Date:   Thu, 14 Oct 2021 10:48:08 -0700
From:   "Darrick J. Wong" <djwong@...nel.org>
To:     Shiyang Ruan <ruansy.fnst@...itsu.com>
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 1/8] dax: Use rwsem for dax_{read,write}_lock()

On Fri, Sep 24, 2021 at 09:09:52PM +0800, Shiyang Ruan wrote:
> In order to introduce dax holder registration, we need a write lock for
> dax.  Because of the rarity of notification failures and the infrequency
> of registration events, it would be better to be a global lock rather
> than per-device.  So, change the current lock to rwsem and introduce a
> write lock for registration.

Urgh, I totally thought dax_read_lock was a global lock on something
relating to the global dax_device state until I noticed this comment
above kill_dax():

/*
 * Note, rcu is not protecting the liveness of dax_dev, rcu is ensuring
 * that any fault handlers or operations that might have seen
 * dax_alive(), have completed.  Any operations that start after
 * synchronize_srcu() has run will abort upon seeing !dax_alive().
 */

So dax_srcu ensures stability in the dax_device's ALIVE state while any
code that relies on that aliveness runs.  As a side effect, it'll block
kill_dax (and I guess run_dax) while those functions run.  It doesn't
protect any global state at all... but this isn't made obvious in the
API by (for example) passing the dax_device into dax_read_lock.

IOWs, It's not protecting against the dax_device getting freed or
anything resembling global state.  So that's probably why you note above
that this /could/ be a per-device synchronization primitive, right?

If that's the case, then why shouldn't this be a per-device item?  As
written here, any code that takes dax_write_lock() will block every dax
device in the system while it does some work on a single dax device.
Being an rwsem, it  will also have to wait for every other dax device
access to complete before it can begin.  That seems excessive,
particularly if in the future we start hooking up lots of pmem to a
single host.

I have more to say around kill_dax() below.

