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SubjectRFC: Cleanup / small fixes to hugetlb fault handling
Hi, Adam, Bill, Hugh,

Does this look like a reasonable patch to send to akpm for -mm.

This patch makes some slight tweaks / cleanups to the fault handling
path for huge pages in -mm. My main motivation is to make it simpler
to fit COW in, but along the way it addresses a few minor problems
with the existing code:

- The check against i_size was duplicated: once in
find_lock_huge_page() and again in hugetlb_fault() after taking the
page_table_lock. We only really need the locked one, so remove the
other.

- find_lock_huge_page() didn't, in fact, lock the page if it newly
allocated one, rather than finding it in the page cache already. As
far as I can tell this is a bug, so the patch corrects it.

- find_lock_huge_page() isn't a great name, since it does extra things
not analagous to find_lock_page(). Rename it
find_or_alloc_huge_page() which is closer to the mark.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

Index: working-2.6/mm/hugetlb.c
===================================================================
--- working-2.6.orig/mm/hugetlb.c 2005-10-26 11:18:39.000000000 +1000
+++ working-2.6/mm/hugetlb.c 2005-10-26 11:34:32.000000000 +1000
@@ -336,8 +336,8 @@
flush_tlb_range(vma, start, end);
}

-static struct page *find_lock_huge_page(struct address_space *mapping,
- unsigned long idx)
+static struct page *find_or_alloc_huge_page(struct address_space *mapping,
+ unsigned long idx)
{
struct page *page;
int err;
@@ -347,21 +347,19 @@
retry:
page = find_lock_page(mapping, idx);
if (page)
- goto out;
-
- /* Check to make sure the mapping hasn't been truncated */
- size = i_size_read(inode) >> HPAGE_SHIFT;
- if (idx >= size)
- goto out;
+ return page;

if (hugetlb_get_quota(mapping))
- goto out;
+ return NULL;
+
page = alloc_huge_page();
if (!page) {
hugetlb_put_quota(mapping);
- goto out;
+ return NULL;
}

+ lock_page(page);
+
err = add_to_page_cache(page, mapping, idx, GFP_KERNEL);
if (err) {
put_page(page);
@@ -370,50 +368,49 @@
goto retry;
page = NULL;
}
-out:
+
return page;
}

-int hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
- unsigned long address, int write_access)
+int hugetlb_no_page(struct mm_struct *mm, struct vm_area_struct *vma,
+ unsigned long address, int write_access, pte_t *ptep)
{
- int ret = VM_FAULT_SIGBUS;
+ int ret;
unsigned long idx;
unsigned long size;
- pte_t *pte;
struct page *page;
struct address_space *mapping;

- pte = huge_pte_alloc(mm, address);
- if (!pte)
- goto out;
-
mapping = vma->vm_file->f_mapping;
idx = ((address - vma->vm_start) >> HPAGE_SHIFT)
+ (vma->vm_pgoff >> (HPAGE_SHIFT - PAGE_SHIFT));

- /*
- * Use page lock to guard against racing truncation
- * before we get page_table_lock.
- */
- page = find_lock_huge_page(mapping, idx);
+ /* This returns a locked page, which keeps us safe in the
+ * event of a race with truncate() */
+ page = find_or_alloc_huge_page(mapping, idx);
if (!page)
- goto out;
+ return VM_FAULT_SIGBUS;

spin_lock(&mm->page_table_lock);
+
+ ret = VM_FAULT_SIGBUS;
+
size = i_size_read(mapping->host) >> HPAGE_SHIFT;
if (idx >= size)
goto backout;

ret = VM_FAULT_MINOR;
+
if (!pte_none(*pte))
+ /* oops, someone instantiated this PTE before us */
goto backout;

add_mm_counter(mm, file_rss, HPAGE_SIZE / PAGE_SIZE);
set_huge_pte_at(mm, address, pte, make_huge_pte(vma, page));
+
spin_unlock(&mm->page_table_lock);
unlock_page(page);
-out:
+
return ret;

backout:
@@ -421,7 +418,29 @@
hugetlb_put_quota(mapping);
unlock_page(page);
put_page(page);
- goto out;
+
+ return ret;
+}
+
+int hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
+ unsigned long address, int write_access)
+{
+ pte_t *ptep;
+ pte_t entry;
+
+ ptep = huge_pte_alloc(mm, address);
+ if (! ptep)
+ goto out;
+
+ entry = *ptep;
+
+ if (pte_none(entry))
+ return hugetlb_no_page(mm, vma, address, ptep);
+
+ /* we could get here if another thread instantiated the pte
+ * before the test above */
+
+ return VM_FAULT_SIGBUS;
}

int follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,

--
David Gibson | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_
| _way_ _around_!
http://www.ozlabs.org/people/dgibson
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