Messages in this thread | | | Date | Fri, 05 Apr 2013 11:01:02 +0200 | From | Stefan Bader <> | Subject | x86/mm/pageattr: Code without effect? |
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When looking through some mm code I stumbled over one part in arch/x86/mm/pageattr.c that looks somewhat bogus to me. Cannot say what exactly the effects are, but maybe you do (or you could explain to me why I am wrong :)).
commit a8aed3e0752b4beb2e37cbed6df69faae88268da Author: Andrea Arcangeli <aarcange@redhat.com> Date: Fri Feb 22 15:11:51 2013 -0800
x86/mm/pageattr: Prevent PSE and GLOABL leftovers to confuse pmd/pte_present and pmd_huge
added the following to try_preserve_large_page:
/* + * Set the PSE and GLOBAL flags only if the PRESENT flag is + * set otherwise pmd_present/pmd_huge will return true even on + * a non present pmd. The canon_pgprot will clear _PAGE_GLOBAL + * for the ancient hardware that doesn't support it. + */ + if (pgprot_val(new_prot) & _PAGE_PRESENT) + pgprot_val(new_prot) |= _PAGE_PSE | _PAGE_GLOBAL; + else + pgprot_val(new_prot) &= ~(_PAGE_PSE | _PAGE_GLOBAL); + + new_prot = canon_pgprot(new_prot); + + /*
but (extending what follows after the changes)
* old_pte points to the large page base address. So we need * to add the offset of the virtual address: */ pfn = pte_pfn(old_pte) + ((address & (psize - 1)) >> PAGE_SHIFT); cpa->pfn = pfn;
new_prot = static_protections(req_prot, address, pfn);
So new_prot gets completely replaced by req_prot and all changes done to new_prot before look to be lost (the PSE and GLOBAL bit settings as well as the canon_pgprot call.
Maybe the hunk is useless anyway, or the breakage is subtle, or I miss something...
Thanks, Stefan
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