> Signed-off-by: Shiyang Ruan <ruansy.fnst@...itsu.com>
> ---
>  drivers/dax/device.c       | 11 +++++-----
>  drivers/dax/super.c        | 43 ++++++++++++++++++++++----------------
>  drivers/md/dm-writecache.c |  7 +++----
>  fs/dax.c                   | 26 +++++++++++------------
>  include/linux/dax.h        |  9 ++++----
>  5 files changed, 49 insertions(+), 47 deletions(-)
> 
> diff --git a/drivers/dax/device.c b/drivers/dax/device.c
> index dd8222a42808..cc7b835509f9 100644
> --- a/drivers/dax/device.c
> +++ b/drivers/dax/device.c
> @@ -198,7 +198,6 @@ static vm_fault_t dev_dax_huge_fault(struct vm_fault *vmf,
>  	struct file *filp = vmf->vma->vm_file;
>  	unsigned long fault_size;
>  	vm_fault_t rc = VM_FAULT_SIGBUS;
> -	int id;
>  	pfn_t pfn;
>  	struct dev_dax *dev_dax = filp->private_data;
>  
> @@ -206,7 +205,7 @@ static vm_fault_t dev_dax_huge_fault(struct vm_fault *vmf,
>  			(vmf->flags & FAULT_FLAG_WRITE) ? "write" : "read",
>  			vmf->vma->vm_start, vmf->vma->vm_end, pe_size);
>  
> -	id = dax_read_lock();
> +	dax_read_lock();
>  	switch (pe_size) {
>  	case PE_SIZE_PTE:
>  		fault_size = PAGE_SIZE;
> @@ -246,7 +245,7 @@ static vm_fault_t dev_dax_huge_fault(struct vm_fault *vmf,
>  			page->index = pgoff + i;
>  		}
>  	}
> -	dax_read_unlock(id);
> +	dax_read_unlock();
>  
>  	return rc;
>  }
> @@ -284,7 +283,7 @@ static const struct vm_operations_struct dax_vm_ops = {
>  static int dax_mmap(struct file *filp, struct vm_area_struct *vma)
>  {
>  	struct dev_dax *dev_dax = filp->private_data;
> -	int rc, id;
> +	int rc;
>  
>  	dev_dbg(&dev_dax->dev, "trace\n");
>  
> @@ -292,9 +291,9 @@ static int dax_mmap(struct file *filp, struct vm_area_struct *vma)
>  	 * We lock to check dax_dev liveness and will re-check at
>  	 * fault time.
>  	 */
> -	id = dax_read_lock();
> +	dax_read_lock();
>  	rc = check_vma(dev_dax, vma, __func__);
> -	dax_read_unlock(id);
> +	dax_read_unlock();
>  	if (rc)
>  		return rc;
>  
> diff --git a/drivers/dax/super.c b/drivers/dax/super.c
> index fc89e91beea7..48ce86501d93 100644
> --- a/drivers/dax/super.c
> +++ b/drivers/dax/super.c
> @@ -36,7 +36,7 @@ struct dax_device {
>  };
>  
>  static dev_t dax_devt;
> -DEFINE_STATIC_SRCU(dax_srcu);
> +static DECLARE_RWSEM(dax_rwsem);
>  static struct vfsmount *dax_mnt;
>  static DEFINE_IDA(dax_minor_ida);
>  static struct kmem_cache *dax_cache __read_mostly;
> @@ -46,18 +46,28 @@ static struct super_block *dax_superblock __read_mostly;
>  static struct hlist_head dax_host_list[DAX_HASH_SIZE];
>  static DEFINE_SPINLOCK(dax_host_lock);
>  
> -int dax_read_lock(void)
> +void dax_read_lock(void)
>  {
> -	return srcu_read_lock(&dax_srcu);
> +	down_read(&dax_rwsem);
>  }
>  EXPORT_SYMBOL_GPL(dax_read_lock);
>  
> -void dax_read_unlock(int id)
> +void dax_read_unlock(void)
>  {
> -	srcu_read_unlock(&dax_srcu, id);
> +	up_read(&dax_rwsem);
>  }
>  EXPORT_SYMBOL_GPL(dax_read_unlock);
>  
