Messages in this thread | | | Subject | Re: [RFC PATCH v1 00/17] ban the use of _PAGE_XXX flags outside platform specific code | From | Christophe LEROY <> | Date | Wed, 12 Sep 2018 18:07:57 +0200 |
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Le 06/09/2018 à 11:58, Aneesh Kumar K.V a écrit : > Christophe Leroy <christophe.leroy@c-s.fr> 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@linux.ibm.com> >
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
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