On Wed, 30 May 2007, KAMEZAWA Hiroyuki wrote:
> This is my latest version.
> (not tested because caller of this function is being rewritten now..)
>
> I'll move this patch to the top of my series and prepare to post this patch as
> a single patch.
>
> ==
> page migration by kernel v2.
>
> Changelog V1 -> V2
> *removed atomic ops.
> *removes changes in anon_vma_free() and add check before calling it.
> *reflected feedback of review.
> *remove CONFIG_MIGRATION_BY_KERNEL
>
> In usual, migrate_pages(page,,) is called with holoding mm->sem by systemcall.
> (mm here is a mm_struct which maps the migration target page.)
> This semaphore helps avoiding some race conditions.
>
> But, if we want to migrate a page by some kernel codes, we have to avoid
> some races. This patch adds check code for following race condition.
>
> 1. A page which is not mapped can be target of migration. Then, we have
> to check page_mapped() before calling try_to_unmap().
>
> 2. We can't trust page->mapping if page_mapcount() can goes down to 0.
> But when we map newpage back to original ptes, we have to access
> anon_vma from a page, which page_mapcount() is 0.
> This patch adds a special refcnt to anon_vma, which is synced by
> anon_vma->lock and delays freeing anon_vma.
>
> Signed-Off-By: KAMEZAWA Hiroyuki <[email protected]>
I've taken a look at last. It looks like a good fix to a real problem,
but may I suggest a simpler version? The anon_vma isn't usually held
by a refcount, but by having a vma on its linked list: why not just
put a dummy vma into that linked list? No need to add a refcount.
The NUMA shmem_alloc_page already uses a dummy vma on its stack,
so you can reasonably declare a vm_area_struct on unmap_and_move's
stack. No need for a special anon_vma_release, anon_vma_unlink
should do fine. I've not reworked your whole patch, but show
what I think the mm/rmap.c part would be at the bottom.
A few comments on your version below...
>
>
> ---
> include/linux/migrate.h | 5 ++++-
> include/linux/rmap.h | 11 +++++++++++
> mm/migrate.c | 35 +++++++++++++++++++++++++++++------
> mm/rmap.c | 36 +++++++++++++++++++++++++++++++++++-
> 4 files changed, 79 insertions(+), 8 deletions(-)
>
> Index: linux-2.6.22-rc2-mm1/mm/migrate.c
> ===================================================================
> --- linux-2.6.22-rc2-mm1.orig/mm/migrate.c
> +++ linux-2.6.22-rc2-mm1/mm/migrate.c
> @@ -607,11 +607,12 @@ static int move_to_new_page(struct page
> * to the newly allocated page in newpage.
> */
> static int unmap_and_move(new_page_t get_new_page, unsigned long private,
> - struct page *page, int force)
> + struct page *page, int force, int nocontext)
> {
An "int context" would be a lot better than the negative "int nocontext";
even better would be "int holds_mmap_sem". Or even skip the additional
argument completely, use the anon_vma_hold method always without relying
on whether or not mmap_sem is held. I don't know how significant it is
to avoid extra locking here: on the one hand we like to avoid unnecessary
locking; on the other hand there's probably a thousand commoner places in
the kernel where we could pass down an arg to say, actually you won't
need to lock in such and such a case.
> int rc = 0;
> int *result = NULL;
> struct page *newpage = get_new_page(page, private, &result);
> + struct anon_vma *anon_vma = NULL;
>
> if (!newpage)
> return -ENOMEM;
> @@ -632,17 +633,23 @@ static int unmap_and_move(new_page_t get
> goto unlock;
> wait_on_page_writeback(page);
> }
> -
> + /* hold this anon_vma until page migration ends */
> + if (nocontext && PageAnon(page) && page_mapped(page))
> + anon_vma = anon_vma_hold(page);
> /*
> * Establish migration ptes or remove ptes
> */
> - try_to_unmap(page, 1);
> + if (page_mapped(page))
> + try_to_unmap(page, 1);
> +
All these preliminary tests: yes, I suppose they avoid unnecessary
locking, so I guess they're good; but it should work without them.
> if (!page_mapped(page))
> rc = move_to_new_page(newpage, page);
>
> if (rc)
> remove_migration_ptes(page, page);
>
> + anon_vma_release(anon_vma);
> +
> unlock:
> unlock_page(page);
>
> @@ -686,8 +693,8 @@ move_newpage:
> *
> * Return: Number of pages not migrated or error code.
> */
> -int migrate_pages(struct list_head *from,
> - new_page_t get_new_page, unsigned long private)
> +int __migrate_pages(struct list_head *from,
> + new_page_t get_new_page, unsigned long private, int nocontext)
> {
Remarks on nocontext as above: mmm, I think keep the patch small
and don't add that extra argument at all.
> int retry = 1;
> int nr_failed = 0;
> @@ -707,7 +714,7 @@ int migrate_pages(struct list_head *from
> cond_resched();
>
> rc = unmap_and_move(get_new_page, private,
> - page, pass > 2);
> + page, pass > 2, nocontext);
>
> switch(rc) {
> case -ENOMEM:
> @@ -737,6 +744,22 @@ out:
> return nr_failed + retry;
> }
>
> +int migrate_pages(struct list_head *from,
> + new_page_t get_new_page, unsigned long private)
> +{
> + return __migrate_pages(from, get_new_page, private, 0);
> +}
> +
> +/*
> + * When page migration is issued by the kernel itself without page mapper's
> + * mm->sem, we have to be more careful to do page migration.
