On 08/01/15 17:01, Juergen Gross wrote:
When allocating a new pmd for the linear mapped p2m list a check is
done for not introducing another pmd when this just happened on
another cpu. In this case the old pte pointer was returned which
points to the p2m_missing or p2m_identity page. The correct value
would be the pointer to the found new page.
Looking at the check I don't see how it works at all.
alloc_p2m() looks up the address of the PTE page
ptep = lookup_address(addr, &level);
pte_pg = (pte_t *)((unsigned long)ptep & ~(PAGE_SIZE - 1));
But the check under the lock that is still true does:
ptechk = lookup_address(vaddr, &level);
if (ptechk == pte_pg) {
So I don't see how this works unless (by chance) we happen to get the
first entry in the PTE page.
It also doesn't handle racing with p2m_missing_pte to p2m_identity_pte
(or vice-versa) change.
Something like:
ptechk = lookup_address(vaddr, &level);
ptechk = (pte_t *)((unsigned long)ptechk & ~(PAGE_SIZE - 1));
if (ptechk == p2m_missing_pte || ptechk == p2m_identity) {
set_pmd(..)
Perhaps?
David
--- a/arch/x86/xen/p2m.c
+++ b/arch/x86/xen/p2m.c
@@ -440,10 +440,9 @@ EXPORT_SYMBOL_GPL(get_phys_to_machine);
* a new pmd is to replace p2m_missing_pte or p2m_identity_pte by a individual
* pmd. In case of PAE/x86-32 there are multiple pmds to allocate!
*/
-static pte_t *alloc_p2m_pmd(unsigned long addr, pte_t *ptep, pte_t *pte_pg)
+static pte_t *alloc_p2m_pmd(unsigned long addr, pte_t *pte_pg)
{
pte_t *ptechk;
- pte_t *pteret = ptep;
pte_t *pte_newpg[PMDS_PER_MID_PAGE];
pmd_t *pmdp;
unsigned int level;
@@ -477,8 +476,6 @@ static pte_t *alloc_p2m_pmd(unsigned long addr, pte_t *ptep, pte_t *pte_pg)
if (ptechk == pte_pg) {
set_pmd(pmdp,
__pmd(__pa(pte_newpg[i]) | _KERNPG_TABLE));
- if (vaddr == (addr & ~(PMD_SIZE - 1)))
- pteret = pte_offset_kernel(pmdp, addr);
pte_newpg[i] = NULL;
}
@@ -492,7 +489,7 @@ static pte_t *alloc_p2m_pmd(unsigned long addr, pte_t *ptep, pte_t *pte_pg)
vaddr += PMD_SIZE;
}
- return pteret;
+ return lookup_address(addr, &level);
}
/*
@@ -521,7 +518,7 @@ static bool alloc_p2m(unsigned long pfn)
if (pte_pg == p2m_missing_pte || pte_pg == p2m_identity_pte) {
/* PMD level is missing, allocate a new one */
- ptep = alloc_p2m_pmd(addr, ptep, pte_pg);
+ ptep = alloc_p2m_pmd(addr, pte_pg);
if (!ptep)
return false;
}