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

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Date:   Fri, 23 Mar 2018 19:34:52 +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:

> On 03/23/2018 12:15 PM, Nadav Amit wrote:
>>> 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...
> 
> This was a typo/thinko.  It should be pfn_pte().
> 
>>> +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?
> 
> I wanted a message.  VM_WARN_ON_ONCE() doesn't let you give a message.

Right (my bad). But VM_WARN_ONCE() lets you.