Re: [PATCH v3 13/16] zsmalloc: migrate head page of zspage

Minchan Kim
2016-04-19T06:14:38Z

Hello Chulmin,

On Tue, Apr 19, 2016 at 03:08:48PM +0900, Chulmin Kim wrote:
> On 2016년 03월 30일 16:12, Minchan Kim wrote:
> >This patch introduces run-time migration feature for zspage.
> >To begin with, it supports only head page migration for
> >easy review(later patches will support tail page migration).
> >
> >For migration, it supports three functions
> >
> >* zs_page_isolate
> >
> >It isolates a zspage which includes a subpage VM want to migrate
> >from class so anyone cannot allocate new object from the zspage.
> >IOW, allocation freeze
> >
> >* zs_page_migrate
> >
> >First of all, it freezes zspage to prevent zspage destrunction
> >so anyone cannot free object. Then, It copies content from oldpage
> >to newpage and create new page-chain with new page.
> >If it was successful, drop the refcount of old page to free
> >and putback new zspage to right data structure of zsmalloc.
> >Lastly, unfreeze zspages so we allows object allocation/free
> >from now on.
> >
> >* zs_page_putback
> >
> >It returns isolated zspage to right fullness_group list
> >if it fails to migrate a page.
> >
> >NOTE: A hurdle to support migration is that destroying zspage
> >while migration is going on. Once a zspage is isolated,
> >anyone cannot allocate object from the zspage but can deallocate
> >object freely so a zspage could be destroyed until all of objects
> >in zspage are freezed to prevent deallocation. The problem is
> >large window betwwen zs_page_isolate and freeze_zspage
> >in zs_page_migrate so the zspage could be destroyed.
> >
> >A easy approach to solve the problem is that object freezing
> >in zs_page_isolate but it has a drawback that any object cannot
> >be deallocated until migration fails after isolation. However,
> >There is large time gab between isolation and migration so
> >any object freeing in other CPU should spin by pin_tag which
> >would cause big latency. So, this patch introduces lock_zspage
> >which holds PG_lock of all pages in a zspage right before
> >freeing the zspage. VM migration locks the page, too right
> >before calling ->migratepage so such race doesn't exist any more.
> >
> >Signed-off-by: Minchan Kim <minchan@kernel.org>
> >---
> > include/uapi/linux/magic.h | 1 +
> > mm/zsmalloc.c | 332 +++++++++++++++++++++++++++++++++++++++++++--
> > 2 files changed, 318 insertions(+), 15 deletions(-)
> >
> >diff --git a/include/uapi/linux/magic.h b/include/uapi/linux/magic.h
> >index e1fbe72c39c0..93b1affe4801 100644
> >--- a/include/uapi/linux/magic.h
> >+++ b/include/uapi/linux/magic.h
> >@@ -79,5 +79,6 @@
> > #define NSFS_MAGIC 0x6e736673
> > #define BPF_FS_MAGIC 0xcafe4a11
> > #define BALLOON_KVM_MAGIC 0x13661366
> >+#define ZSMALLOC_MAGIC 0x58295829
> >
> > #endif /* __LINUX_MAGIC_H__ */
> >diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
> >index ac8ca7b10720..f6c9138c3be0 100644
> >--- a/mm/zsmalloc.c
> >+++ b/mm/zsmalloc.c
> >@@ -56,6 +56,8 @@
> > #include <linux/debugfs.h>
> > #include <linux/zsmalloc.h>
> > #include <linux/zpool.h>
> >+#include <linux/mount.h>
> >+#include <linux/migrate.h>
> >
> > /*
> > * This must be power of 2 and greater than of equal to sizeof(link_free).
> >@@ -182,6 +184,8 @@ struct zs_size_stat {
> > static struct dentry *zs_stat_root;
> > #endif
> >
> >+static struct vfsmount *zsmalloc_mnt;
> >+
> > /*
> > * number of size_classes
> > */
> >@@ -263,6 +267,7 @@ struct zs_pool {
> > #ifdef CONFIG_ZSMALLOC_STAT
> > struct dentry *stat_dentry;
> > #endif
> >+ struct inode *inode;
> > };
> >
> > struct zs_meta {
> >@@ -412,6 +417,29 @@ static int is_last_page(struct page *page)
> > return PagePrivate2(page);
> > }
> >
> >+/*
> >+ * Indicate that whether zspage is isolated for page migration.
> >+ * Protected by size_class lock
> >+ */
> >+static void SetZsPageIsolate(struct page *first_page)
> >+{
> >+ VM_BUG_ON_PAGE(!is_first_page(first_page), first_page);
> >+ SetPageUptodate(first_page);
> >+}
> >+
> >+static int ZsPageIsolate(struct page *first_page)
> >+{
> >+ VM_BUG_ON_PAGE(!is_first_page(first_page), first_page);
> >+
> >+ return PageUptodate(first_page);
> >+}
> >+
