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Subject[PATCH 2/3] mm, dax, pmem: Introduce dev_pagemap_failure()
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Jason wondered why the get_user_pages_fast() path takes references on a
@pgmap object. The rationale was to protect against accessing a 'struct
page' that might be in the process of being removed by the driver, but
he rightly points out that should be solved the same way all gup-fast
synchronization is solved which is invalidate the mapping and let the
gup slow path do @pgmap synchronization [1].

To achieve that it means that new user mappings need to stop being
created and all existing user mappings need to be invalidated.

For device-dax this is already the case as kill_dax() prevents future
faults from installing a pte, and the single device-dax inode
address_space can be trivially unmapped.

The situation is different for filesystem-dax where device pages could
be mapped by any number of inode address_space instances. An initial
thought was to treat the device removal event like a drop_pagecache_sb()
event that walks superblocks and unmaps all inodes. However, Dave points
out that it is not just the filesystem user-mappings that need to react
to global DAX page-unmap events, it is also filesystem metadata
(proposed DAX metadata access), and other drivers (upstream
DM-writecache) that need to react to this event [2].

The only kernel facility that is meant to globally broadcast the loss of
a page (via corruption or surprise remove) is memory_failure(). The
downside of memory_failure() is that it is a pfn-at-a-time interface.
However, the events that would trigger the need to call memory_failure()
over a full PMEM device should be rare. Remove should always be
coordinated by the administrator with the filesystem. If someone force
removes a device from underneath a mounted filesystem the driver assumes
they have a good reason, or otherwise get to keep the pieces. Since
->remove() callbacks can not fail the only option is to trigger the mass
memory_failure().

The mechanism to determine whether memory_failure() triggers at
pmem->remove() time is whether the associated dax_device has an elevated
reference at @pgmap ->kill() time.

With this in place the get_user_pages_fast() path can drop its
half-measure synchronization with an @pgmap reference.

Link: http://lore.kernel.org/r/20210224010017.GQ2643399@ziepe.ca [1]
Link: http://lore.kernel.org/r/20210302075736.GJ4662@dread.disaster.area [2]
Reported-by: Jason Gunthorpe <jgg@ziepe.ca>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Shiyang Ruan <ruansy.fnst@fujitsu.com>
Cc: Vishal Verma <vishal.l.verma@intel.com>
Cc: Dave Jiang <dave.jiang@intel.com>
Cc: Ira Weiny <ira.weiny@intel.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Jan Kara <jack@suse.cz>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Naoya Horiguchi <naoya.horiguchi@nec.com>
Cc: "Darrick J. Wong" <djwong@kernel.org>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
---
drivers/dax/super.c | 15 +++++++++++++++
drivers/nvdimm/pmem.c | 10 +++++++++-
drivers/nvdimm/pmem.h | 1 +
include/linux/dax.h | 5 +++++
include/linux/memremap.h | 5 +++++
include/linux/mm.h | 3 +++
mm/memory-failure.c | 11 +++++++++--
mm/memremap.c | 11 +++++++++++
8 files changed, 58 insertions(+), 3 deletions(-)

diff --git a/drivers/dax/super.c b/drivers/dax/super.c
index 5fa6ae9dbc8b..5ebcedf4a68c 100644
--- a/drivers/dax/super.c
+++ b/drivers/dax/super.c
@@ -624,6 +624,21 @@ void put_dax(struct dax_device *dax_dev)
}
EXPORT_SYMBOL_GPL(put_dax);

+bool dax_is_idle(struct dax_device *dax_dev)
+{
+ struct inode *inode;
+
+ if (!dax_dev)
+ return true;
+
+ WARN_ONCE(test_bit(DAXDEV_ALIVE, &dax_dev->flags),
+ "dax idle check on live device.\n");
+
+ inode = &dax_dev->inode;
+ return atomic_read(&inode->i_count) < 2;
+}
+EXPORT_SYMBOL_GPL(dax_is_idle);
+
/**
* dax_get_by_host() - temporary lookup mechanism for filesystem-dax
* @host: alternate name for the device registered by a dax driver
diff --git a/drivers/nvdimm/pmem.c b/drivers/nvdimm/pmem.c
index b8a85bfb2e95..e8822c9262ee 100644
--- a/drivers/nvdimm/pmem.c
+++ b/drivers/nvdimm/pmem.c
@@ -348,15 +348,21 @@ static void pmem_pagemap_kill(struct dev_pagemap *pgmap)
{
struct request_queue *q =
container_of(pgmap->ref, struct request_queue, q_usage_counter);
+ struct pmem_device *pmem = q->queuedata;

blk_freeze_queue_start(q);
+ kill_dax(pmem->dax_dev);
+ if (!dax_is_idle(pmem->dax_dev)) {
+ dev_warn(pmem->dev,
+ "DAX active at remove, trigger mass memory failure\n");
+ dev_pagemap_failure(pgmap);
+ }
}

static void pmem_release_disk(void *__pmem)
{
struct pmem_device *pmem = __pmem;

