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powerpc/mm/thp: Make page table walk safe against thp split/collapse

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We can disable a THP split or a hugepage collapse by disabling irq.
We do send IPI to all the cpus in the early part of split/collapse,
and disabling local irq ensure we don't make progress with
split/collapse. If the THP is getting split we return NULL from
find_linux_pte_or_hugepte(). For all the current callers it should be ok.
We need to be careful if we want to use returned pte_t pointer outside
the irq disabled region. W.r.t to THP split, the pfn remains the same,
but then a hugepage collapse will result in a pfn change. There are
few steps we can take to avoid a hugepage collapse.One way is to take page
reference inside the irq disable region. Other option is to take
mmap_sem so that a parallel collapse will not happen. We can also
disable collapse by taking pmd_lock. Another method used by kvm
subsystem is to check whether we had a mmu_notifer update in between
using mmu_notifier_retry().

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
This commit is contained in:
Aneesh Kumar K.V 2015-03-30 10:41:03 +05:30 • committed by Michael Ellerman
parent dac5657067
commit 691e95fd73
9 changed files with 88 additions and 36 deletions
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11
arch/powerpc/include/asm/pgtable.h
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@ -247,8 +247,17 @@ extern int gup_hugepte(pte_t *ptep, unsigned long sz, unsigned long addr,
#define pmd_large(pmd) 0
#define has_transparent_hugepage() 0
#endif
pte_t *find_linux_pte_or_hugepte(pgd_t *pgdir, unsigned long ea,
pte_t *__find_linux_pte_or_hugepte(pgd_t *pgdir, unsigned long ea,
unsigned *shift);
static inline pte_t *find_linux_pte_or_hugepte(pgd_t *pgdir, unsigned long ea,
unsigned *shift)
{
if (!arch_irqs_disabled()) {
pr_info("%s called with irq enabled\n", __func__);
dump_stack();
}
return __find_linux_pte_or_hugepte(pgdir, ea, shift);
}
#endif /* __ASSEMBLY__ */
#endif /* __KERNEL__ */