linux-kernel - Re: [PATCH 05/11] x86/mm: do not auto-massage page protections

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Message-ID: <224464E0-1D3A-4ED8-88E0-A8E84C4265FC@vmware.com>
Date:   Fri, 23 Mar 2018 19:15:55 +0000
From:   Nadav Amit <namit@...are.com>
To:     Dave Hansen <dave.hansen@...ux.intel.com>
CC:     LKML <linux-kernel@...r.kernel.org>,
        "open list:MEMORY MANAGEMENT" <linux-mm@...ck.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        "luto@...nel.org" <luto@...nel.org>,
        "torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
        "keescook@...gle.com" <keescook@...gle.com>,
        "hughd@...gle.com" <hughd@...gle.com>,
        "jgross@...e.com" <jgross@...e.com>,
        "x86@...nel.org" <x86@...nel.org>
Subject: Re: [PATCH 05/11] x86/mm: do not auto-massage page protections

Dave Hansen <dave.hansen@...ux.intel.com> wrote:

> 
> From: Dave Hansen <dave.hansen@...ux.intel.com>
> 
> A PTE is constructed from a physical address and a pgprotval_t.
> __PAGE_KERNEL, for instance, is a pgprot_t and must be converted
> into a pgprotval_t before it can be used to create a PTE.  This is
> done implicitly within functions like set_pte() by massage_pgprot().
> 
> However, this makes it very challenging to set bits (and keep them
> set) if your bit is being filtered out by massage_pgprot().
> 
> This moves the bit filtering out of set_pte() and friends.  For

I don’t see that set_pte() filters the bits, so I am confused by this
sentence...

> +static inline pgprotval_t check_pgprot(pgprot_t pgprot)
> +{
> +	pgprotval_t massaged_val = massage_pgprot(pgprot);
> +
> +	/* mmdebug.h can not be included here because of dependencies */
> +#ifdef CONFIG_DEBUG_VM
> +	WARN_ONCE(pgprot_val(pgprot) != massaged_val,
> +		  "attempted to set unsupported pgprot: %016lx "
> +		  "bits: %016lx supported: %016lx\n",
> +		  pgprot_val(pgprot),
> +		  pgprot_val(pgprot) ^ massaged_val,
> +		  __supported_pte_mask);
> +#endif
Why not to use VM_WARN_ON_ONCE() and avoid the ifdef?