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Message-ID: <5d4fb139-ea0c-7b43-ba7d-ac52d1bb462c@fujitsu.com>
Date: Wed, 20 Oct 2021 14:58:43 +0800
From: Shiyang Ruan <ruansy.fnst@...itsu.com>
To: "Darrick J. Wong" <djwong@...nel.org>
CC: <linux-kernel@...r.kernel.org>, <linux-xfs@...r.kernel.org>,
<nvdimm@...ts.linux.dev>, <linux-mm@...ck.org>,
<linux-fsdevel@...r.kernel.org>, <dan.j.williams@...el.com>,
<david@...morbit.com>, <hch@...radead.org>, <jane.chu@...cle.com>
Subject: Re: [PATCH v7 2/8] dax: Introduce holder for dax_device
在 2021/10/15 2:00, Darrick J. Wong 写道:
> On Fri, Sep 24, 2021 at 09:09:53PM +0800, Shiyang Ruan wrote:
>> To easily track filesystem from a pmem device, we introduce a holder for
>> dax_device structure, and also its operation. This holder is used to
>> remember who is using this dax_device:
>> - When it is the backend of a filesystem, the holder will be the
>> superblock of this filesystem.
>> - When this pmem device is one of the targets in a mapped device, the
>> holder will be this mapped device. In this case, the mapped device
>> has its own dax_device and it will follow the first rule. So that we
>> can finally track to the filesystem we needed.
>>
>> The holder and holder_ops will be set when filesystem is being mounted,
>> or an target device is being activated.
>>
>> Signed-off-by: Shiyang Ruan <ruansy.fnst@...itsu.com>
>> ---
>> drivers/dax/super.c | 59 +++++++++++++++++++++++++++++++++++++++++++++
>> include/linux/dax.h | 29 ++++++++++++++++++++++
>> 2 files changed, 88 insertions(+)
>>
>> diff --git a/drivers/dax/super.c b/drivers/dax/super.c
>> index 48ce86501d93..7d4a11dcba90 100644
>> --- a/drivers/dax/super.c
>> +++ b/drivers/dax/super.c
>> @@ -23,7 +23,10 @@
>> * @cdev: optional character interface for "device dax"
>> * @host: optional name for lookups where the device path is not available
>> * @private: dax driver private data
>> + * @holder_data: holder of a dax_device: could be filesystem or mapped device
>> * @flags: state and boolean properties
>> + * @ops: operations for dax_device
>> + * @holder_ops: operations for the inner holder
>> */
>> struct dax_device {
>> struct hlist_node list;
>> @@ -31,8 +34,10 @@ struct dax_device {
>> struct cdev cdev;
>> const char *host;
>> void *private;
>> + void *holder_data;
>> unsigned long flags;
>> const struct dax_operations *ops;
>> + const struct dax_holder_operations *holder_ops;
>> };
>>
>> static dev_t dax_devt;
>> @@ -374,6 +379,29 @@ int dax_zero_page_range(struct dax_device *dax_dev, pgoff_t pgoff,
>> }
>> EXPORT_SYMBOL_GPL(dax_zero_page_range);
>>
>> +int dax_holder_notify_failure(struct dax_device *dax_dev, loff_t offset,
>> + size_t size, int flags)
>> +{
>> + int rc;
>> +
>> + dax_read_lock();
>> + if (!dax_alive(dax_dev)) {
>> + rc = -ENXIO;
>> + goto out;
>> + }
>> +
>> + if (!dax_dev->holder_data) {
>> + rc = -EOPNOTSUPP;
>> + goto out;
>> + }
>> +
>> + rc = dax_dev->holder_ops->notify_failure(dax_dev, offset, size, flags);
>
> Shouldn't this check if dax_dev->holder_ops != NULL before dereferencing
> it for the function call? Imagine an implementation that wants to
> attach a ->notify_failure function to a dax_device, maintains its own
> lookup table, and decides that it doesn't need to set holder_data.
>
> (Or, imagine someone who writes a garbage into holder_data and *boom*)
My mistake. I should check @holder_ops instead of @holder_data.
>
> How does the locking work here? If there's a media failure, we'll take
> dax_rwsem and call ->notify_failure. If the ->notify_failure function
> wants to access the pmem to handle the error by calling back into the
> dax code, will that cause nested locking on dax_rwsem?
Won't for now. I have tested it with my simple testcases.
>
> Jumping ahead a bit, I think the rmap btree accesses that the xfs
> implementation performs can cause xfs_buf(fer) cache IO, which would
> trigger that if the buffers aren't already in memory, if I'm reading
> this correctly?
I didn't think of this case. But I think this uses read lock too. It
won't be blocked. Only dax_set_holder() takes write lock.
