Re: [RFC PATCH v8 04/14] swiotlb: Map the buffer if it was unmapped by XPFO

From: Khalid Aziz <khalid.aziz@oracle.com>
To: Christoph Hellwig <hch@lst.de>
Cc: juergh@gmail.com, tycho@tycho.ws, jsteckli@amazon.de,
	ak@linux.intel.com, torvalds@linux-foundation.org,
	liran.alon@oracle.com, keescook@google.com,
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	konrad.wilk@oracle.com,
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	tyhicks@canonical.com, dwmw@amazon.co.uk,
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	dave.hansen@intel.com, peterz@infradead.org,
	kernel-hardening@lists.openwall.com, linux-mm@kvack.org,
	x86@kernel.org, linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org, Tycho Andersen <tycho@docker.com>
Subject: Re: [RFC PATCH v8 04/14] swiotlb: Map the buffer if it was unmapped by XPFO
Date: Thu, 14 Feb 2019 09:56:24 -0700	[thread overview]
Message-ID: <3c75c46c-2a5a-cd75-83d4-f77d96d22f7d@oracle.com> (raw)
In-Reply-To: <20190214074747.GA10666@lst.de>

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On 2/14/19 12:47 AM, Christoph Hellwig wrote:
> On Wed, Feb 13, 2019 at 05:01:27PM -0700, Khalid Aziz wrote:
>> +++ b/kernel/dma/swiotlb.c
>> @@ -396,8 +396,9 @@ static void swiotlb_bounce(phys_addr_t orig_addr, phys_addr_t tlb_addr,
>>  {
>>  	unsigned long pfn = PFN_DOWN(orig_addr);
>>  	unsigned char *vaddr = phys_to_virt(tlb_addr);
>> +	struct page *page = pfn_to_page(pfn);
>>  
>> -	if (PageHighMem(pfn_to_page(pfn))) {
>> +	if (PageHighMem(page) || xpfo_page_is_unmapped(page)) {
> 
> I think this just wants a page_unmapped or similar helper instead of
> needing the xpfo_page_is_unmapped check.  We actually have quite
> a few similar construct in the arch dma mapping code for architectures
> that require cache flushing.

As I am not the original author of this patch, I am interpreting the
original intent. I think xpfo_page_is_unmapped() was added to account
for kernel build without CONFIG_XPFO. xpfo_page_is_unmapped() has an
alternate definition to return false if CONFIG_XPFO is not defined.
xpfo_is_unmapped() is cleaned up further in patch 11 ("xpfo, mm: remove
dependency on CONFIG_PAGE_EXTENSION") to a one-liner "return
PageXpfoUnmapped(page);". xpfo_is_unmapped() can be eliminated entirely
by adding an else clause to the following code added by that patch:

--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -398,6 +402,15 @@ TESTCLEARFLAG(Young, young, PF_ANY)
 PAGEFLAG(Idle, idle, PF_ANY)
 #endif

+#ifdef CONFIG_XPFO
+PAGEFLAG(XpfoUser, xpfo_user, PF_ANY)
+TESTCLEARFLAG(XpfoUser, xpfo_user, PF_ANY)
+TESTSETFLAG(XpfoUser, xpfo_user, PF_ANY)
+PAGEFLAG(XpfoUnmapped, xpfo_unmapped, PF_ANY)
+TESTCLEARFLAG(XpfoUnmapped, xpfo_unmapped, PF_ANY)
+TESTSETFLAG(XpfoUnmapped, xpfo_unmapped, PF_ANY)
+#endif
+
 /*
  * On an anonymous page mapped into a user virtual memory area,
  * page->mapping points to its anon_vma, not to a struct address_space;


Adding the following #else to above conditional:

#else
TESTPAGEFLAG_FALSE(XpfoUser)
TESTPAGEFLAG_FALSE(XpfoUnmapped)

should allow us to eliminate xpfo_is_unmapped(). Right?

Thanks,
Khalid

> 
>> +bool xpfo_page_is_unmapped(struct page *page)
>> +{
>> +	struct xpfo *xpfo;
>> +
>> +	if (!static_branch_unlikely(&xpfo_inited))
>> +		return false;
>> +
>> +	xpfo = lookup_xpfo(page);
>> +	if (unlikely(!xpfo) && !xpfo->inited)
>> +		return false;
>> +
>> +	return test_bit(XPFO_PAGE_UNMAPPED, &xpfo->flags);
>> +}
>> +EXPORT_SYMBOL(xpfo_page_is_unmapped);
> 
> And at least for swiotlb there is no need to export this helper,
> as it is always built in.
> 


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