[patch] Re: Interaction between Xen and XFS: stray RW mappings

From: David Chinner
Date: Tue Oct 23 2007 - 03:05:12 EST

On Tue, Oct 23, 2007 at 10:36:41AM +1000, David Chinner wrote:
> On Tue, Oct 23, 2007 at 01:35:14AM +0200, Andi Kleen wrote:
> > On Tue, Oct 23, 2007 at 08:32:25AM +1000, David Chinner wrote:
> > > Could vmap()/vunmap() take references to the pages that are mapped? That
> > > way delaying the unmap would also delay the freeing of the pages and hence
> > > we'd have no problems with the pages being reused before the mapping is
> > > torn down. That'd work for Xen even with XFS's lazy unmapping scheme, and
> > > would allow Nick's more advanced methods to work as well....
> >
> > You could always just keep around an array of the pages and then drop the
> > reference count after unmap. Or walk the vmalloc mapping and generate such
> > an array before freeing, then unmap and then drop the reference counts.
>
> You mean like vmap() could record the pages passed to it in the area->pages
> array, and we walk and release than in __vunmap() like it already does
> for vfree()?
>
> If we did this, it would probably be best to pass a page release function
> into the vmap or vunmap call - we'd need page_cache_release() called on
> the page rather than __free_page()....
>
> The solution belongs behind the vmap/vunmap interface, not in XFS....

Lightly tested(*) patch that does this with lazy unmapping
below for comment.

(*) a) kernel boots, b) made an XFS filesystem with 64k directory
blocks, created ~100,000 files in a directory to get a wide btree
(~1700 blocks, still only a single level) and run repeated finds
across it dropping caches in between. Each traversal maps and
unmaps every btree block.

Cheers,

Dave.
--
Dave Chinner
Principal Engineer
SGI Australian Software Group

---
fs/xfs/linux-2.6/xfs_buf.c | 21 +----
include/linux/vmalloc.h | 4 +
mm/vmalloc.c | 160 +++++++++++++++++++++++++++++++++++----------
3 files changed, 133 insertions(+), 52 deletions(-)

Index: 2.6.x-xfs-new/fs/xfs/linux-2.6/xfs_buf.c
===================================================================
--- 2.6.x-xfs-new.orig/fs/xfs/linux-2.6/xfs_buf.c 2007-10-18 17:11:06.000000000 +1000
+++ 2.6.x-xfs-new/fs/xfs/linux-2.6/xfs_buf.c 2007-10-23 14:48:32.131088616 +1000
@@ -187,19 +187,6 @@ free_address(
{
a_list_t *aentry;

-#ifdef CONFIG_XEN
- /*
- * Xen needs to be able to make sure it can get an exclusive
- * RO mapping of pages it wants to turn into a pagetable. If
- * a newly allocated page is also still being vmap()ed by xfs,
- * it will cause pagetable construction to fail. This is a
- * quick workaround to always eagerly unmap pages so that Xen
- * is happy.
- */
- vunmap(addr);
- return;
-#endif
-
aentry = kmalloc(sizeof(a_list_t), GFP_NOWAIT);
if (likely(aentry)) {
spin_lock(&as_lock);
@@ -209,7 +196,7 @@ free_address(
as_list_len++;
spin_unlock(&as_lock);
} else {
- vunmap(addr);
+ vunmap_pages(addr, put_page);
}
}

@@ -228,7 +215,7 @@ purge_addresses(void)
spin_unlock(&as_lock);

