Hi Aneesh,
On Wed, Sep 04, 2013 at 12:43:19PM +0530, Aneesh Kumar K.V wrote:
> Naoya Horiguchi <n-horigu...@ah.jp.nec.com> writes:
>
> > Currently all of page table handling by hugetlbfs code are done under
> > mm->page_table_lock. So when a process have many threads and they heavily
> > access to the memory, lock contention happens and impacts the performance.
> >
> > This patch makes hugepage support split page table lock so that we use
> > page->ptl of the leaf node of page table tree which is pte for normal pages
> > but can be pmd and/or pud for hugepages of some architectures.
> >
> > ChangeLog v2:
> > - add split ptl on other archs missed in v1
> >
> > Signed-off-by: Naoya Horiguchi <n-horigu...@ah.jp.nec.com>
> > ---
> > arch/powerpc/mm/hugetlbpage.c | 6 ++-
> > arch/tile/mm/hugetlbpage.c | 6 ++-
> > include/linux/hugetlb.h | 20 ++++++++++
> > mm/hugetlb.c | 92
> > ++++++++++++++++++++++++++-----------------
> > mm/mempolicy.c | 5 ++-
> > mm/migrate.c | 4 +-
> > mm/rmap.c | 2 +-
> > 7 files changed, 90 insertions(+), 45 deletions(-)
> >
> > diff --git v3.11-rc3.orig/arch/powerpc/mm/hugetlbpage.c
> > v3.11-rc3/arch/powerpc/mm/hugetlbpage.c
> > index d67db4b..7e56cb7 100644
> > --- v3.11-rc3.orig/arch/powerpc/mm/hugetlbpage.c
> > +++ v3.11-rc3/arch/powerpc/mm/hugetlbpage.c
> > @@ -124,6 +124,7 @@ static int __hugepte_alloc(struct mm_struct *mm,
> > hugepd_t *hpdp,
> > {
> > struct kmem_cache *cachep;
> > pte_t *new;
> > + spinlock_t *ptl;
> >
> > #ifdef CONFIG_PPC_FSL_BOOK3E
> > int i;
> > @@ -141,7 +142,8 @@ static int __hugepte_alloc(struct mm_struct *mm,
> > hugepd_t *hpdp,
> > if (! new)
> > return -ENOMEM;
> >
> > - spin_lock(&mm->page_table_lock);
> > + ptl = huge_pte_lockptr(mm, new);
> > + spin_lock(ptl);
>
>
> Are you sure we can do that for ppc ?
> new = kmem_cache_zalloc(cachep, GFP_KERNEL|__GFP_REPEAT);
Ah, thanks. new is not a pointer to one full page occupied by page
table entries, so trying to use struct page of it is totally wrong.
> The page for new(pte_t) could be shared right ? which mean a deadlock ?
Yes, that's disastrous.
> May be you should do it at the pmd level itself for ppc
Yes, that's possible, but I simply drop the changes in __hugepte_alloc()
for now because this lock seems to protect us from the race between concurrent
calls of __hugepte_alloc(), not between allocation and read/write access.
Split ptl is used to avoid race between read/write accesses, so I think
that using different types of locks here is not dangerous.
# I guess that that's why we now use mm->page_table_lock for __pte_alloc()
# and its family even if USE_SPLIT_PTLOCKS is true.
A bit off-topic, but I found that we have a bogus comment on
hugetlb_free_pgd_range in arch/powerpc/mm/hugetlbpage.c saying
"Must be called with pagetable lock held."
This seems not true because the caller free_pgtables() and its
callers (unmap_region() and exit_mmap()) never hold it.
I guess that it's just copied from free_pgd_range() and it's also
false for this function. I'll post a patch to remove this later.
Anyway, thank you for valuable comments!
Thanks,
Naoya Horiguchi
> > #ifdef CONFIG_PPC_FSL_BOOK3E
> > /*
> > * We have multiple higher-level entries that point to the same
> > @@ -174,7 +176,7 @@ static int __hugepte_alloc(struct mm_struct *mm,
> > hugepd_t *hpdp,
> > #endif
> > }
> > #endif
> > - spin_unlock(&mm->page_table_lock);
> > + spin_unlock(ptl);
> > return 0;
> > }
> >
>
>
> -aneesh
>
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