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Subject[PATCH] swap 07/13 objrmap page_table_lock
A recent update removed the page_table_lock from objrmap.  That was
simply wrong: it's necessary to guard mm->rss (in try_to_unmap), and
it's necessary to ensure the page tables don't get freed beneath us
(in which case any modification of the pte, by page_referenced or
by try_to_unmap, might be corrupting a freed and reused page).

Then restore the original SWAP_AGAIN semantics in try_to_unmap:
simplest for inner levels to return SWAP_AGAIN or SWAP_FAILED,
outer level to decide SWAP_SUCCESS if all pages were unmapped.
Stop searching when all have been unmapped.

page_convert_anon left for the moment with FIXMEs rather than
page_table_lock. I believe it can be done without page_table_lock
(which would be problematic there), but have noticed several other
bugs there, more patches will follow at a later date (though I'd
much rather anobjrmap, which handles all this so much more cleanly).

--- swap06/mm/rmap.c Tue Mar 25 20:43:51 2003
+++ swap07/mm/rmap.c Tue Mar 25 20:44:02 2003
@@ -143,9 +143,13 @@
static int
page_referenced_obj_one(struct vm_area_struct *vma, struct page *page)
{
+ struct mm_struct *mm = vma->vm_mm;
pte_t *pte;
int referenced = 0;

+ if (!spin_trylock(&mm->page_table_lock))
+ return 1;
+
pte = find_pte(vma, page, NULL);
if (pte) {
if (ptep_test_and_clear_young(pte))
@@ -153,6 +157,7 @@
pte_unmap(pte);
}

+ spin_unlock(&mm->page_table_lock);
return referenced;
}

@@ -489,7 +494,10 @@
unsigned long address;
pte_t *pte;
pte_t pteval;
- int ret = SWAP_SUCCESS;
+ int ret = SWAP_AGAIN;
+
+ if (!spin_trylock(&mm->page_table_lock))
+ return ret;

pte = find_pte(vma, page, &address);
if (!pte)
@@ -518,6 +526,7 @@
pte_unmap(pte);

out:
+ spin_unlock(&mm->page_table_lock);
return ret;
}

@@ -538,7 +547,7 @@
{
struct address_space *mapping = page->mapping;
struct vm_area_struct *vma;
- int ret = SWAP_SUCCESS;
+ int ret = SWAP_AGAIN;

if (!mapping)
BUG();
@@ -547,23 +556,20 @@
BUG();

if (down_trylock(&mapping->i_shared_sem))
- return SWAP_AGAIN;
+ return ret;

list_for_each_entry(vma, &mapping->i_mmap, shared) {
ret = try_to_unmap_obj_one(vma, page);
- if (ret != SWAP_SUCCESS)
+ if (ret == SWAP_FAIL || !page->pte.mapcount)
goto out;
}

list_for_each_entry(vma, &mapping->i_mmap_shared, shared) {
ret = try_to_unmap_obj_one(vma, page);
- if (ret != SWAP_SUCCESS)
+ if (ret == SWAP_FAIL || !page->pte.mapcount)
goto out;
}

- if (page->pte.mapcount)
- BUG();
-
out:
up(&mapping->i_shared_sem);
return ret;
@@ -753,8 +759,10 @@
}
}
out:
- if (!page_mapped(page))
+ if (!page_mapped(page)) {
dec_page_state(nr_mapped);
+ ret = SWAP_SUCCESS;
+ }
return ret;
}

@@ -839,6 +847,7 @@

index = NRPTE-1;
list_for_each_entry(vma, &mapping->i_mmap, shared) {
+ /* FIXME: unsafe without page_table_lock */
pte = find_pte(vma, page, NULL);
if (pte) {
pte_paddr = ptep_to_paddr(pte);
@@ -855,6 +864,7 @@
}
}
list_for_each_entry(vma, &mapping->i_mmap_shared, shared) {
+ /* FIXME: unsafe without page_table_lock */
pte = find_pte(vma, page, NULL);
if (pte) {
pte_paddr = ptep_to_paddr(pte);
-
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