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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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24
arch/powerpc/perf/callchain.c
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@@ -111,41 +111,45 @@ perf_callchain_kernel(struct perf_callchain_entry *entry, struct pt_regs *regs)
* interrupt context, so if the access faults, we read the page tables
* to find which page (if any) is mapped and access it directly.
*/
static int read_user_stack_slow(void __user *ptr, void *ret, int nb)
static int read_user_stack_slow(void __user *ptr, void *buf, int nb)
{
int ret = -EFAULT;
pgd_t *pgdir;
pte_t *ptep, pte;
unsigned shift;
unsigned long addr = (unsigned long) ptr;
unsigned long offset;
unsigned long pfn;
unsigned long pfn, flags;
void *kaddr;
pgdir = current->mm->pgd;
if (!pgdir)
return -EFAULT;
local_irq_save(flags);
ptep = find_linux_pte_or_hugepte(pgdir, addr, &shift);
if (!ptep)
goto err_out;
if (!shift)
shift = PAGE_SHIFT;
/* align address to page boundary */
offset = addr & ((1UL << shift) - 1);
addr -= offset;
if (ptep == NULL)
return -EFAULT;
pte = *ptep;
pte = READ_ONCE(*ptep);
if (!pte_present(pte) || !(pte_val(pte) & _PAGE_USER))
return -EFAULT;
goto err_out;
pfn = pte_pfn(pte);
if (!page_is_ram(pfn))
return -EFAULT;
goto err_out;
/* no highmem to worry about here */
kaddr = pfn_to_kaddr(pfn);
memcpy(ret, kaddr + offset, nb);
return 0;
memcpy(buf, kaddr + offset, nb);
ret = 0;
err_out:
local_irq_restore(flags);
return ret;
}
static int read_user_stack_64(unsigned long __user *ptr, unsigned long *ret)