> +void dax_write_lock(void)
> +{
> +	down_write(&dax_rwsem);
> +}
> +
> +void dax_write_unlock(void)
> +{
> +	up_write(&dax_rwsem);
> +}
> +
>  static int dax_host_hash(const char *host)
>  {
>  	return hashlen_hash(hashlen_string("DAX", host)) % DAX_HASH_SIZE;
> @@ -70,14 +80,14 @@ static int dax_host_hash(const char *host)
>  static struct dax_device *dax_get_by_host(const char *host)
>  {
>  	struct dax_device *dax_dev, *found = NULL;
> -	int hash, id;
> +	int hash;
>  
>  	if (!host)
>  		return NULL;
>  
>  	hash = dax_host_hash(host);
>  
> -	id = dax_read_lock();
> +	dax_read_lock();
>  	spin_lock(&dax_host_lock);
>  	hlist_for_each_entry(dax_dev, &dax_host_list[hash], list) {
>  		if (!dax_alive(dax_dev)
> @@ -89,7 +99,7 @@ static struct dax_device *dax_get_by_host(const char *host)
>  		break;
>  	}
>  	spin_unlock(&dax_host_lock);
> -	dax_read_unlock(id);
> +	dax_read_unlock();
>  
>  	return found;
>  }
> @@ -130,7 +140,7 @@ bool generic_fsdax_supported(struct dax_device *dax_dev,
>  	pfn_t pfn, end_pfn;
>  	sector_t last_page;
>  	long len, len2;
> -	int err, id;
> +	int err;
>  
>  	if (blocksize != PAGE_SIZE) {
>  		pr_info("%pg: error: unsupported blocksize for dax\n", bdev);
> @@ -155,14 +165,14 @@ bool generic_fsdax_supported(struct dax_device *dax_dev,
>  		return false;
>  	}
>  
> -	id = dax_read_lock();
> +	dax_read_lock();
>  	len = dax_direct_access(dax_dev, pgoff, 1, &kaddr, &pfn);
>  	len2 = dax_direct_access(dax_dev, pgoff_end, 1, &end_kaddr, &end_pfn);
>  
>  	if (len < 1 || len2 < 1) {
>  		pr_info("%pg: error: dax access failed (%ld)\n",
>  				bdev, len < 1 ? len : len2);
> -		dax_read_unlock(id);
> +		dax_read_unlock();
>  		return false;
>  	}
>  
> @@ -192,7 +202,7 @@ bool generic_fsdax_supported(struct dax_device *dax_dev,
>  		put_dev_pagemap(end_pgmap);
>  
>  	}
> -	dax_read_unlock(id);
> +	dax_read_unlock();
>  
>  	if (!dax_enabled) {
>  		pr_info("%pg: error: dax support not enabled\n", bdev);
> @@ -206,16 +216,15 @@ bool dax_supported(struct dax_device *dax_dev, struct block_device *bdev,
>  		int blocksize, sector_t start, sector_t len)
>  {
>  	bool ret = false;
> -	int id;
>  
>  	if (!dax_dev)
>  		return false;
>  
> -	id = dax_read_lock();
> +	dax_read_lock();
>  	if (dax_alive(dax_dev) && dax_dev->ops->dax_supported)
>  		ret = dax_dev->ops->dax_supported(dax_dev, bdev, blocksize,
>  						  start, len);
> -	dax_read_unlock(id);
> +	dax_read_unlock();
>  	return ret;
>  }
>  EXPORT_SYMBOL_GPL(dax_supported);
> @@ -410,7 +419,7 @@ EXPORT_SYMBOL_GPL(__set_dax_synchronous);
>  
>  bool dax_alive(struct dax_device *dax_dev)
>  {
> -	lockdep_assert_held(&dax_srcu);
> +	lockdep_assert_held(&dax_rwsem);
>  	return test_bit(DAXDEV_ALIVE, &dax_dev->flags);
>  }
>  EXPORT_SYMBOL_GPL(dax_alive);
> @@ -428,8 +437,6 @@ void kill_dax(struct dax_device *dax_dev)
>  
>  	clear_bit(DAXDEV_ALIVE, &dax_dev->flags);
>  
> -	synchronize_srcu(&dax_srcu);

