Skip to contentSkip to conversation

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

PATCH Applied [email protected] 4 participants 4 messages Nov 15, 2018 – Dec 4, 2018

Subsystems (2)
Memory management VFS
lore
Share & tools
lore

[email protected]

author
#0 lore raw

PATCH v2 1/1 iomap: get/put the page in iomap_page_create/release()

From: Piotr Jaroszynski <[email protected]>

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().

Fixes 82cb14175e7d ("xfs: add support for sub-pagesize writeback without buffer_heads")
Signed-off-by Piotr Jaroszynski <[email protected]>
Reviewed-by Christoph Hellwig <[email protected]>
---
 fs/iomap.c | 7 +++++++
 1 file changed, 7 insertions(+)
fs/iomap.c +7 -0
index 90c2febc93ac..7c369faea1dc 100644
@@ -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);
+
+	/*
+	 * 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);
 }
Signature · 1 line
2.11.0.262.g4b0a5b2.dirty
#1 lore raw

The V2 fixes look good to me.

William Kucharski

Quoted 37 lines · On Nov 15, 2018, at 11:41 AM, [email protected] wrote:
Fixes 82cb14175e7d ("xfs: add support for sub-pagesize writeback without buffer_heads")
Signed-off-by Piotr Jaroszynski <[email protected]>
Reviewed-by Christoph Hellwig <[email protected]>

---
fs/iomap.c | 7 +++++++

1 file changed, 7 insertions(+)
fs/iomap.c +7 -0
index 90c2febc93ac..7c369faea1dc 100644
@@ -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);
+
+	/*
+	 * 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);
 }
Signature · 1 line
2.11.0.262.g4b0a5b2.dirty
maintainer
#2 lore raw
In reply to [email protected]
Quoted 10 lines · On Thu, 15 Nov 2018 10:41:40 -0800 [email protected] wrote:

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().

What are the real-world end-user effects of this bug?

Is a -stable backport warranted?

#3 lore raw
Quoted 9 lines · Andrew Morton
Quoted 7 lines · On Thu, 15 Nov 2018 10:41:40 -0800 [email protected] wrote:

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.

Quoted 5 lines
Quoted 3 lines

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().

What are the real-world end-user effects of this bug?

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, see [1].

I only hit this in tests that verify that move_pages() actually moved the pages. The test also got confused by the positive return from move_pages() (it got treated as a success as positive numbers were not expected and not handled) making it a bit harder to track down what's going on.

Quoted 3 lines

Is a -stable backport warranted?

I would say yes, but this is my first kernel contribution so others would be probably better judges of that.

[1] - https://lkml.kernel.org/r/[email protected]

Thanks,
Piotr