[PATCH 07/11] unpaged: COW on VM_UNPAGED

Hugh Dickins
2005-11-17T19:38:43Z

Remove the BUG_ON(vma->vm_flags & VM_UNPAGED) from do_wp_page, and let
it do Copy-On-Write without touching the VM_UNPAGED's page counts - but
this is incomplete, because the anonymous page it inserts will itself
need to be handled, here and in other functions - next patch.

We still don't copy the page if the pfn is invalid, because the
copy_user_highpage interface does not allow it. But that's not been
a problem in the past: can be added in later if the need arises.

Signed-off-by: Hugh Dickins <hugh@veritas.com>
---

mm/memory.c | 37 +++++++++++++++++++++++++------------
1 files changed, 25 insertions(+), 12 deletions(-)

--- unpaged06/mm/memory.c 2005-11-17 15:11:02.000000000 +0000
+++ unpaged07/mm/memory.c 2005-11-17 15:11:30.000000000 +0000
@@ -1277,22 +1277,28 @@ static int do_wp_page(struct mm_struct *
unsigned long address, pte_t *page_table, pmd_t *pmd,
spinlock_t *ptl, pte_t orig_pte)
{
- struct page *old_page, *new_page;
+ struct page *old_page, *src_page, *new_page;
unsigned long pfn = pte_pfn(orig_pte);
pte_t entry;
int ret = VM_FAULT_MINOR;

- BUG_ON(vma->vm_flags & VM_UNPAGED);
-
if (unlikely(!pfn_valid(pfn))) {
/*
* Page table corrupted: show pte and kill process.
+ * Or it's an attempt to COW an out-of-map VM_UNPAGED
+ * entry, which copy_user_highpage does not support.
*/
print_bad_pte(vma, orig_pte, address);
ret = VM_FAULT_OOM;
goto unlock;
}
old_page = pfn_to_page(pfn);
+ src_page = old_page;
+
+ if (unlikely(vma->vm_flags & VM_UNPAGED)) {
+ old_page = NULL;
+ goto gotten;
+ }

if (PageAnon(old_page) && !TestSetPageLocked(old_page)) {
int reuse = can_share_swap_page(old_page);
@@ -1313,11 +1319,12 @@ static int do_wp_page(struct mm_struct *
* Ok, we need to copy. Oh, well..
*/
page_cache_get(old_page);
+gotten:
pte_unmap_unlock(page_table, ptl);

if (unlikely(anon_vma_prepare(vma)))
goto oom;
- if (old_page == ZERO_PAGE(address)) {
+ if (src_page == ZERO_PAGE(address)) {
new_page = alloc_zeroed_user_highpage(vma, address);
if (!new_page)
goto oom;
@@ -1325,7 +1332,7 @@ static int do_wp_page(struct mm_struct *
new_page = alloc_page_vma(GFP_HIGHUSER, vma, address);
if (!new_page)
goto oom;
- copy_user_highpage(new_page, old_page, address);
+ copy_user_highpage(new_page, src_page, address);
}

/*
@@ -1333,11 +1340,14 @@ static int do_wp_page(struct mm_struct *
*/
page_table = pte_offset_map_lock(mm, pmd, address, &ptl);
if (likely(pte_same(*page_table, orig_pte))) {
- page_remove_rmap(old_page);
- if (!PageAnon(old_page)) {
+ if (old_page) {
+ page_remove_rmap(old_page);
+ if (!PageAnon(old_page)) {
+ dec_mm_counter(mm, file_rss);
+ inc_mm_counter(mm, anon_rss);
+ }
+ } else
inc_mm_counter(mm, anon_rss);
- dec_mm_counter(mm, file_rss);
- }
flush_cache_page(vma, address, pfn);
entry = mk_pte(new_page, vma->vm_page_prot);
entry = maybe_mkwrite(pte_mkdirty(entry), vma);
@@ -1351,13 +1361,16 @@ static int do_wp_page(struct mm_struct *
new_page = old_page;
ret |= VM_FAULT_WRITE;
}
- page_cache_release(new_page);
- page_cache_release(old_page);
+ if (new_page)
+ page_cache_release(new_page);
+ if (old_page)
+ page_cache_release(old_page);
unlock:
pte_unmap_unlock(page_table, ptl);
return ret;
oom:
- page_cache_release(old_page);
+ if (old_page)
+ page_cache_release(old_page);
return VM_FAULT_OOM;
}