>
>> +out:
>> + dax_read_unlock();
>> + return rc;
>> +}
>> +EXPORT_SYMBOL_GPL(dax_holder_notify_failure);
>> +
>> #ifdef CONFIG_ARCH_HAS_PMEM_API
>> void arch_wb_cache_pmem(void *addr, size_t size);
>> void dax_flush(struct dax_device *dax_dev, void *addr, size_t size)
>> @@ -618,6 +646,37 @@ void put_dax(struct dax_device *dax_dev)
>> }
>> EXPORT_SYMBOL_GPL(put_dax);
>>
>> +void dax_set_holder(struct dax_device *dax_dev, void *holder,
>> + const struct dax_holder_operations *ops)
>> +{
>> + dax_write_lock();
>> + if (!dax_alive(dax_dev)) {
>> + dax_write_unlock();
>> + return;
>> + }
>> +
>> + dax_dev->holder_data = holder;
>> + dax_dev->holder_ops = ops;
>> + dax_write_unlock();
>
> I guess this means that the holder has to detach itself before anyone
> calls kill_dax, or else a dead dax device ends up with a dangling
> reference to the holder?
Yes.
>
>> +}
>> +EXPORT_SYMBOL_GPL(dax_set_holder);
>> +
>> +void *dax_get_holder(struct dax_device *dax_dev)
>> +{
>> + void *holder;
>> +
>> + dax_read_lock();
>> + if (!dax_alive(dax_dev)) {
>> + dax_read_unlock();
>> + return NULL;
>> + }
>> +
>> + holder = dax_dev->holder_data;
>> + dax_read_unlock();
>> + return holder;
>> +}
>> +EXPORT_SYMBOL_GPL(dax_get_holder);
>> +
>> /**
>> * inode_dax: convert a public inode into its dax_dev
>> * @inode: An inode with i_cdev pointing to a dax_dev
>> diff --git a/include/linux/dax.h b/include/linux/dax.h
>> index 097b3304f9b9..d273d59723cd 100644
>> --- a/include/linux/dax.h
>> +++ b/include/linux/dax.h
>> @@ -38,9 +38,24 @@ struct dax_operations {
>> int (*zero_page_range)(struct dax_device *, pgoff_t, size_t);
>> };
>>
>> +struct dax_holder_operations {
>> + /*
>> + * notify_failure - notify memory failure into inner holder device
>> + * @dax_dev: the dax device which contains the holder
>> + * @offset: offset on this dax device where memory failure occurs
>> + * @size: length of this memory failure event
>> + * @flags: action flags for memory failure handler
>> + */
>> + int (*notify_failure)(struct dax_device *dax_dev, loff_t offset,
>> + size_t size, int flags);
>
> Shouldn't size be u64 or something? Let's say that 8GB of your pmem go
> bad, wouldn't you want a single call? Though I guess the current
> implementation only goes a single page at a time, doesn't it?
Right.
>
>> +};
>> +
>> extern struct attribute_group dax_attribute_group;
>>
>> #if IS_ENABLED(CONFIG_DAX)
>> +void dax_set_holder(struct dax_device *dax_dev, void *holder,
>> + const struct dax_holder_operations *ops);
>> +void *dax_get_holder(struct dax_device *dax_dev);
>> 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);
>> @@ -70,6 +85,18 @@ static inline bool daxdev_mapping_supported(struct vm_area_struct *vma,
>> return dax_synchronous(dax_dev);
>> }
>> #else
>> +static inline struct dax_device *dax_get_by_host(const char *host)
>
> Not sure why this is being added here? AFAICT none of the patches call
> this function...?
It's mistake when I rebase my code to the latest. These lines were
deleted but I didn't notice. Will fix it.
--
Thanks,
Ruan.
>
> --D
>
>> +{
>> + return NULL;
>> +}
>> +static inline void dax_set_holder(struct dax_device *dax_dev, void *holder,
>> + const struct dax_holder_operations *ops)
>> +{
>> +}
>> +static inline void *dax_get_holder(struct dax_device *dax_dev)
>> +{
>> + return NULL;
>> +}
>> static inline struct dax_device *alloc_dax(void *private, const char *host,
>> const struct dax_operations *ops, unsigned long flags)
>> {
>> @@ -198,6 +225,8 @@ size_t dax_copy_to_iter(struct dax_device *dax_dev, pgoff_t pgoff, void *addr,
>> size_t bytes, struct iov_iter *i);
>> int dax_zero_page_range(struct dax_device *dax_dev, pgoff_t pgoff,
>> size_t nr_pages);
>> +int dax_holder_notify_failure(struct dax_device *dax_dev, loff_t offset,
>> + size_t size, int flags);
>> void dax_flush(struct dax_device *dax_dev, void *addr, size_t size);
>>
>> ssize_t dax_iomap_rw(struct kiocb *iocb, struct iov_iter *iter,
>> --
>> 2.33.0
>>
>>
>>
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