[PATCH] iomap: get/put the page in iomap_page_create/release()

[PATCH] iomap: get/put the page in iomap_page_create/release()

[Christoph Hellwig]

From: Piotr Jaroszynski <pjaroszynski@nvidia.com>

migrate_page_move_mapping() expects pages with private data set to have
a page_count elevated by 1.  This is what used to happen for xfs through
the buffer_heads code before the switch to iomap in commit 82cb14175e7d
("xfs: add support for sub-pagesize writeback without buffer_heads").
Not having the count elevated causes move_pages() to fail on memory
mapped files coming from xfs.

Make iomap compatible with the migrate_page_move_mapping() assumption by
elevating the page count as part of iomap_page_create() and lowering it
in iomap_page_release().

It causes the move_pages() syscall to misbehave on memory mapped files
from xfs.  It does not not move any pages, which I suppose is "just" a
perf issue, but it also ends up returning a positive number which is out
of spec for the syscall.  Talking to Michal Hocko, it sounds like
returning positive numbers might be a necessary update to move_pages()
anyway though.

Fixes: 82cb14175e7d ("xfs: add support for sub-pagesize writeback without buffer_heads")
Signed-off-by: Piotr Jaroszynski <pjaroszynski@nvidia.com>
[hch: actually get/put the page iomap_migrate_page() to make it work
      properly]
Signed-off-by: Christoph Hellwig <hch@lst.de>
---
 fs/iomap.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/fs/iomap.c b/fs/iomap.c
index 987fefc054b4..47362397cb82 100644
--- a/fs/iomap.c
+++ b/fs/iomap.c
@@ -116,6 +116,12 @@ iomap_page_create(struct inode *inode, struct page *page)
 	atomic_set(&iop->read_count, 0);
 	atomic_set(&iop->write_count, 0);
 	bitmap_zero(iop->uptodate, PAGE_SIZE / SECTOR_SIZE);
+
+	/*
+	 * migrate_page_move_mapping() assumes that pages with private data have
+	 * their count elevated by 1.
+	 */
+	get_page(page);
 	set_page_private(page, (unsigned long)iop);
 	SetPagePrivate(page);
 	return iop;
@@ -132,6 +138,7 @@ iomap_page_release(struct page *page)
 	WARN_ON_ONCE(atomic_read(&iop->write_count));
 	ClearPagePrivate(page);
 	set_page_private(page, 0);
+	put_page(page);
 	kfree(iop);
 }
 
@@ -569,8 +576,10 @@ iomap_migrate_page(struct address_space *mapping, struct page *newpage,
 
 	if (page_has_private(page)) {
 		ClearPagePrivate(page);
+		get_page(newpage);
 		set_page_private(newpage, page_private(page));
 		set_page_private(page, 0);
+		put_page(page);
 		SetPagePrivate(newpage);
 	}
 
-- 
2.20.1

Re: [PATCH] iomap: get/put the page in iomap_page_create/release()

[Darrick J. Wong]

On Mon, Jan 21, 2019 at 04:17:02PM +0100, Christoph Hellwig wrote:
> From: Piotr Jaroszynski <pjaroszynski@nvidia.com>
> 
> migrate_page_move_mapping() expects pages with private data set to have
> a page_count elevated by 1.  This is what used to happen for xfs through
> the buffer_heads code before the switch to iomap in commit 82cb14175e7d
> ("xfs: add support for sub-pagesize writeback without buffer_heads").
> Not having the count elevated causes move_pages() to fail on memory
> mapped files coming from xfs.
> 
> Make iomap compatible with the migrate_page_move_mapping() assumption by
> elevating the page count as part of iomap_page_create() and lowering it
> in iomap_page_release().
> 
> It causes the move_pages() syscall to misbehave on memory mapped files
> from xfs.  It does not not move any pages, which I suppose is "just" a
> perf issue, but it also ends up returning a positive number which is out
> of spec for the syscall.  Talking to Michal Hocko, it sounds like
> returning positive numbers might be a necessary update to move_pages()
> anyway though.
> 
> Fixes: 82cb14175e7d ("xfs: add support for sub-pagesize writeback without buffer_heads")
> Signed-off-by: Piotr Jaroszynski <pjaroszynski@nvidia.com>
> [hch: actually get/put the page iomap_migrate_page() to make it work
>       properly]
> Signed-off-by: Christoph Hellwig <hch@lst.de>

Looks ok (apparently I forgot to ack this explicitly...)
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>