- kill_dax(pmem->dax_dev);
put_dax(pmem->dax_dev);
del_gendisk(pmem->disk);
put_disk(pmem->disk);
@@ -406,6 +412,7 @@ static int pmem_attach_disk(struct device *dev,
devm_namespace_disable(dev, ndns);

dev_set_drvdata(dev, pmem);
+ pmem->dev = dev;
pmem->phys_addr = res->start;
pmem->size = resource_size(res);
fua = nvdimm_has_flush(nd_region);
@@ -467,6 +474,7 @@ static int pmem_attach_disk(struct device *dev,
blk_queue_flag_set(QUEUE_FLAG_NONROT, q);
if (pmem->pfn_flags & PFN_MAP)
blk_queue_flag_set(QUEUE_FLAG_DAX, q);
+ q->queuedata = pmem;

disk = alloc_disk_node(0, nid);
if (!disk)
diff --git a/drivers/nvdimm/pmem.h b/drivers/nvdimm/pmem.h
index 59cfe13ea8a8..1222088a569a 100644
--- a/drivers/nvdimm/pmem.h
+++ b/drivers/nvdimm/pmem.h
@@ -23,6 +23,7 @@ struct pmem_device {
struct badblocks bb;
struct dax_device *dax_dev;
struct gendisk *disk;
+ struct device *dev;
struct dev_pagemap pgmap;
};

diff --git a/include/linux/dax.h b/include/linux/dax.h
index b52f084aa643..015f1d9a8232 100644
--- a/include/linux/dax.h
+++ b/include/linux/dax.h
@@ -46,6 +46,7 @@ struct dax_device *alloc_dax(void *private, const char *host,
const struct dax_operations *ops, unsigned long flags);
void put_dax(struct dax_device *dax_dev);
void kill_dax(struct dax_device *dax_dev);
+bool dax_is_idle(struct dax_device *dax_dev);
void dax_write_cache(struct dax_device *dax_dev, bool wc);
bool dax_write_cache_enabled(struct dax_device *dax_dev);
bool __dax_synchronous(struct dax_device *dax_dev);
@@ -92,6 +93,10 @@ static inline void put_dax(struct dax_device *dax_dev)
static inline void kill_dax(struct dax_device *dax_dev)
{
}
+static inline bool dax_is_idle(struct dax_device *dax_dev)
+{
+ return true;
+}
static inline void dax_write_cache(struct dax_device *dax_dev, bool wc)
{
}
diff --git a/include/linux/memremap.h b/include/linux/memremap.h
index f5b464daeeca..d52cdc6c5313 100644
--- a/include/linux/memremap.h
+++ b/include/linux/memremap.h
@@ -137,6 +137,7 @@ void *devm_memremap_pages(struct device *dev, struct dev_pagemap *pgmap);
void devm_memunmap_pages(struct device *dev, struct dev_pagemap *pgmap);
struct dev_pagemap *get_dev_pagemap(unsigned long pfn,
struct dev_pagemap *pgmap);
+void dev_pagemap_failure(struct dev_pagemap *pgmap);
bool pgmap_pfn_valid(struct dev_pagemap *pgmap, unsigned long pfn);

unsigned long vmem_altmap_offset(struct vmem_altmap *altmap);
@@ -160,6 +161,10 @@ static inline void devm_memunmap_pages(struct device *dev,
{
}

+static inline void dev_pagemap_failure(struct dev_pagemap *pgmap)
+{
+}
+
static inline struct dev_pagemap *get_dev_pagemap(unsigned long pfn,
struct dev_pagemap *pgmap)
{
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 77e64e3eac80..95f79f457bab 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -3002,8 +3002,11 @@ enum mf_flags {
MF_ACTION_REQUIRED = 1 << 1,
MF_MUST_KILL = 1 << 2,
MF_SOFT_OFFLINE = 1 << 3,
+ MF_MEM_REMOVE = 1 << 4,
};
extern int memory_failure(unsigned long pfn, int flags);
+extern int memory_failure_dev_pagemap(unsigned long pfn, int flags,
+ struct dev_pagemap *pgmap);
extern void memory_failure_queue(unsigned long pfn, int flags);
extern void memory_failure_queue_kick(int cpu);
extern int unpoison_memory(unsigned long pfn);
diff --git a/mm/memory-failure.c b/mm/memory-failure.c
index 43ba4307c526..8f557beb19ee 100644
--- a/mm/memory-failure.c
+++ b/mm/memory-failure.c
@@ -1296,8 +1296,8 @@ static int memory_failure_hugetlb(unsigned long pfn, int flags)
return res;
}

-static int memory_failure_dev_pagemap(unsigned long pfn, int flags,
- struct dev_pagemap *pgmap)
+int memory_failure_dev_pagemap(unsigned long pfn, int flags,
+ struct dev_pagemap *pgmap)
{
struct page *page = pfn_to_page(pfn);
const bool unmap_success = true;
@@ -1377,6 +1377,13 @@ static int memory_failure_dev_pagemap(unsigned long pfn, int flags,
unlock:
dax_unlock_page(page, cookie);
out:
+ /*
+ * In the removal case, given unmap is always successful, and
+ * the driver is responsible for the direct map the recovery is
+ * always successful
+ */
+ if (flags & MF_MEM_REMOVE)
+ rc = 0;
action_result(pfn, MF_MSG_DAX, rc ? MF_FAILED : MF_RECOVERED);
return rc;
}
diff --git a/mm/memremap.c b/mm/memremap.c
index 7aa7d6e80ee5..f34da1e14b52 100644
--- a/mm/memremap.c
+++ b/mm/memremap.c
@@ -165,6 +165,17 @@ static void pageunmap_range(struct dev_pagemap *pgmap, int range_id)
pgmap_array_delete(range);
}

+void dev_pagemap_failure(struct dev_pagemap *pgmap)
+{
+ unsigned long pfn;
+ int i;
+
+ for (i = 0; i < pgmap->nr_range; i++)
+ for_each_device_pfn(pfn, pgmap, i)
+ memory_failure_dev_pagemap(pfn, MF_MEM_REMOVE, pgmap);
+}
+EXPORT_SYMBOL_GPL(dev_pagemap_failure);
+
void memunmap_pages(struct dev_pagemap *pgmap)
{
unsigned long pfn;
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