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SubjectRe: [PATCH] rmap: add exclusively owned pages to the newest anon_vma


On Wed, 14 Apr 2010, Rik van Riel wrote:
> - /*
> - * We must use the _oldest_ possible anon_vma for the page mapping!
> - *
> - * So take the last AVC chain entry in the vma, which is the deepest
> - * ancestor, and use the anon_vma from that.
> - */
> - avc = list_entry(vma->anon_vma_chain.prev, struct anon_vma_chain, same_vma);
> - anon_vma = avc->anon_vma;
> + if (exclusive)
> + anon_vma = vma->anon_vma;
> + else {
> + /*
> + * The page may be shared between multiple processes.
> + * We must use the _oldest_ possible anon_vma for the
> + * page mapping! That anon_vma is guaranteed to be
> + * present in all processes that could share this page.
> + *
> + * So take the last AVC chain entry in the vma, which is the
> + * deepest ancestor, and use the anon_vma from that.
> + */
> + avc = list_entry(vma->anon_vma_chain.prev, struct anon_vma_chain, same_vma);
> + anon_vma = avc->anon_vma;
> + }

I really dislike your coding style.

If we do this conditionally, we're _much_ better off declaring the
variables we only use inside that conditional block inside the block
itself. And since we access "vma->anon_vma" in either case, just move that
case outside the conditional statement, and avoid a pointless
if/then/else.

IOW, something like this. Totally untested.

Linus

---
mm/rmap.c | 26 +++++++++++++++-----------
1 files changed, 15 insertions(+), 11 deletions(-)
diff --git a/mm/rmap.c b/mm/rmap.c
index 4bad326..78d4730 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -732,21 +732,25 @@ void page_move_anon_rmap(struct page *page,
* @address: the user virtual address mapped
*/
static void __page_set_anon_rmap(struct page *page,
- struct vm_area_struct *vma, unsigned long address)
+ struct vm_area_struct *vma, unsigned long address, int exclusive)
{
- struct anon_vma_chain *avc;
- struct anon_vma *anon_vma;
+ struct anon_vma *anon_vma = vma->anon_vma;

- BUG_ON(!vma->anon_vma);
+ BUG_ON(!anon_vma);

/*
- * We must use the _oldest_ possible anon_vma for the page mapping!
+ * If the page isn't exclusively mapped into this vma,
+ * we must use the _oldest_ possible anon_vma for the
+ * page mapping!
*
- * So take the last AVC chain entry in the vma, which is the deepest
- * ancestor, and use the anon_vma from that.
+ * So take the last AVC chain entry in the vma, which is
+ * the deepest ancestor, and use the anon_vma from that.
*/
- avc = list_entry(vma->anon_vma_chain.prev, struct anon_vma_chain, same_vma);
- anon_vma = avc->anon_vma;
+ if (!exclusive) {
+ struct anon_vma_chain *avc;
+ avc = list_entry(vma->anon_vma_chain.prev, struct anon_vma_chain, same_vma);
+ anon_vma = avc->anon_vma;
+ }

anon_vma = (void *) anon_vma + PAGE_MAPPING_ANON;
page->mapping = (struct address_space *) anon_vma;
@@ -802,7 +806,7 @@ void page_add_anon_rmap(struct page *page,
VM_BUG_ON(!PageLocked(page));
VM_BUG_ON(address < vma->vm_start || address >= vma->vm_end);
if (first)
- __page_set_anon_rmap(page, vma, address);
+ __page_set_anon_rmap(page, vma, address, 0);
else
__page_check_anon_rmap(page, vma, address);
}
@@ -824,7 +828,7 @@ void page_add_new_anon_rmap(struct page *page,
SetPageSwapBacked(page);
atomic_set(&page->_mapcount, 0); /* increment count (starts at -1) */
__inc_zone_page_state(page, NR_ANON_PAGES);
- __page_set_anon_rmap(page, vma, address);
+ __page_set_anon_rmap(page, vma, address, 1);
if (page_evictable(page, vma))
lru_cache_add_lru(page, LRU_ACTIVE_ANON);
else

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