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

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Date:   Thu, 14 Feb 2019 09:56:24 -0700
From:   Khalid Aziz <khalid.aziz@...cle.com>
To:     Christoph Hellwig <hch@....de>
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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        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,
        steven.sistare@...cle.com, labbott@...hat.com, luto@...nel.org,
        dave.hansen@...el.com, peterz@...radead.org,
        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>
Subject: Re: [RFC PATCH v8 04/14] swiotlb: Map the buffer if it was unmapped
 by XPFO

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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