while ((old = aentry) != NULL) {
- vunmap(aentry->vm_addr);
+ vunmap_pages(aentry->vm_addr, put_page);
aentry = aentry->next;
kfree(old);
}
@@ -456,8 +443,8 @@ _xfs_buf_map_pages(
} else if (flags & XBF_MAPPED) {
if (as_list_len > 64)
purge_addresses();
- bp->b_addr = vmap(bp->b_pages, bp->b_page_count,
- VM_MAP, PAGE_KERNEL);
+ bp->b_addr = vmap_pages(bp->b_pages, bp->b_page_count,
+ VM_MAP, PAGE_KERNEL, xb_to_gfp(flags));
if (unlikely(bp->b_addr == NULL))
return -ENOMEM;
bp->b_addr += bp->b_offset;
Index: 2.6.x-xfs-new/include/linux/vmalloc.h
===================================================================
--- 2.6.x-xfs-new.orig/include/linux/vmalloc.h 2007-09-12 15:41:41.000000000 +1000
+++ 2.6.x-xfs-new/include/linux/vmalloc.h 2007-10-23 14:36:56.264860921 +1000
@@ -51,6 +51,10 @@ extern void *vmap(struct page **pages, u
unsigned long flags, pgprot_t prot);
extern void vunmap(void *addr);

+extern void *vmap_pages(struct page **pages, unsigned int count,
+ unsigned long flags, pgprot_t prot, gfp_t gfp_mask);
+extern void vunmap_pages(void *addr, void (*release)(struct page *page));
+
extern int remap_vmalloc_range(struct vm_area_struct *vma, void *addr,
unsigned long pgoff);
void vmalloc_sync_all(void);
Index: 2.6.x-xfs-new/mm/vmalloc.c
===================================================================
--- 2.6.x-xfs-new.orig/mm/vmalloc.c 2007-09-12 15:41:48.000000000 +1000
+++ 2.6.x-xfs-new/mm/vmalloc.c 2007-10-23 16:29:40.130384957 +1000
@@ -318,7 +318,56 @@ struct vm_struct *remove_vm_area(void *a
return v;
}

-static void __vunmap(void *addr, int deallocate_pages)
+static void vm_area_release_page(struct page *page)
+{
+ __free_page(page);
+}
+
+static int vm_area_alloc_pagearray(struct vm_struct *area, gfp_t gfp_mask,
+ unsigned int nr_pages, int node)
+{
+ struct page **pages;
+ unsigned int array_size;
+
+ array_size = (nr_pages * sizeof(struct page *));
+
+ area->nr_pages = nr_pages;
+ /* Please note that the recursion is strictly bounded. */
+ if (array_size > PAGE_SIZE) {
+ pages = __vmalloc_node(array_size, gfp_mask | __GFP_ZERO,
+ PAGE_KERNEL, node);
+ area->flags |= VM_VPAGES;
+ } else {
+ pages = kmalloc_node(array_size,
+ (gfp_mask & GFP_LEVEL_MASK) | __GFP_ZERO,
+ node);
+ }
+ area->pages = pages;
+ if (!area->pages) {
+ remove_vm_area(area->addr);
+ kfree(area);
+ return -ENOMEM;
+ }
+ return 0;
+}
+
+static void vm_area_release_pagearray(struct vm_struct *area,
+ void (*release)(struct page *))
+{
+ int i;
+
+ for (i = 0; i < area->nr_pages; i++) {
+ BUG_ON(!area->pages[i]);
+ release(area->pages[i]);
+ }
+
+ if (area->flags & VM_VPAGES)
+ vfree(area->pages);
+ else
+ kfree(area->pages);
+}
+
+static void __vunmap(void *addr, void (*release)(struct page *))
{
struct vm_struct *area;

@@ -341,19 +390,8 @@ static void __vunmap(void *addr, int dea

debug_check_no_locks_freed(addr, area->size);

- if (deallocate_pages) {
- int i;
-
- for (i = 0; i < area->nr_pages; i++) {
- BUG_ON(!area->pages[i]);
- __free_page(area->pages[i]);
- }
-
- if (area->flags & VM_VPAGES)
- vfree(area->pages);
- else
- kfree(area->pages);
- }
+ if (release)
+ vm_area_release_pagearray(area, release);

kfree(area);
return;
@@ -372,7 +410,7 @@ static void __vunmap(void *addr, int dea
void vfree(void *addr)
{
BUG_ON(in_interrupt());
- __vunmap(addr, 1);
+ __vunmap(addr, vm_area_release_page);
}
EXPORT_SYMBOL(vfree);