--D

> ---
>  fs/iomap.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/fs/iomap.c b/fs/iomap.c
> index 987fefc054b4..47362397cb82 100644
> --- a/fs/iomap.c
> +++ b/fs/iomap.c
> @@ -116,6 +116,12 @@ iomap_page_create(struct inode *inode, struct page *page)
>  	atomic_set(&iop->read_count, 0);
>  	atomic_set(&iop->write_count, 0);
>  	bitmap_zero(iop->uptodate, PAGE_SIZE / SECTOR_SIZE);
> +
> +	/*
> +	 * migrate_page_move_mapping() assumes that pages with private data have
> +	 * their count elevated by 1.
> +	 */
> +	get_page(page);
>  	set_page_private(page, (unsigned long)iop);
>  	SetPagePrivate(page);
>  	return iop;
> @@ -132,6 +138,7 @@ iomap_page_release(struct page *page)
>  	WARN_ON_ONCE(atomic_read(&iop->write_count));
>  	ClearPagePrivate(page);
>  	set_page_private(page, 0);
> +	put_page(page);
>  	kfree(iop);
>  }
>  
> @@ -569,8 +576,10 @@ iomap_migrate_page(struct address_space *mapping, struct page *newpage,
>  
>  	if (page_has_private(page)) {
>  		ClearPagePrivate(page);
> +		get_page(newpage);
>  		set_page_private(newpage, page_private(page));
>  		set_page_private(page, 0);
> +		put_page(page);
>  		SetPagePrivate(newpage);
>  	}
>  
> -- 
> 2.20.1
> 

[PATCH] iomap: get/put the page in iomap_page_create/release()

[p.jaroszynski]

From: Piotr Jaroszynski <pjaroszynski@nvidia.com>

migrate_page_move_mapping() expects pages with private data set to have
a page_count elevated by 1. This is what used to happen for xfs through
the buffer_heads code before the switch to iomap in 82cb14175e7d ("xfs:
add support for sub-pagesize writeback without buffer_heads"). Not
having the count elevated causes move_pages() to fail on memory mapped
files coming from xfs.

Make iomap compatible with the migrate_page_move_mapping() assumption
by elevating the page count as part of iomap_page_create() and lowering
it in iomap_page_release().

Fixes: 82cb14175e7d ("xfs: add support for sub-pagesize writeback
                      without buffer_heads")
Signed-off-by: Piotr Jaroszynski <pjaroszynski@nvidia.com>
---
 fs/iomap.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/fs/iomap.c b/fs/iomap.c
index 90c2febc93ac..23977f9f23a2 100644
--- a/fs/iomap.c
+++ b/fs/iomap.c
@@ -117,6 +117,12 @@ iomap_page_create(struct inode *inode, struct page *page)
 	atomic_set(&iop->read_count, 0);
 	atomic_set(&iop->write_count, 0);
 	bitmap_zero(iop->uptodate, PAGE_SIZE / SECTOR_SIZE);
+
+	/*
+	 * At least migrate_page_move_mapping() assumes that pages with private
+	 * data have their count elevated by 1.
+	 */
+	get_page(page);
 	set_page_private(page, (unsigned long)iop);
 	SetPagePrivate(page);
 	return iop;
@@ -133,6 +139,7 @@ iomap_page_release(struct page *page)
 	WARN_ON_ONCE(atomic_read(&iop->write_count));
 	ClearPagePrivate(page);
 	set_page_private(page, 0);
+	put_page(page);
 	kfree(iop);
 }
 
-- 
2.11.0.262.g4b0a5b2.dirty

Re: [PATCH] iomap: get/put the page in iomap_page_create/release()

[Christoph Hellwig]

> Fixes: 82cb14175e7d ("xfs: add support for sub-pagesize writeback
>                       without buffer_heads")

I've never seen line breaks in Fixes tags, is this really a valid format?

> +	/*
> +	 * At least migrate_page_move_mapping() assumes that pages with private
> +	 * data have their count elevated by 1.
> +	 */

I'd drop the "At least".

Otherwise this looks fine to me:

Reviewed-by: Christoph Hellwig <hch@lst.de>

Re: [PATCH] iomap: get/put the page in iomap_page_create/release()

[Piotr Jaroszynski]

On 11/15/18 1:30 AM, Christoph Hellwig wrote:
>> Fixes: 82cb14175e7d ("xfs: add support for sub-pagesize writeback
>>                       without buffer_heads")
> 
> I've never seen line breaks in Fixes tags, is this really a valid format?

Probably not, fixed in v2.

> 
>> +	/*
>> +	 * At least migrate_page_move_mapping() assumes that pages with private
>> +	 * data have their count elevated by 1.
>> +	 */
> 
> I'd drop the "At least".

Fixed in v2.

> 
> Otherwise this looks fine to me:
> 
> Reviewed-by: Christoph Hellwig <hch@lst.de>
> 

Thanks!

Thanks,
Piotr