Shouldn't you take the dax_write_lock() around the clear_bit call to
maintain the behavior that kill_dax can't proceed until all the
functions that depend on DAXDEV_ALIVE state have finished?

--D

> -
>  	spin_lock(&dax_host_lock);
>  	hlist_del_init(&dax_dev->list);
>  	spin_unlock(&dax_host_lock);
> diff --git a/drivers/md/dm-writecache.c b/drivers/md/dm-writecache.c
> index 18320444fb0a..1067b3e98220 100644
> --- a/drivers/md/dm-writecache.c
> +++ b/drivers/md/dm-writecache.c
> @@ -260,7 +260,6 @@ static int persistent_memory_claim(struct dm_writecache *wc)
>  	loff_t s;
>  	long p, da;
>  	pfn_t pfn;
> -	int id;
>  	struct page **pages;
>  	sector_t offset;
>  
> @@ -284,7 +283,7 @@ static int persistent_memory_claim(struct dm_writecache *wc)
>  	}
>  	offset >>= PAGE_SHIFT - 9;
>  
> -	id = dax_read_lock();
> +	dax_read_lock();
>  
>  	da = dax_direct_access(wc->ssd_dev->dax_dev, offset, p, &wc->memory_map, &pfn);
>  	if (da < 0) {
> @@ -334,7 +333,7 @@ static int persistent_memory_claim(struct dm_writecache *wc)
>  		wc->memory_vmapped = true;
>  	}
>  
> -	dax_read_unlock(id);
> +	dax_read_unlock();
>  
>  	wc->memory_map += (size_t)wc->start_sector << SECTOR_SHIFT;
>  	wc->memory_map_size -= (size_t)wc->start_sector << SECTOR_SHIFT;
> @@ -343,7 +342,7 @@ static int persistent_memory_claim(struct dm_writecache *wc)
>  err3:
>  	kvfree(pages);
>  err2:
> -	dax_read_unlock(id);
> +	dax_read_unlock();
>  err1:
>  	return r;
>  }
> diff --git a/fs/dax.c b/fs/dax.c
> index 4e3e5a283a91..798c43f09eee 100644
> --- a/fs/dax.c
> +++ b/fs/dax.c
> @@ -715,22 +715,21 @@ static int copy_cow_page_dax(struct block_device *bdev, struct dax_device *dax_d
>  	void *vto, *kaddr;
>  	pgoff_t pgoff;
>  	long rc;
> -	int id;
>  
>  	rc = bdev_dax_pgoff(bdev, sector, PAGE_SIZE, &pgoff);
>  	if (rc)
>  		return rc;
>  
> -	id = dax_read_lock();
> +	dax_read_lock();
>  	rc = dax_direct_access(dax_dev, pgoff, 1, &kaddr, NULL);
>  	if (rc < 0) {
> -		dax_read_unlock(id);
> +		dax_read_unlock();
>  		return rc;
>  	}
>  	vto = kmap_atomic(to);
>  	copy_user_page(vto, (void __force *)kaddr, vaddr, to);
>  	kunmap_atomic(vto);
> -	dax_read_unlock(id);
> +	dax_read_unlock();
>  	return 0;
>  }
>  
> @@ -1015,13 +1014,13 @@ static int dax_iomap_pfn(const struct iomap *iomap, loff_t pos, size_t size,
>  {
>  	const sector_t sector = dax_iomap_sector(iomap, pos);
>  	pgoff_t pgoff;
> -	int id, rc;
> +	int rc;
>  	long length;
>  
>  	rc = bdev_dax_pgoff(iomap->bdev, sector, size, &pgoff);
>  	if (rc)
>  		return rc;
> -	id = dax_read_lock();
> +	dax_read_lock();
>  	length = dax_direct_access(iomap->dax_dev, pgoff, PHYS_PFN(size),
>  				   NULL, pfnp);
>  	if (length < 0) {
> @@ -1038,7 +1037,7 @@ static int dax_iomap_pfn(const struct iomap *iomap, loff_t pos, size_t size,
>  		goto out;
>  	rc = 0;
>  out:
> -	dax_read_unlock(id);
> +	dax_read_unlock();
>  	return rc;
>  }
>  
> @@ -1130,7 +1129,7 @@ s64 dax_iomap_zero(loff_t pos, u64 length, struct iomap *iomap)
>  {
>  	sector_t sector = iomap_sector(iomap, pos & PAGE_MASK);
>  	pgoff_t pgoff;
> -	long rc, id;
> +	long rc;
>  	void *kaddr;
>  	bool page_aligned = false;
>  	unsigned offset = offset_in_page(pos);
> @@ -1144,14 +1143,14 @@ s64 dax_iomap_zero(loff_t pos, u64 length, struct iomap *iomap)
>  	if (rc)
>  		return rc;
>  
> -	id = dax_read_lock();
> +	dax_read_lock();
>  
>  	if (page_aligned)
>  		rc = dax_zero_page_range(iomap->dax_dev, pgoff, 1);
>  	else
>  		rc = dax_direct_access(iomap->dax_dev, pgoff, 1, &kaddr, NULL);
>  	if (rc < 0) {
> -		dax_read_unlock(id);
> +		dax_read_unlock();
>  		return rc;
>  	}
>  
> @@ -1159,7 +1158,7 @@ s64 dax_iomap_zero(loff_t pos, u64 length, struct iomap *iomap)
>  		memset(kaddr + offset, 0, size);
>  		dax_flush(iomap->dax_dev, kaddr + offset, size);
>  	}
> -	dax_read_unlock(id);
> +	dax_read_unlock();
>  	return size;
>  }
>  
> @@ -1174,7 +1173,6 @@ static loff_t dax_iomap_iter(const struct iomap_iter *iomi,
>  	loff_t end = pos + length, done = 0;
>  	ssize_t ret = 0;
>  	size_t xfer;
> -	int id;
>  
>  	if (iov_iter_rw(iter) == READ) {
>  		end = min(end, i_size_read(iomi->inode));
> @@ -1199,7 +1197,7 @@ static loff_t dax_iomap_iter(const struct iomap_iter *iomi,
>  					      (end - 1) >> PAGE_SHIFT);
>  	}
>  
> -	id = dax_read_lock();
> +	dax_read_lock();
>  	while (pos < end) {
>  		unsigned offset = pos & (PAGE_SIZE - 1);
>  		const size_t size = ALIGN(length + offset, PAGE_SIZE);
> @@ -1251,7 +1249,7 @@ static loff_t dax_iomap_iter(const struct iomap_iter *iomi,
>  		if (xfer < map_len)
>  			break;
>  	}
> -	dax_read_unlock(id);
> +	dax_read_unlock();
>  
>  	return done ? done : ret;
>  }
> diff --git a/include/linux/dax.h b/include/linux/dax.h
> index 2619d94c308d..097b3304f9b9 100644
> --- a/include/linux/dax.h
> +++ b/include/linux/dax.h
> @@ -177,15 +177,14 @@ static inline void dax_unlock_page(struct page *page, dax_entry_t cookie)
>  #endif
>  
>  #if IS_ENABLED(CONFIG_DAX)
> -int dax_read_lock(void);
> -void dax_read_unlock(int id);
> +void dax_read_lock(void);
> +void dax_read_unlock(void);
>  #else
> -static inline int dax_read_lock(void)
> +static inline void dax_read_lock(void)
>  {
> -	return 0;
>  }
>  
> -static inline void dax_read_unlock(int id)
> +static inline void dax_read_unlock(void)
>  {
>  }
>  #endif /* CONFIG_DAX */
> -- 
> 2.33.0
> 
> 
> 

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