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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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6
arch/powerpc/kernel/eeh.c
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@@ -334,9 +334,11 @@ static inline unsigned long eeh_token_to_phys(unsigned long token)
int hugepage_shift;
/*
* We won't find hugepages here, iomem
* We won't find hugepages here(this is iomem). Hence we are not
* worried about _PAGE_SPLITTING/collapse. Also we will not hit
* page table free, because of init_mm.
*/
ptep = find_linux_pte_or_hugepte(init_mm.pgd, token, &hugepage_shift);
ptep = __find_linux_pte_or_hugepte(init_mm.pgd, token, &hugepage_shift);
if (!ptep)
return token;
WARN_ON(hugepage_shift);