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Subject[patch - repost] do_no_pfn handler
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Hi,

This is a repost of the lobotomized do_no_pfn patch from last week.

It adds the BUG_ON for !is_cow_mapping() as well as addresses the
issue around negative pfn numbers as you pointed out.

No changes since the last posting as I haven't received any other
comments since.

Cheers,
Jes

Implement do_no_pfn() for handling mapping of memory without a struct
page backing it. This avoids creating fake page table entries for
regions which are not backed by real memory.

This version uses specific NOPFN_{SIGBUS,OOM} return values, rather
than expect all negative pfn values would be an error. It also bugs on
cow mappings as this would not work with the VM.
Signed-off-by: Jes Sorensen <jes@sgi.com>

---
include/linux/mm.h | 7 +++++
mm/memory.c | 62 ++++++++++++++++++++++++++++++++++++++++++++++++-----
2 files changed, 64 insertions(+), 5 deletions(-)
Index: linux-2.6/include/linux/mm.h
===================================================================
--- linux-2.6.orig/include/linux/mm.h
+++ linux-2.6/include/linux/mm.h
@@ -199,6 +199,7 @@
void (*open)(struct vm_area_struct * area);
void (*close)(struct vm_area_struct * area);
struct page * (*nopage)(struct vm_area_struct * area, unsigned long address, int *type);
+ long (*nopfn)(struct vm_area_struct * area, unsigned long address);
int (*populate)(struct vm_area_struct * area, unsigned long address, unsigned long len, pgprot_t prot, unsigned long pgoff, int nonblock);
#ifdef CONFIG_NUMA
int (*set_policy)(struct vm_area_struct *vma, struct mempolicy *new);
@@ -612,6 +613,12 @@
#define NOPAGE_OOM ((struct page *) (-1))

/*
+ * Error return values for the *_nopfn functions
+ */
+#define NOPFN_SIGBUS ((unsigned long) -1)
+#define NOPFN_OOM ((unsigned long) -2)
+
+/*
* Different kinds of faults, as returned by handle_mm_fault().
* Used to decide whether a process gets delivered SIGBUS or
* just gets major/minor fault counters bumped up.
Index: linux-2.6/mm/memory.c
===================================================================
--- linux-2.6.orig/mm/memory.c
+++ linux-2.6/mm/memory.c
@@ -2146,6 +2146,52 @@
}

/*
+ * do_no_pfn() tries to create a new page mapping for a page without
+ * a struct_page backing it
+ *
+ * As this is called only for pages that do not currently exist, we
+ * do not need to flush old virtual caches or the TLB.
+ *
+ * We enter with non-exclusive mmap_sem (to exclude vma changes,
+ * but allow concurrent faults), and pte mapped but not yet locked.
+ * We return with mmap_sem still held, but pte unmapped and unlocked.
+ *
+ * It is expected that the ->nopfn handler always returns the same pfn
+ * for a given virtual mapping.
+ */
+static int do_no_pfn(struct mm_struct *mm, struct vm_area_struct *vma,
+ unsigned long address, pte_t *page_table, pmd_t *pmd,
+ int write_access)
+{
+ spinlock_t *ptl;
+ pte_t entry;
+ unsigned long pfn;
+ int ret = VM_FAULT_MINOR;
+
+ pte_unmap(page_table);
+ BUG_ON(!(vma->vm_flags & VM_PFNMAP));
+ BUG_ON(is_cow_mapping(vma->vm_flags));
+
+ pfn = vma->vm_ops->nopfn(vma, address & PAGE_MASK);
+ if (pfn == NOPFN_OOM)
+ return VM_FAULT_OOM;
+ if (pfn == NOPFN_SIGBUS)
+ return VM_FAULT_SIGBUS;
+
+ page_table = pte_offset_map_lock(mm, pmd, address, &ptl);
+
+ /* Only go through if we didn't race with anybody else... */
+ if (pte_none(*page_table)) {
+ entry = pfn_pte(pfn, vma->vm_page_prot);
+ if (write_access)
+ entry = maybe_mkwrite(pte_mkdirty(entry), vma);
+ set_pte_at(mm, address, page_table, entry);
+ }
+ pte_unmap_unlock(page_table, ptl);
+ return ret;
+}
+
+/*
* Fault of a previously existing named mapping. Repopulate the pte
* from the encoded file_pte if possible. This enables swappable
* nonlinear vmas.
@@ -2207,11 +2253,17 @@
old_entry = entry = *pte;
if (!pte_present(entry)) {
if (pte_none(entry)) {
- if (!vma->vm_ops || !vma->vm_ops->nopage)
- return do_anonymous_page(mm, vma, address,
- pte, pmd, write_access);
- return do_no_page(mm, vma, address,
- pte, pmd, write_access);
+ if (vma->vm_ops) {
+ if (vma->vm_ops->nopage)
+ return do_no_page(mm, vma, address,
+ pte, pmd,
+ write_access);
+ if (vma->vm_ops->nopfn)
+ return do_no_pfn(mm, vma, address, pte,
+ pmd, write_access);
+ }
+ return do_anonymous_page(mm, vma, address,
+ pte, pmd, write_access);
}
if (pte_file(entry))
return do_file_page(mm, vma, address,
-
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