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

[Christophe LEROY]

Le 06/09/2018 Ã 11:58, Aneesh Kumar K.V a ÃcritÂ:
> Christophe Leroy <christophe.leroy@xxxxxx> writes:
>
> > 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.
>
> The series is really good. It also helps in code readability. Few things
> i am not sure there is a way to reduce the overhead
>
> -		access = _PAGE_EXEC;
> +		access = pte_val(pte_mkexec(__pte(0)));
>
> Considering we have multiple big endian to little endian coversion there
> for book3s 64.
>
> Other thing is __ioremap_at where we do
>
> +       pte_t pte = __pte(flags);
>   
>          /* Make sure we have the base flags */
> -       if ((flags & _PAGE_PRESENT) == 0)
> +       if (!pte_present(pte))
>
> -               err = map_kernel_page(v+i, p+i, flags);
> +               err = map_kernel_page(v + i, p + i, pte_val(pte));

Finally, for that I now ensure that all base flags are set by the 
callers and I have removed that hack which adds PAGE_KERNEL flags when 
_PAGE_PRESENT is not in the handedover flags.

> But otherwise for the series.
>
> Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@xxxxxxxxxxxxx>

Thanks, I added it to all unchanged patches of the serie. You are 
welcome to give a feedback on the new ones if you have time.

Christophe