linux-kernel - Re: [RFC PATCH v8 03/14] mm, x86: Add support for eXclusive Page Frame Ownership (XPFO)

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Date:   Thu, 14 Feb 2019 10:13:54 -0700
From:   Khalid Aziz <khalid.aziz@...cle.com>
To:     Peter Zijlstra <peterz@...radead.org>, juergh@...il.com,
        jsteckli@...zon.de
Cc:     tycho@...ho.ws, ak@...ux.intel.com, torvalds@...ux-foundation.org,
        liran.alon@...cle.com, keescook@...gle.com,
        akpm@...ux-foundation.org, mhocko@...e.com,
        catalin.marinas@....com, 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 2/14/19 3:56 AM, Peter Zijlstra wrote:
> 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.
> 

Ah, the spin_lock in in xpfo_kmap() can be problematic in interrupt
context. I will see if I can fix that.

Juerg, you wrote the original code and understand what you were trying
to do here. If you have ideas on how to tackle this, I would very much
appreciate it.

>> +/* 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.
> 

Patch 11 ("xpfo, mm: remove dependency on CONFIG_PAGE_EXTENSION") cleans
all this up. If the original authors of these two patches, Juerg
Haefliger and Julian Stecklina, are ok with it, I would like to combine
the two patches in one.

>> +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.
> 

Got it. I will work on this.

Thanks,
Khalid


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