xpfo {
> + unsigned long flags; /* Page state */
> + bool inited; /* Map counter and lock initialized */
What's sizeof(_Bool) ? Why can't you use a bit in that flags word?
> + atomic_t mapcount; /* Counter for balancing map/unmap requests */
> + spinlock_t maplock; /* Lock to serialize map/unmap requests */
> +};
Without that bool, the structure would be 16 bytes on 64bit, which seems
like a good number.
> +void xpfo_kmap(void *kaddr, struct page *page)
> +{
> + struct xpfo *xpfo;
> +
> + if (!static_branch_unlikely(&xpfo_inited))
> + return;
> +
> + xpfo = lookup_xpfo(page);
> +
> + /*
> + * The page was allocated before page_ext was initialized (which means
> + * it's a kernel page) or it's allocated to the kernel, so nothing to
> + * do.
> + */
> + if (!xpfo || unlikely(!xpfo->inited) ||
> + !test_bit(XPFO_PAGE_USER, &xpfo->flags))
> + return;
> +
> + spin_lock(&xpfo->maplock);
> +
> + /*
> + * The page was previously allocated to user space, so map it back
> + * into the kernel. No TLB flush required.
> + */
> + if ((atomic_inc_return(&xpfo->mapcount) == 1) &&
> + test_and_clear_bit(XPFO_PAGE_UNMAPPED, &xpfo->flags))
> + set_kpte(kaddr, page, PAGE_KERNEL);
> +
> + spin_unlock(&xpfo->maplock);
> +}
> +EXPORT_SYMBOL(xpfo_kmap);
> +
> +void xpfo_kunmap(void *kaddr, struct page *page)
> +{
> + struct xpfo *xpfo;
> +
> + if (!static_branch_unlikely(&xpfo_inited))
> + return;
> +
> + xpfo = lookup_xpfo(page);
> +
> + /*
> + * The page was allocated before page_ext was initialized (which means
> + * it's a kernel page) or it's allocated to the kernel, so nothing to
> + * do.
> + */
> + if (!xpfo || unlikely(!xpfo->inited) ||
> + !test_bit(XPFO_PAGE_USER, &xpfo->flags))
> + return;
> +
> + spin_lock(&xpfo->maplock);
> +
> + /*
> + * The page is to be allocated back to user space, so unmap it from the
> + * kernel, flush the TLB and tag it as a user page.
> + */
> + if (atomic_dec_return(&xpfo->mapcount) == 0) {
> + WARN(test_bit(XPFO_PAGE_UNMAPPED, &xpfo->flags),
> + "xpfo: unmapping already unmapped page\n");
> + set_bit(XPFO_PAGE_UNMAPPED, &xpfo->flags);
> + set_kpte(kaddr, page, __pgprot(0));
> + xpfo_flush_kernel_tlb(page, 0);
> + }
> +
> + spin_unlock(&xpfo->maplock);
> +}
> +EXPORT_SYMBOL(xpfo_kunmap);
And these here things are most definitely not IRQ-safe.
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