> >+static void ClearZsPageIsolate(struct page *first_page)
> >+{
> >+ VM_BUG_ON_PAGE(!is_first_page(first_page), first_page);
> >+ ClearPageUptodate(first_page);
> >+}
> >+
> > static int get_zspage_inuse(struct page *first_page)
> > {
> > struct zs_meta *m;
> >@@ -783,8 +811,11 @@ static enum fullness_group fix_fullness_group(struct size_class *class,
> > if (newfg == currfg)
> > goto out;
> >
> >- remove_zspage(class, currfg, first_page);
> >- insert_zspage(class, newfg, first_page);
> >+ /* Later, putback will insert page to right list */
> >+ if (!ZsPageIsolate(first_page)) {
> >+ remove_zspage(class, currfg, first_page);
> >+ insert_zspage(class, newfg, first_page);
> >+ }
> > set_zspage_mapping(first_page, class_idx, newfg);
> >
> > out:
> >@@ -950,12 +981,31 @@ static void unpin_tag(unsigned long handle)
> >
> > static void reset_page(struct page *page)
> > {
> >+ if (!PageIsolated(page))
> >+ __ClearPageMovable(page);
> >+ ClearPageIsolated(page);
> > clear_bit(PG_private, &page->flags);
> > clear_bit(PG_private_2, &page->flags);
> > set_page_private(page, 0);
> > page->freelist = NULL;
> > }
> >
> >+/**
> >+ * lock_zspage - lock all pages in the zspage
> >+ * @first_page: head page of the zspage
> >+ *
> >+ * To prevent destroy during migration, zspage freeing should
> >+ * hold locks of all pages in a zspage
> >+ */
> >+void lock_zspage(struct page *first_page)
> >+{
> >+ struct page *cursor = first_page;
> >+
> >+ do {
> >+ while (!trylock_page(cursor));
> >+ } while ((cursor = get_next_page(cursor)) != NULL);
> >+}
> >+
> > static void free_zspage(struct zs_pool *pool, struct page *first_page)
> > {
> > struct page *nextp, *tmp, *head_extra;
> >@@ -963,26 +1013,31 @@ static void free_zspage(struct zs_pool *pool, struct page *first_page)
> > VM_BUG_ON_PAGE(!is_first_page(first_page), first_page);
> > VM_BUG_ON_PAGE(get_zspage_inuse(first_page), first_page);
> >
> >+ lock_zspage(first_page);
> > head_extra = (struct page *)page_private(first_page);
> >
> >- reset_page(first_page);
> >- __free_page(first_page);
> >-
> > /* zspage with only 1 system page */
> > if (!head_extra)
> >- return;
> >+ goto out;
> >
> > list_for_each_entry_safe(nextp, tmp, &head_extra->lru, lru) {
> > list_del(&nextp->lru);
> > reset_page(nextp);
> >- __free_page(nextp);
> >+ unlock_page(nextp);
> >+ put_page(nextp);
> > }
> > reset_page(head_extra);
> >- __free_page(head_extra);
> >+ unlock_page(head_extra);
> >+ put_page(head_extra);
> >+out:
> >+ reset_page(first_page);
> >+ unlock_page(first_page);
> >+ put_page(first_page);
> > }
> >
> > /* Initialize a newly allocated zspage */
> >-static void init_zspage(struct size_class *class, struct page *first_page)
> >+static void init_zspage(struct size_class *class, struct page *first_page,
> >+ struct address_space *mapping)
> > {
> > int freeobj = 1;
> > unsigned long off = 0;
> >@@ -991,6 +1046,9 @@ static void init_zspage(struct size_class *class, struct page *first_page)
> > first_page->freelist = NULL;
> > INIT_LIST_HEAD(&first_page->lru);
> > set_zspage_inuse(first_page, 0);
> >+ BUG_ON(!trylock_page(first_page));
> >+ __SetPageMovable(first_page, mapping);
> >+ unlock_page(first_page);
> >
> > while (page) {
> > struct page *next_page;
> >@@ -1065,10 +1123,45 @@ static void create_page_chain(struct page *pages[], int nr_pages)
> > }
> > }
> >
> >+static void replace_sub_page(struct size_class *class, struct page *first_page,
> >+ struct page *newpage, struct page *oldpage)
> >+{
> >+ struct page *page;
> >+ struct page *pages[ZS_MAX_PAGES_PER_ZSPAGE] = {NULL,};
> >+ int idx = 0;
> >+
> >+ page = first_page;
> >+ do {
> >+ if (page == oldpage)
> >+ pages[idx] = newpage;
> >+ else
> >+ pages[idx] = page;
> >+ idx++;
> >+ } while ((page = get_next_page(page)) != NULL);
> >+
> >+ create_page_chain(pages, class->pages_per_zspage);
> >+
> >+ if (is_first_page(oldpage)) {
> >+ enum fullness_group fg;
> >+ int class_idx;
> >+
> >+ SetZsPageIsolate(newpage);
> >+ get_zspage_mapping(oldpage, &class_idx, &fg);
> >+ set_zspage_mapping(newpage, class_idx, fg);
> >+ set_freeobj(newpage, get_freeobj(oldpage));
> >+ set_zspage_inuse(newpage, get_zspage_inuse(oldpage));
> >+ if (class->huge)
