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SubjectRe: [PATCH 2/3] mm, dax, pmem: Introduce dev_pagemap_failure()
On Wed, Mar 17, 2021 at 09:08:23PM -0700, Dan Williams wrote:
> 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);

So my 6TB memory chassis falls off the desk and we have to call
memory_failure_dev_pagemap for 1.6 billion PFNs?

Honestly if you're going offline the /entire/ device then just tell us
sb->memory_failure(dev, 0, -1ULL) and we'll just kill everything all at
once. That was where I was trying to push Shiyang's patchset, and I had
nearly succeeded when you NAKd the whole thing.

In the meantime, I estimate that there are ~45 months worth of deferred
XFS patch review that I can make progress on, so that's where I'm going
to focus.

--D

> +}
> +EXPORT_SYMBOL_GPL(dev_pagemap_failure);
> +
> void memunmap_pages(struct dev_pagemap *pgmap)
> {
> unsigned long pfn;
>

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