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

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Message-ID: <20190214174451.GA3338@lst.de>
Date:   Thu, 14 Feb 2019 18:44:51 +0100
From:   Christoph Hellwig <hch@....de>
To:     Khalid Aziz <khalid.aziz@...cle.com>
Cc:     Christoph Hellwig <hch@....de>, juergh@...il.com, tycho@...ho.ws,
        jsteckli@...zon.de, 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,
        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 Thu, Feb 14, 2019 at 09:56:24AM -0700, Khalid Aziz wrote:
> 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:

The point I'm making it that just about every PageHighMem() check
before code that does a kmap* later needs to account for xpfo as well.

So instead of opencoding the above, be that using xpfo_page_is_unmapped
or PageXpfoUnmapped, we really need one self-describing helper that
checks if a page is unmapped for any reason and needs a kmap to access
it.