Re: [PATCH] UBIFS: Implement ->migratepage()

From: Vlastimil Babka
Date: Thu Mar 17 2016 - 05:57:53 EST

+CC Hugh, Mel

On 03/16/2016 11:55 PM, Richard Weinberger wrote:
From: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>

When using CMA during page migrations UBIFS might get confused

It shouldn't be CMA specific, the same code runs from compaction, autonuma balancing...

and the following assert triggers:
UBIFS assert failed in ubifs_set_page_dirty at 1451 (pid 436)

UBIFS is using PagePrivate() which can have different meanings across
filesystems. Therefore the generic page migration code cannot handle this
case correctly.
We have to implement our own migration function which basically does a
plain copy but also duplicates the page private flag.

Lack of PagePrivate() migration is surely a bug, but at a glance of how UBIFS uses the flag, it's more about accounting, it shouldn't prevent a page from being marked PageDirty()?
I suspect your initial bug (which is IIUC the fact that there's a dirty pte, but PageDirty(page) is false) comes from the generic fallback_migrate_page() which does:

if (PageDirty(page)) {
/* Only writeback pages in full synchronous migration */
if (mode != MIGRATE_SYNC)
return -EBUSY;
return writeout(mapping, page);

And writeout() seems to Clear PageDirty() through clear_page_dirty_for_io() but I'm not so sure about the pte (or pte's in all rmaps). But this comment in the latter function:

* Yes, Virginia, this is indeed insane.

scared me enough to not investigate further. Hopefully the people I CC'd understand more about page migration than me. I'm just an user :)

In any case, this patch would solve both lack of PageDirty() transfer, and avoid the path leading from fallback_migrate_page() to writeout(). But I'm not confident enough here to ack it.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
[rw: Massaged changelog]
Signed-off-by: Richard Weinberger <richard@xxxxxx>
fs/ubifs/file.c | 20 ++++++++++++++++++++
1 file changed, 20 insertions(+)

diff --git a/fs/ubifs/file.c b/fs/ubifs/file.c
index 0edc128..48b2944 100644
--- a/fs/ubifs/file.c
+++ b/fs/ubifs/file.c
@@ -52,6 +52,7 @@
#include "ubifs.h"
#include <linux/mount.h>
#include <linux/slab.h>
+#include <linux/migrate.h>

static int read_block(struct inode *inode, void *addr, unsigned int block,
struct ubifs_data_node *dn)
@@ -1452,6 +1453,24 @@ static int ubifs_set_page_dirty(struct page *page)
return ret;

+static int ubifs_migrate_page(struct address_space *mapping,
+ struct page *newpage, struct page *page, enum migrate_mode mode)
+ int rc;
+ rc = migrate_page_move_mapping(mapping, newpage, page, NULL, mode, 0);
+ return rc;
+ if (PagePrivate(page)) {
+ ClearPagePrivate(page);
+ SetPagePrivate(newpage);
+ }
+ migrate_page_copy(newpage, page);
static int ubifs_releasepage(struct page *page, gfp_t unused_gfp_flags)
@@ -1591,6 +1610,7 @@ const struct address_space_operations ubifs_file_address_operations = {
.write_end = ubifs_write_end,
.invalidatepage = ubifs_invalidatepage,
.set_page_dirty = ubifs_set_page_dirty,
+ .migratepage = ubifs_migrate_page,
.releasepage = ubifs_releasepage,