[RFC PATCH v1 00/17] ban the use of _PAGE_XXX flags outside platform specific code

[Aneesh Kumar K.V]

On 09/05/2018 06:06 PM, Christophe Leroy wrote:
> Today flags like for instance _PAGE_RW or _PAGE_USER are used through
> common parts of code.
> Using those directly in common parts of code have proven to lead to
> mistakes or misbehaviour, because their use is not always as trivial
> as one could think.
> 
> For instance, (flags & _PAGE_USER) == 0 isn't enough to tell
> that a page is a kernel page, because some targets are using
> _PAGE_PRIVILEDGED and not _PAGE_USER, so the test has to be
> (flags & (_PAGE_USER | _PAGE_PRIVILEDGED)) == _PAGE_PRIVILEDGED
> This has to (bad) consequences:
> 
>   - All targets must define every bit, even the unsupported ones,
>     leading to a lot of useless #define _PAGE_XXX 0
>   - If someone forgets to take into account all possible _PAGE_XXX bits
>     for the case, we can get unexpected behaviour on some targets.
> 
> This becomes even more complex when we come to using _PAGE_RW.
> Testing (flags & _PAGE_RW) is not enough to test whether a page
> if writable or not, because:
> 
>   - Some targets have _PAGE_RO instead, which has to be unset to tell
>     a page is writable
>   - Some targets have _PAGE_R and _PAGE_W, in which case
>     _PAGE_RW = _PAGE_R | _PAGE_W
>   - Even knowing whether a page is readable is not always trivial because:
>     - Some targets requires to check that _PAGE_R is set to ensure page
>     is readable
>     - Some targets requires to check that _PAGE_NA is not set
>     - Some targets requires to check that _PAGE_RO or _PAGE_RW is set
> 
> Etc ....
> 
> In order to work around all those issues and minimise the risks of errors,
> this serie aims at removing all use of _PAGE_XXX flags from powerpc code
> and always use pte_xxx() and pte_mkxxx() accessors instead. Those accessors
> are then defined in target specific parts of the kernel code.

We recently did on book3s 64.

static inline int pte_present(pte_t pte)
{
	/*
	 * A pte is considerent present if _PAGE_PRESENT is set.
	 * We also need to consider the pte present which is marked
	 * invalid during ptep_set_access_flags. Hence we look for _PAGE_INVALID
	 * if we find _PAGE_PRESENT cleared.
	 */
	return !!(pte_raw(pte) & cpu_to_be64(_PAGE_PRESENT | _PAGE_INVALID));
}

So I guess with that pte_present conversion we need to be careful.

Do you have a git tree which I can use to double check?

-aneesh