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SubjectRe: [PATCH] UBIFS: Implement ->migratepage()
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+CC Hugh, Mel

On 03/16/2016 11:55 PM, Richard Weinberger wrote:
> From: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
>
> 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@linux.intel.com>
> [rw: Massaged changelog]
> Signed-off-by: Richard Weinberger <richard@nod.at>
> ---
> 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);
> + if (rc != MIGRATEPAGE_SUCCESS)
> + return rc;
> +
> + if (PagePrivate(page)) {
> + ClearPagePrivate(page);
> + SetPagePrivate(newpage);
> + }
> +
> + migrate_page_copy(newpage, page);
> + return MIGRATEPAGE_SUCCESS;
> +}
> +
> 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,
> };
>
>

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