netdev - Re: [PATCH v2 4/4] x86/vmalloc: Add TLB efficient x86 arch

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Date:   Wed, 12 Dec 2018 21:05:52 +0000
From:   "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To:     "namit@...are.com" <namit@...are.com>
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        "Hansen, Dave" <dave.hansen@...el.com>
Subject: Re: [PATCH v2 4/4] x86/vmalloc: Add TLB efficient x86 arch_vunmap

On Wed, 2018-12-12 at 06:30 +0000, Nadav Amit wrote:
> > On Dec 11, 2018, at 4:03 PM, Rick Edgecombe <rick.p.edgecombe@...el.com>
> > wrote:
> > 
> > This adds a more efficient x86 architecture specific implementation of
> > arch_vunmap, that can free any type of special permission memory with only 1
> > TLB
> > flush.
> > 
> > In order to enable this, _set_pages_p and _set_pages_np are made non-static
> > and
> > renamed set_pages_p_noflush and set_pages_np_noflush to better communicate
> > their different (non-flushing) behavior from the rest of the set_pages_*
> > functions.
> > 
> > The method for doing this with only 1 TLB flush was suggested by Andy
> > Lutomirski.
> > 
> 
> [snip]
> 
> > +	/*
> > +	 * If the vm being freed has security sensitive capabilities such as
> > +	 * executable we need to make sure there is no W window on the directmap
> > +	 * before removing the X in the TLB. So we set not present first so we
> > +	 * can flush without any other CPU picking up the mapping. Then we reset
> > +	 * RW+P without a flush, since NP prevented it from being cached by
> > +	 * other cpus.
> > +	 */
> > +	set_area_direct_np(area);
> > +	vm_unmap_aliases();
> 
> Does vm_unmap_aliases() flush in the TLB the direct mapping range as well? I
> can only find the flush of the vmalloc range.
Hmmm. It should usually (I tested), but now I wonder if there are cases where it
doesn't and it could depend on architecture as well. I'll have to trace through
this to verify, thanks.

Rick