> >+ set_page_private(newpage, page_private(oldpage));
> >+ }
> >+
> >+ __SetPageMovable(newpage, oldpage->mapping);
> >+}
> >+
> > /*
> > * Allocate a zspage for the given size class
> > */
> >-static struct page *alloc_zspage(struct size_class *class, gfp_t flags)
> >+static struct page *alloc_zspage(struct zs_pool *pool,
> >+ struct size_class *class)
> > {
> > int i;
> > struct page *first_page = NULL;
> >@@ -1088,7 +1181,7 @@ static struct page *alloc_zspage(struct size_class *class, gfp_t flags)
> > for (i = 0; i < class->pages_per_zspage; i++) {
> > struct page *page;
> >
> >- page = alloc_page(flags);
> >+ page = alloc_page(pool->flags);
> > if (!page) {
> > while (--i >= 0)
> > __free_page(pages[i]);
> >@@ -1100,7 +1193,7 @@ static struct page *alloc_zspage(struct size_class *class, gfp_t flags)
> >
> > create_page_chain(pages, class->pages_per_zspage);
> > first_page = pages[0];
> >- init_zspage(class, first_page);
> >+ init_zspage(class, first_page, pool->inode->i_mapping);
> >
> > return first_page;
> > }
> >@@ -1499,7 +1592,7 @@ unsigned long zs_malloc(struct zs_pool *pool, size_t size)
> >
> > if (!first_page) {
> > spin_unlock(&class->lock);
> >- first_page = alloc_zspage(class, pool->flags);
> >+ first_page = alloc_zspage(pool, class);
> > if (unlikely(!first_page)) {
> > free_handle(pool, handle);
> > return 0;
> >@@ -1559,6 +1652,7 @@ void zs_free(struct zs_pool *pool, unsigned long handle)
> > if (unlikely(!handle))
> > return;
> >
> >+ /* Once handle is pinned, page|object migration cannot work */
> > pin_tag(handle);
> > obj = handle_to_obj(handle);
> > obj_to_location(obj, &f_page, &f_objidx);
> >@@ -1714,6 +1808,9 @@ static enum fullness_group putback_zspage(struct size_class *class,
> > {
> > enum fullness_group fullness;
> >
> >+ VM_BUG_ON_PAGE(!list_empty(&first_page->lru), first_page);
> >+ VM_BUG_ON_PAGE(ZsPageIsolate(first_page), first_page);
> >+
> > fullness = get_fullness_group(class, first_page);
> > insert_zspage(class, fullness, first_page);
> > set_zspage_mapping(first_page, class->index, fullness);
> >@@ -2059,6 +2156,173 @@ static int zs_register_shrinker(struct zs_pool *pool)
> > return register_shrinker(&pool->shrinker);
> > }
> >
> >+bool zs_page_isolate(struct page *page, isolate_mode_t mode)
> >+{
> >+ struct zs_pool *pool;
> >+ struct size_class *class;
> >+ int class_idx;
> >+ enum fullness_group fullness;
> >+ struct page *first_page;
> >+
> >+ /*
> >+ * The page is locked so it couldn't be destroyed.
> >+ * For detail, look at lock_zspage in free_zspage.
> >+ */
> >+ VM_BUG_ON_PAGE(!PageLocked(page), page);
> >+ VM_BUG_ON_PAGE(PageIsolated(page), page);
> >+ /*
> >+ * In this implementation, it allows only first page migration.
> >+ */
> >+ VM_BUG_ON_PAGE(!is_first_page(page), page);
> >+ first_page = page;
> >+
> >+ /*
> >+ * Without class lock, fullness is meaningless while constant
> >+ * class_idx is okay. We will get it under class lock at below,
> >+ * again.
> >+ */
> >+ get_zspage_mapping(first_page, &class_idx, &fullness);
> >+ pool = page->mapping->private_data;
> >+ class = pool->size_class[class_idx];
> >+
> >+ if (!spin_trylock(&class->lock))
> >+ return false;
> >+
> >+ get_zspage_mapping(first_page, &class_idx, &fullness);
> >+ remove_zspage(class, fullness, first_page);
> >+ SetZsPageIsolate(first_page);
> >+ SetPageIsolated(page);
> >+ spin_unlock(&class->lock);
> >+
> >+ return true;
> >+}
>
> Hello, Minchan.
>
> We found another race condition.
>
> When there is alloc_zspage(), which is not protected by any lock, in-flight,
> a migrate context can isolate the zs subpage which is being
> initiated by alloc_zspage().
>
> We detected VM_BUG_ON during remove_zspage() above in consequence of
> "page->index" being set to NULL wrongly. (seems uninitialized yet)
>
> Though it is a real problem,
> as this race issue is somewhat similar with the one we detected last time,
> this seems to be fixed in the next version hopefully.
>
> I report this just for note.

I found problem you reported and already fixed it in my WIP version.
With your report, I am convinced my analysis was right, too. :)

Thanks for the analysis and reporting.
I really apprecaite your help, Chulmin!