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SubjectRe: [PATCH 2.5.66-mm3] New page_convert_anon

--On Thursday, April 03, 2003 14:24:41 -0800 Andrew Morton <akpm@digeo.com>
wrote:

> It's more conventional to lock the page in the caller. And we forgot the
> whole reason for locking it: to keep truncate away. We need to check that
> the page is still on the address_space after the page lock has been
> acquired.
>
> This applies on top of your first.

Ok, that all makes sense. Here's a patch with all your changes applied.

Dave

======================================================================
Dave McCracken IBM Linux Base Kernel Team 1-512-838-3059
dmccr@us.ibm.com T/L 678-3059--- 2.5.66-mm3/./mm/fremap.c 2003-04-03 10:37:42.000000000 -0600
+++ 2.5.66-mm3-objfix/./mm/fremap.c 2003-04-03 16:35:27.000000000 -0600
@@ -73,7 +73,10 @@ int install_page(struct mm_struct *mm, s
pgidx += vma->vm_pgoff;
pgidx >>= PAGE_CACHE_SHIFT - PAGE_SHIFT;
if (!PageAnon(page) && (page->index != pgidx)) {
- if (page_convert_anon(page) < 0)
+ lock_page(page);
+ err = page_convert_anon(page);
+ unlock_page(page);
+ if (err < 0)
goto err_free;
}

--- 2.5.66-mm3/./mm/rmap.c 2003-04-03 10:37:42.000000000 -0600
+++ 2.5.66-mm3-objfix/./mm/rmap.c 2003-04-03 16:36:17.000000000 -0600
@@ -460,23 +460,6 @@ void page_remove_rmap(struct page * page
}
}
}
-#ifdef DEBUG_RMAP
- /* Not found. This should NEVER happen! */
- printk(KERN_ERR "page_remove_rmap: pte_chain %p not present.\n", ptep);
- printk(KERN_ERR "page_remove_rmap: only found: ");
- if (PageDirect(page)) {
- printk("%llx", (u64)page->pte.direct);
- } else {
- for (pc = page->pte.chain; pc; pc = pc->next) {
- int i;
- for (i = 0; i < NRPTE; i++)
- printk(" %d:%llx", i, (u64)pc->ptes[i]);
- }
- }
- printk("\n");
- printk(KERN_ERR "page_remove_rmap: driver cleared PG_reserved ?\n");
-#endif
-
out:
pte_chain_unlock(page);
if (!page_mapped(page))
@@ -781,152 +764,100 @@ out:
* Find all the mappings for an object-based page and convert them
* to 'anonymous', ie create a pte_chain and store all the pte pointers there.
*
- * This function takes the address_space->i_shared_sem and the pte_chain_lock
- * for the page. It jumps through some hoops to preallocate the correct number
- * of pte_chain structures to ensure that it can complete without releasing
- * the lock.
+ * This function takes the address_space->i_shared_sem, sets the PageAnon flag,
+ * then sets the mm->page_table_lock for each vma and calls page_add_rmap. This
+ * means there is a period when PageAnon is set, but still has some mappings
+ * with no pte_chain entry. This is in fact safe, since page_remove_rmap will
+ * simply not find it. try_to_unmap might erroneously return success, but it
+ * will never be called because the page_convert_anon() caller has locked the
+ * page.
+ *
+ * page_referenced() may fail to scan all the appropriate pte's and may return
+ * an inaccurate result. This is so rare that it does not matter.
*/
int page_convert_anon(struct page *page)
{
- struct address_space *mapping = page->mapping;
+ struct address_space *mapping;
struct vm_area_struct *vma;
- struct pte_chain *pte_chain = NULL, *ptec;
+ struct pte_chain *pte_chain = NULL;
pte_t *pte;
- pte_addr_t pte_paddr = 0;
- int mapcount;
- int ptecount;
- int index = 1;
int err = 0;

+ mapping = page->mapping;
+ if (mapping == NULL)
+ goto out; /* truncate won the lock_page() race */
+
down(&mapping->i_shared_sem);
- pte_chain_lock(page);

+ /* Take this only during setup */
+ pte_chain_lock(page);
/*
* Has someone else done it for us before we got the lock?
* If so, pte.direct or pte.chain has replaced pte.mapcount.
*/
- if (PageAnon(page))
+ if (PageAnon(page)) {
+ pte_chain_unlock(page);
goto out_unlock;
+ }

