Re: [External] Re: [PATCH 1/1] dax: Fix stack overflow when mounting fsdax pmem device

[Jan Kara <jack@suse.cz>]

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On Wed 16-09-20 07:02:12, Adrian Huang12 wrote:
> > -----Original Message-----
> > From: Jan Kara <jack@suse.cz>
> > 
> > I'm not sure how you can get __generic_fsdax_supported() called for dm-0?
> > Possibly because there's another dm device stacked on top of it and
> > dm_table_supports_dax() calls generic_fsdax_supported()? That actually seems
> > to be a bug in dm_table_supports_dax() (device_supports_dax() in particular).
> > I'd think it should be calling dax_supported() instead of
> > generic_fsdax_supported() so that proper device callback gets called when
> > determining whether a device supports DAX or not.
> > 
> 
> Yes, you're right. There's another dm device stacked on top of it. 
> 
> When applying the following patch and running 'lvm2-testsuite --only activate-minor.sh', the following error messages are observed.
> 
> dm-3: error: dax access failed (-95)
> dm-3: error: dax access failed (-95)
> dm-3: error: dax access failed (-95)

Right, and that's result of the problem I also describe above. Attached
patch should fix these errors.

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR

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From edb67c5b213526a169c13cefbebc26b3ce8ad959 Mon Sep 17 00:00:00 2001
From: Jan Kara <jack@suse.cz>
Date: Wed, 16 Sep 2020 13:08:44 +0200
Subject: [PATCH] dm: Call proper helper to determine dax support

DM was calling generic_fsdax_supported() to determine whether a device
referenced in the DM table supports DAX. However this is a helper for "leaf" device drivers so that
they don't have to duplicate common generic checks. High level code
should call dax_supported() helper which that calls into appropriate
helper for the particular device. This problem manifested itself as
kernel messages:

dm-3: error: dax access failed (-95)

when lvm2-testsuite run in cases where a DM device was stacked on top of
another DM device.

Signed-off-by: Jan Kara <jack@suse.cz>
---
 drivers/dax/super.c   |  1 +
 drivers/md/dm-table.c |  3 +--
 include/linux/dax.h   | 11 +++++++++--
 3 files changed, 11 insertions(+), 4 deletions(-)

diff --git a/drivers/dax/super.c b/drivers/dax/super.c
index e5767c83ea23..533230bef33c 100644
--- a/drivers/dax/super.c
+++ b/drivers/dax/super.c
@@ -330,6 +330,7 @@ bool dax_supported(struct dax_device *dax_dev, struct block_device *bdev,
 
 	return dax_dev->ops->dax_supported(dax_dev, bdev, blocksize, start, len);
 }
+EXPORT_SYMBOL_GPL(dax_supported);
 
 size_t dax_copy_from_iter(struct dax_device *dax_dev, pgoff_t pgoff, void *addr,
 		size_t bytes, struct iov_iter *i)
diff --git a/drivers/md/dm-table.c b/drivers/md/dm-table.c
index 5edc3079e7c1..bed1ff0744ec 100644
--- a/drivers/md/dm-table.c
+++ b/drivers/md/dm-table.c
@@ -862,8 +862,7 @@ int device_supports_dax(struct dm_target *ti, struct dm_dev *dev,
 {
 	int blocksize = *(int *) data;
 
-	return generic_fsdax_supported(dev->dax_dev, dev->bdev, blocksize,
-				       start, len);
+	return dax_supported(dev->dax_dev, dev->bdev, blocksize, start, len);
 }
 
 /* Check devices support synchronous DAX */
diff --git a/include/linux/dax.h b/include/linux/dax.h
index 6904d4e0b2e0..9f916326814a 100644
--- a/include/linux/dax.h
+++ b/include/linux/dax.h
@@ -130,6 +130,8 @@ static inline bool generic_fsdax_supported(struct dax_device *dax_dev,
 	return __generic_fsdax_supported(dax_dev, bdev, blocksize, start,
 			sectors);
 }
+bool dax_supported(struct dax_device *dax_dev, struct block_device *bdev,
+		int blocksize, sector_t start, sector_t len);
 
 static inline void fs_put_dax(struct dax_device *dax_dev)
 {
@@ -157,6 +159,13 @@ static inline bool generic_fsdax_supported(struct dax_device *dax_dev,
 	return false;
 }
 
+static inline bool dax_supported(struct dax_device *dax_dev,
+		struct block_device *bdev, int blocksize, sector_t start,
+		sector_t len)
+{
+	return false;
+}
+
 static inline void fs_put_dax(struct dax_device *dax_dev)
 {
 }
@@ -195,8 +204,6 @@ bool dax_alive(struct dax_device *dax_dev);
 void *dax_get_private(struct dax_device *dax_dev);
 long dax_direct_access(struct dax_device *dax_dev, pgoff_t pgoff, long nr_pages,
 		void **kaddr, pfn_t *pfn);
-bool dax_supported(struct dax_device *dax_dev, struct block_device *bdev,
-		int blocksize, sector_t start, sector_t len);
 size_t dax_copy_from_iter(struct dax_device *dax_dev, pgoff_t pgoff, void *addr,
 		size_t bytes, struct iov_iter *i);
 size_t dax_copy_to_iter(struct dax_device *dax_dev, pgoff_t pgoff, void *addr,
-- 
2.16.4


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