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SubjectRe: [PATCH]: Handling spurious page fault for hugetlb region for 2.6.14-rc4-git5
Rohit Seth <rohit.seth@intel.com> wrote:
>
> The prefetching problem is handled OK for regular pages because we can
> handle page faults corresponding to those pages. That is currently not
> true for hugepages. Currently the kernel assumes that PAGE_FAULT
> happening against a hugetlb page is caused by truncate and returns
> SIGBUS.

Doh. No fault handler. The penny finally drops.

Adam, I think this patch is temporary?


From: "Seth, Rohit" <rohit.seth@intel.com>

We prefault hugepages at mmap() time, but hardware TLB prefetching may mean
that the TLB has NULL pagetable entries in the places where the pagetable
in fact has the desired virtual->physical translation.

For regular pages this problem is resolved via the resulting pagefault, in
the pagefault handler. But hugepages don't support pagefaults - they're
supposed to be prefaulted.

So we need minimal pagefault handling for these stale hugepage TLB entries.

An alternative is to invalidate the relevant TLB entries at hugepage
mmap()-time, but this is apparently too expensive.

Note: Adam Litke <agl@us.ibm.com>'s demand-paging-for-hugepages patches are
now in -mm. If/when these are merged up, this fix should probably be
reverted.

Signed-off-by: Rohit Seth <rohit.seth@intel.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
---
include/linux/hugetlb.h | 13 +++++++++++++
mm/memory.c | 14 ++++++++++++--
2 files changed, 25 insertions(+), 2 deletions(-)
diff -puN include/linux/hugetlb.h~handling-spurious-page-fault-for-hugetlb-region include/linux/hugetlb.h
--- devel/include/linux/hugetlb.h~handling-spurious-page-fault-for-hugetlb-region 2005-10-18 21:04:34.000000000 -0700
+++ devel-akpm/include/linux/hugetlb.h 2005-10-18 21:04:34.000000000 -0700
@@ -155,11 +155,24 @@ static inline void set_file_hugepages(st
{
file->f_op = &hugetlbfs_file_operations;
}
+
+static inline int valid_hugetlb_file_off(struct vm_area_struct *vma,
+ unsigned long address)
+{
+ struct inode *inode = vma->vm_file->f_dentry->d_inode;
+ loff_t file_off = address - vma->vm_start;
+
+ file_off += (vma->vm_pgoff << PAGE_SHIFT);
+
+ return (file_off < inode->i_size);
+}
+
#else /* !CONFIG_HUGETLBFS */

#define is_file_hugepages(file) 0
#define set_file_hugepages(file) BUG()
#define hugetlb_zero_setup(size) ERR_PTR(-ENOSYS)
+#define valid_hugetlb_file_off(vma, address) 0

#endif /* !CONFIG_HUGETLBFS */

diff -puN mm/memory.c~handling-spurious-page-fault-for-hugetlb-region mm/memory.c
--- devel/mm/memory.c~handling-spurious-page-fault-for-hugetlb-region 2005-10-18 21:04:34.000000000 -0700
+++ devel-akpm/mm/memory.c 2005-10-18 21:04:34.000000000 -0700
@@ -2045,8 +2045,18 @@ int __handle_mm_fault(struct mm_struct *

inc_page_state(pgfault);

- if (is_vm_hugetlb_page(vma))
- return VM_FAULT_SIGBUS; /* mapping truncation does this. */
+ if (unlikely(is_vm_hugetlb_page(vma))) {
+ if (valid_hugetlb_file_off(vma, address))
+ /* We get here only if there was a stale(zero) TLB entry
+ * (because of HW prefetching).
+ * Low-level arch code (if needed) should have already
+ * purged the stale entry as part of this fault handling.
+ * Here we just return.
+ */
+ return VM_FAULT_MINOR;
+ else
+ return VM_FAULT_SIGBUS; /* mapping truncation does this. */
+ }

/*
* We need the page table lock to synchronize with kswapd
_
-
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