@@ -388,11 +426,30 @@ EXPORT_SYMBOL(vfree);
void vunmap(void *addr)
{
BUG_ON(in_interrupt());
- __vunmap(addr, 0);
+ __vunmap(addr, NULL);
}
EXPORT_SYMBOL(vunmap);

/**
+ * vunmap_pages - release virtual mapping obtained by vmap_pages()
+ * @addr: memory base address
+ * @release: release page callback function
+ *
+ * Free the virtually contiguous memory area starting at @addr,
+ * which was created from the page array passed to vmap_pages(),
+ * then call the release function on each page in the associated
+ * array of page attached to the area.
+ *
+ * Must not be called in interrupt context.
+ */
+void vunmap_pages(void *addr, void (*release)(struct page *))
+{
+ BUG_ON(in_interrupt());
+ __vunmap(addr, release);
+}
+EXPORT_SYMBOL(vunmap_pages);
+
+/**
* vmap - map an array of pages into virtually contiguous space
* @pages: array of page pointers
* @count: number of pages to map
@@ -422,32 +479,63 @@ void *vmap(struct page **pages, unsigned
}
EXPORT_SYMBOL(vmap);

+/**
+ * vmap_pages - map an array of pages into virtually contiguous space
+ * @pages: array of page pointers
+ * @count: number of pages to map
+ * @flags: vm_area->flags
+ * @prot: page protection for the mapping
+ * @gfp_mask: flags for the page level allocator
+ *
+ * Maps @count pages from @pages into contiguous kernel virtual
+ * space taking a reference to each page and keeping track of all
+ * the pages within the vm area structure.
+ */
+void *vmap_pages(struct page **pages, unsigned int count,
+ unsigned long flags, pgprot_t prot, gfp_t gfp_mask)
+{
+ struct vm_struct *area;
+ struct page **pgp;
+ int i;
+
+ if (count > num_physpages)
+ return NULL;
+
+ area = get_vm_area((count << PAGE_SHIFT), flags);
+ if (!area)
+ return NULL;
+ if (vm_area_alloc_pagearray(area, gfp_mask, count, -1))
+ return NULL;
+
+ /* map_vm_area modifies pgp */
+ pgp = pages;
+ if (map_vm_area(area, prot, &pgp)) {
+ vunmap(area->addr);
+ return NULL;
+ }
+ /*
+ * now that the region is mapped, take a reference to each
+ * page and store them in the area page array.
+ */
+ for (i = 0; i < area->nr_pages; i++) {
+ get_page(pages[i]);
+ area->pages[i] = pages[i];
+ }
+
+ return area->addr;
+}
+EXPORT_SYMBOL(vmap_pages);
+
void *__vmalloc_area_node(struct vm_struct *area, gfp_t gfp_mask,
pgprot_t prot, int node)
{
struct page **pages;
- unsigned int nr_pages, array_size, i;
+ unsigned int nr_pages;
+ int i;

nr_pages = (area->size - PAGE_SIZE) >> PAGE_SHIFT;
- array_size = (nr_pages * sizeof(struct page *));
-
- area->nr_pages = nr_pages;
- /* Please note that the recursion is strictly bounded. */
- if (array_size > PAGE_SIZE) {
- pages = __vmalloc_node(array_size, gfp_mask | __GFP_ZERO,
- PAGE_KERNEL, node);
- area->flags |= VM_VPAGES;
- } else {
- pages = kmalloc_node(array_size,
- (gfp_mask & GFP_LEVEL_MASK) | __GFP_ZERO,
- node);
- }
- area->pages = pages;
- if (!area->pages) {
- remove_vm_area(area->addr);
- kfree(area);
+ if (vm_area_alloc_pagearray(area, gfp_mask, nr_pages, node))
return NULL;
- }

for (i = 0; i < area->nr_pages; i++) {
if (node < 0)
@@ -461,6 +549,8 @@ void *__vmalloc_area_node(struct vm_stru
}
}

+ /* map_vm_area modifies pages */
+ pages = area->pages;
if (map_vm_area(area, prot, &pages))
goto fail;
return area->addr;
-
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