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 <aarca...@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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