- /*
- * Preallocate the pte_chains outside the lock.
- * If mapcount grows while we're allocating here, retry.
- * If mapcount shrinks, we free the excess before returning.
- */
- mapcount = page->pte.mapcount;
- while (index < mapcount) {
+ SetPageAnon(page);
+ if (page->pte.mapcount == 0) {
pte_chain_unlock(page);
- up(&mapping->i_shared_sem);
- for (; index < mapcount; index += NRPTE) {
- if (index == 1)
- index = 0;
- ptec = pte_chain_alloc(GFP_KERNEL);
- if (!ptec) {
- err = -ENOMEM;
- goto out_free;
- }
- ptec->next = pte_chain;
- pte_chain = ptec;
- }
- down(&mapping->i_shared_sem);
- pte_chain_lock(page);
- /*
- * Has someone else done it while we were allocating?
- */
- if (PageAnon(page))
- goto out_unlock;
- mapcount = page->pte.mapcount;
+ goto out_unlock;
}
- if (!mapcount)
- goto set_anon;
+ /* This is gonna get incremented by page_add_rmap */
+ dec_page_state(nr_mapped);
+ page->pte.mapcount = 0;

-again:
/*
- * We don't try for page_table_lock (what would we do when a
- * trylock fails?), therefore there's a small chance that we
- * catch a vma just as it's being unmapped and its page tables
- * freed. Our pte_chain_lock prevents that on vmas that really
- * contain our page, but not on the others we look at. So we
- * might locate junk that looks just like our page's pfn. It's
- * a transient and very unlikely occurrence (much less likely
- * than a trylock failing), so check how many maps we find,
- * and if too many, start all over again.
+ * Now that the page is marked as anon, unlock it.
+ * page_add_rmap will lock it as necessary.
*/
- ptecount = 0;
- ptec = pte_chain;
-
- /*
- * Arrange for the first pte_chain to be partially filled at
- * the top, and the last (and any intermediates) to be full.
- */
- index = mapcount % NRPTE;
- if (index)
- index = NRPTE - index;
+ pte_chain_unlock(page);

list_for_each_entry(vma, &mapping->i_mmap, shared) {
if (vma->vm_pgoff > (page->index << (PAGE_CACHE_SHIFT - PAGE_SHIFT)))
break;
+ if (!pte_chain) {
+ pte_chain = pte_chain_alloc(GFP_KERNEL);
+ if (!pte_chain) {
+ err = -ENOMEM;
+ goto out_unlock;
+ }
+ }
+ spin_lock(&vma->vm_mm->page_table_lock);
pte = find_pte(vma, page, NULL);
if (pte) {
- ptecount++;
- if (unlikely(ptecount > mapcount))
- goto again;
- pte_paddr = ptep_to_paddr(pte);
- pte_unmap(pte);
- if (ptec) {
- ptec->ptes[index] = pte_paddr;
- index++;
- if (index == NRPTE) {
- ptec = ptec->next;
- index = 0;
- }
- }
+ /* Make sure this isn't a duplicate */
+ page_remove_rmap(page, pte);
+ pte_chain = page_add_rmap(page, pte, pte_chain);
}
+ spin_unlock(&vma->vm_mm->page_table_lock);
}
list_for_each_entry(vma, &mapping->i_mmap_shared, shared) {
if (vma->vm_pgoff > (page->index << (PAGE_CACHE_SHIFT - PAGE_SHIFT)))
break;
+ if (!pte_chain) {
+ pte_chain = pte_chain_alloc(GFP_KERNEL);
+ if (!pte_chain) {
+ err = -ENOMEM;
+ goto out_unlock;
+ }
+ }
+ spin_lock(&vma->vm_mm->page_table_lock);
pte = find_pte(vma, page, NULL);
if (pte) {
- ptecount++;
- if (unlikely(ptecount > mapcount))
- goto again;
- pte_paddr = ptep_to_paddr(pte);
- pte_unmap(pte);
- if (ptec) {
- ptec->ptes[index] = pte_paddr;
- index++;
- if (index == NRPTE) {
- ptec = ptec->next;
- index = 0;
- }
- }
+ /* Make sure this isn't a duplicate */
+ page_remove_rmap(page, pte);
+ pte_chain = page_add_rmap(page, pte, pte_chain);
}
+ spin_unlock(&vma->vm_mm->page_table_lock);
}

- BUG_ON(ptecount != mapcount);
- if (mapcount == 1) {
- SetPageDirect(page);
- page->pte.direct = pte_paddr;
- /* If pte_chain is set, it's all excess to be freed */
- } else {
- page->pte.chain = pte_chain;
- /* Point pte_chain to any excess to be freed */
- pte_chain = ptec;
- BUG_ON(index);
- }
-
-set_anon:
- SetPageAnon(page);
out_unlock:
- pte_chain_unlock(page);
+ pte_chain_free(pte_chain);
up(&mapping->i_shared_sem);
-out_free:
- while (pte_chain) {
- ptec = pte_chain->next;
- pte_chain_free(pte_chain);
- pte_chain = ptec;
- }
+out:
return err;
}
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