path: root/mm/zsmalloc.c
diff options
authorJérôme Glisse <>2017-09-08 16:12:06 -0700
committerLinus Torvalds <>2017-09-08 18:26:46 -0700
commit2916ecc0f9d435d849c98f4da50e453124c87531 (patch)
treee9da172a4df499b85e62a57bce6e6591d26b32cf /mm/zsmalloc.c
parent858b54dabf4363daa3a97b9a722130a8e7cea8c9 (diff)
mm/migrate: new migrate mode MIGRATE_SYNC_NO_COPY
Introduce a new migration mode that allow to offload the copy to a device DMA engine. This changes the workflow of migration and not all address_space migratepage callback can support this. This is intended to be use by migrate_vma() which itself is use for thing like HMM (see include/linux/hmm.h). No additional per-filesystem migratepage testing is needed. I disables MIGRATE_SYNC_NO_COPY in all problematic migratepage() callback and i added comment in those to explain why (part of this patch). The commit message is unclear it should say that any callback that wish to support this new mode need to be aware of the difference in the migration flow from other mode. Some of these callbacks do extra locking while copying (aio, zsmalloc, balloon, ...) and for DMA to be effective you want to copy multiple pages in one DMA operations. But in the problematic case you can not easily hold the extra lock accross multiple call to this callback. Usual flow is: For each page { 1 - lock page 2 - call migratepage() callback 3 - (extra locking in some migratepage() callback) 4 - migrate page state (freeze refcount, update page cache, buffer head, ...) 5 - copy page 6 - (unlock any extra lock of migratepage() callback) 7 - return from migratepage() callback 8 - unlock page } The new mode MIGRATE_SYNC_NO_COPY: 1 - lock multiple pages For each page { 2 - call migratepage() callback 3 - abort in all problematic migratepage() callback 4 - migrate page state (freeze refcount, update page cache, buffer head, ...) } // finished all calls to migratepage() callback 5 - DMA copy multiple pages 6 - unlock all the pages To support MIGRATE_SYNC_NO_COPY in the problematic case we would need a new callback migratepages() (for instance) that deals with multiple pages in one transaction. Because the problematic cases are not important for current usage I did not wanted to complexify this patchset even more for no good reason. Link: Signed-off-by: Jérôme Glisse <> Cc: Aneesh Kumar <> Cc: Balbir Singh <> Cc: Benjamin Herrenschmidt <> Cc: Dan Williams <> Cc: David Nellans <> Cc: Evgeny Baskakov <> Cc: Johannes Weiner <> Cc: John Hubbard <> Cc: Kirill A. Shutemov <> Cc: Mark Hairgrove <> Cc: Michal Hocko <> Cc: Paul E. McKenney <> Cc: Ross Zwisler <> Cc: Sherry Cheung <> Cc: Subhash Gutti <> Cc: Vladimir Davydov <> Cc: Bob Liu <> Signed-off-by: Andrew Morton <> Signed-off-by: Linus Torvalds <>
Diffstat (limited to 'mm/zsmalloc.c')
1 files changed, 8 insertions, 0 deletions
diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
index 62457eb82330..5ad75ec4151c 100644
--- a/mm/zsmalloc.c
+++ b/mm/zsmalloc.c
@@ -1969,6 +1969,14 @@ int zs_page_migrate(struct address_space *mapping, struct page *newpage,
unsigned int obj_idx;
int ret = -EAGAIN;
+ /*
+ * We cannot support the _NO_COPY case here, because copy needs to
+ * happen under the zs lock, which does not work with
+ * MIGRATE_SYNC_NO_COPY workflow.
+ */
+ if (mode == MIGRATE_SYNC_NO_COPY)
+ return -EINVAL;
VM_BUG_ON_PAGE(!PageMovable(page), page);
VM_BUG_ON_PAGE(!PageIsolated(page), page);