> + */
> +int migrate_pages_nocontext(struct list_head *from,
> + new_page_t get_new_page, unsigned long private)
> +{
> + return __migrate_pages(from, get_new_page, private, 1);
> +}
> +
> #ifdef CONFIG_NUMA
> /*
> * Move a list of individual pages
> Index: linux-2.6.22-rc2-mm1/include/linux/rmap.h
> ===================================================================
> --- linux-2.6.22-rc2-mm1.orig/include/linux/rmap.h
> +++ linux-2.6.22-rc2-mm1/include/linux/rmap.h
> @@ -26,6 +26,9 @@
> struct anon_vma {
> spinlock_t lock; /* Serialize access to vma list */
> struct list_head head; /* List of private "related" vmas */
> +#ifdef CONFIG_MIGRATION
> + int ref; /* special refcnt for migration */
> +#endif
> };
>
> #ifdef CONFIG_MMU
> @@ -42,6 +45,14 @@ static inline void anon_vma_free(struct
> kmem_cache_free(anon_vma_cachep, anon_vma);
> }
>
> +#ifdef CONFIG_MIGRATION
> +extern struct anon_vma *anon_vma_hold(struct page *page);
> +extern void anon_vma_release(struct anon_vma *anon_vma);
> +#else
> +#define anon_vma_hold(page) do{}while(0)
> +#define anon_vma_release(anon) do{}while(0)
Rather than change those to "do {} while (0)", to which others
will ask for static inlines, just delete them, can't you -
they're simply not needed in the !CONFIG_MIGRATION case, right?
> +#endif
> +
> static inline void anon_vma_lock(struct vm_area_struct *vma)
> {
> struct anon_vma *anon_vma = vma->anon_vma;
> Index: linux-2.6.22-rc2-mm1/mm/rmap.c
> ===================================================================
> --- linux-2.6.22-rc2-mm1.orig/mm/rmap.c
> +++ linux-2.6.22-rc2-mm1/mm/rmap.c
> @@ -90,6 +90,9 @@ int anon_vma_prepare(struct vm_area_stru
> anon_vma = anon_vma_alloc();
> if (unlikely(!anon_vma))
> return -ENOMEM;
> +#ifdef CONFIG_MIGRATION
> + anon_vma->ref = 0;
> +#endif
> allocated = anon_vma;
> locked = NULL;
> }
> @@ -150,9 +153,13 @@ void anon_vma_unlink(struct vm_area_stru
> spin_lock(&anon_vma->lock);
> validate_anon_vma(vma);
> list_del(&vma->anon_vma_node);
> -
> /* We must garbage collect the anon_vma if it's empty */
> empty = list_empty(&anon_vma->head);
> +#ifdef CONFIG_MIGRATION
> + /* this means migrate_pages() has reference to this */
> + if (anon_vma->ref)
> + empty = 0;
> +#endif
> spin_unlock(&anon_vma->lock);
>
> if (empty)
> @@ -203,6 +210,33 @@ static void page_unlock_anon_vma(struct
> spin_unlock(&anon_vma->lock);
> rcu_read_unlock();
> }
> +#ifdef CONFIG_MIGRATION
> +struct anon_vma *anon_vma_hold(struct page *page) {
> + struct anon_vma *anon_vma = NULL;
> + anon_vma = page_lock_anon_vma(page);
> + if (!anon_vma)
> + return NULL;
> + if (!list_empty(&anon_vma->head))
> + anon_vma->ref++;
> + spin_unlock(&anon_vma->lock);
> + return anon_vma;
> +}
> +
> +void anon_vma_release(struct anon_vma *anon_vma)
> +{
> + int empty;
> + if (!anon_vma) /* noting to do */
> + return;
> + spin_lock(&anon_vma->lock);
> + empty = list_empty(&anon_vma->head);
> + anon_vma->ref--;
> + if (!anon_vma->ref)
> + empty = 0;
> + spin_unlock(&anon_vma->lock);
> + if (empty)
> + anon_vma_free(anon_vma);
> +}
> +#endif
>
> /*
> * At what user virtual address is page expected in vma?
> Index: linux-2.6.22-rc2-mm1/include/linux/migrate.h
> ===================================================================
> --- linux-2.6.22-rc2-mm1.orig/include/linux/migrate.h
> +++ linux-2.6.22-rc2-mm1/include/linux/migrate.h
> @@ -30,7 +30,10 @@ extern int putback_lru_pages(struct list
> extern int migrate_page(struct address_space *,
> struct page *, struct page *);
> extern int migrate_pages(struct list_head *l, new_page_t x, unsigned long);
> -
> +#ifdef CONFIG_MIGRATION_BY_KERNEL
> +extern int migrate_pages_nocontext(struct list_head *l, new_page_t x,
> + unsigned long);
> +#endif
> extern int fail_migrate_page(struct address_space *,
> struct page *, struct page *);
>
--- 2.6.22-rc3/mm/rmap.c 2007-05-19 07:36:34.000000000 +0100
+++ linux/mm/rmap.c 2007-05-30 20:11:21.000000000 +0100
@@ -204,6 +204,23 @@ static void page_unlock_anon_vma(struct
rcu_read_unlock();
}
+#ifdef CONFIG_MIGRATION
+/*
+ * Insert a dummy vm_struct_struct into the page's anon_vma list of vmas,
+ * to hold it from being freed during page migration (lacking mmap_sem).
+ */
+void anon_vma_hold(struct page *page, struct vm_area_struct *holder)
+{
+ holder->anon_vma = page_lock_anon_vma(page);
+ if (holder->anon_vma) {
+ /* Make any call to vma_address() fail */
+ holder->vm_start = holder->vm_end = 0;
+ list_add_tail(&holder->anon_vma_node, &holder->anon_vma->head);
+ page_unlock_anon_vma(holder->anon_vma);
+ }
+}
+#endif
+
/*
* At what user virtual address is page expected in vma?
*/
-
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