Re: [PATCH V3] powerpc/mm: Fix Multi hit ERAT cause by recent THP update

On Mon, 2016-02-15 at 16:31 +0530, Aneesh Kumar K.V wrote:

Balbir Singh [off-list ref] writes:

quoted

quoted

Now we can't depend for mm_cpumask, a parallel find_linux_pte_hugepte
can happen outside that. Now i had a variant for kick_all_cpus_sync that
ignored idle cpus. But then that needs more verification.

http://article.gmane.org/gmane.linux.ports.ppc.embedded/81105

Can be racy as a CPU moves from non-idle to idle

In

quoted

+A A A A A pmd_hugepage_update(vma->vm_mm, address, pmdp, ~0UL, 0);
+A A A A A /*
+A A A A A A * This ensures that generic code that rely on IRQ disabling
+A A A A A A * to prevent a parallel THP split work as expected.
+A A A A A A */
+A A A A A kick_all_cpus_sync();

pmdp_invalidate()->pmd_hugepage_update() can still run in parallel withA 
find_linux_pte_or_hugepte() and race.. Am I missing something?

Yes. But then we make sure that the pte_t returned by
find_linux_pte_or_hugepte doesn't change to a regular pmd entry by using
that kick. Now callers of find_lnux_pte_or_hugepte will check for
_PAGE_PRESENT. So if it called before
pmd_hugepage_update(_PAGE_PRESENT), we wait for the caller to finish the
usage (via kick()). Or they bail out after finding _PAGE_PRESENT cleared.

Makes sense, but I would still check the assumption about checking for
_PAGE_PRESENT

Balbir Singh

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