diff mbox series

[v9,01/10] dax: Use percpu rwsem for dax_{read,write}_lock()

Message ID 20211226143439.3985960-2-ruansy.fnst@fujitsu.com (mailing list archive)
State Superseded
Headers show
Series fsdax: introduce fs query to support reflink | expand

Commit Message

Shiyang Ruan Dec. 26, 2021, 2:34 p.m. UTC 
In order to introduce dax holder registration, we need a write lock for
dax.  The write operation is per dax device job.  So, the global lock is
not suitable.  Change the current lock to percpu_rw_semaphore and introduce
a write lock for registration.

Signed-off-by: Shiyang Ruan <ruansy.fnst@fujitsu.com>
---
 drivers/dax/device.c       | 11 +++++-----
 drivers/dax/super.c        | 42 ++++++++++++++++++++++++++------------
 drivers/md/dm-writecache.c |  7 +++----
 fs/dax.c                   | 31 ++++++++++++++--------------
 fs/fuse/dax.c              |  6 +++---
 include/linux/dax.h        | 19 ++++++++++++-----
 6 files changed, 69 insertions(+), 47 deletions(-)

Comments

Dan Williams Jan. 4, 2022, 10:44 p.m. UTC | #1 
On Sun, Dec 26, 2021 at 6:35 AM Shiyang Ruan <ruansy.fnst@fujitsu.com> wrote:
>
> In order to introduce dax holder registration, we need a write lock for
> dax.

As far as I can see, no, a write lock is not needed while the holder
is being registered.

The synchronization that is needed is to make sure that the device
stays live over the registration event, and that any in-flight holder
operations are flushed before the device transitions from live to
dead, and that in turn relates to the live state of the pgmap.

The dax device cannot switch from live to dead without first flushing
all readers, so holding dax_read_lock() over the register holder event
should be sufficient. If you are worried about 2 or more potential
holders colliding at registration time, I would expect that's already
prevented by block device exclusive holder synchronization, but you
could also use cmpxchg and a single pointer to a 'struct dax_holder {
void *holder_data, struct dax_holder_operations *holder_ops }'. If you
are worried about memory_failure triggering while the filesystem is
shutting down it can do a synchronize_srcu(&dax_srcu) if it really
needs to ensure that the notify path is idle after removing the holder
registration.

...are there any cases remaining not covered by the above suggestions?
Darrick J. Wong Jan. 5, 2022, 5:45 p.m. UTC | #2 
On Tue, Jan 04, 2022 at 02:44:08PM -0800, Dan Williams wrote:
> On Sun, Dec 26, 2021 at 6:35 AM Shiyang Ruan <ruansy.fnst@fujitsu.com> wrote:
> >
> > In order to introduce dax holder registration, we need a write lock for
> > dax.
> 
> As far as I can see, no, a write lock is not needed while the holder
> is being registered.
> 
> The synchronization that is needed is to make sure that the device
> stays live over the registration event, and that any in-flight holder
> operations are flushed before the device transitions from live to
> dead, and that in turn relates to the live state of the pgmap.
> 
> The dax device cannot switch from live to dead without first flushing
> all readers, so holding dax_read_lock() over the register holder event
> should be sufficient.

...and perhaps add a comment describing that this is what the
synchronization primitive is really protecting against?  The first time
I read through this patchset, I assumed the rwsem was protecting
&dax_hosts and was confused when I saw the one use of dax_write_lock.

--D

> If you are worried about 2 or more potential
> holders colliding at registration time, I would expect that's already
> prevented by block device exclusive holder synchronization, but you
> could also use cmpxchg and a single pointer to a 'struct dax_holder {
> void *holder_data, struct dax_holder_operations *holder_ops }'. If you
> are worried about memory_failure triggering while the filesystem is
> shutting down it can do a synchronize_srcu(&dax_srcu) if it really
> needs to ensure that the notify path is idle after removing the holder
> registration.
> 
> ...are there any cases remaining not covered by the above suggestions?
Shiyang Ruan Jan. 6, 2022, 11:06 a.m. UTC | #3 
在 2022/1/5 6:44, Dan Williams 写道:
> On Sun, Dec 26, 2021 at 6:35 AM Shiyang Ruan <ruansy.fnst@fujitsu.com> wrote:
>>
>> In order to introduce dax holder registration, we need a write lock for
>> dax.
> 
> As far as I can see, no, a write lock is not needed while the holder
> is being registered.
> 
> The synchronization that is needed is to make sure that the device
> stays live over the registration event, and that any in-flight holder
> operations are flushed before the device transitions from live to
> dead, and that in turn relates to the live state of the pgmap.
> 
> The dax device cannot switch from live to dead without first flushing
> all readers, so holding dax_read_lock() over the register holder event
> should be sufficient. If you are worried about 2 or more potential
> holders colliding at registration time, I would expect that's already
> prevented by block device exclusive holder synchronization, but you
> could also use cmpxchg and a single pointer to a 'struct dax_holder {
> void *holder_data, struct dax_holder_operations *holder_ops }'. If you
> are worried about memory_failure triggering while the filesystem is
> shutting down it can do a synchronize_srcu(&dax_srcu) if it really
> needs to ensure that the notify path is idle after removing the holder
> registration.
> 
> ...are there any cases remaining not covered by the above suggestions?

OK, I think I didn't get what actual role does the dax lock play 
before...  So, the modification of the lock is unnecessary.  I'll take 
your two suggestions into consideration.


--
Thanks,
Ruan.
diff mbox series

Patch

diff --git a/drivers/dax/device.c b/drivers/dax/device.c
index dd8222a42808..041345f9956d 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(dev_dax->dax_dev);
 	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(dev_dax->dax_dev);
 
 	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(dev_dax->dax_dev);
 	rc = check_vma(dev_dax, vma, __func__);
-	dax_read_unlock(id);
+	dax_read_unlock(dev_dax->dax_dev);
 	if (rc)
 		return rc;
 
diff --git a/drivers/dax/super.c b/drivers/dax/super.c
index e7152a6c4cc4..c46f56e33d40 100644
--- a/drivers/dax/super.c
+++ b/drivers/dax/super.c
@@ -26,29 +26,41 @@  struct dax_device {
 	struct inode inode;
 	struct cdev cdev;
 	void *private;
+	struct percpu_rw_semaphore rwsem;
 	unsigned long flags;
 	const struct dax_operations *ops;
 };
 
 static dev_t dax_devt;
-DEFINE_STATIC_SRCU(dax_srcu);
 static struct vfsmount *dax_mnt;
 static DEFINE_IDA(dax_minor_ida);
 static struct kmem_cache *dax_cache __read_mostly;
 static struct super_block *dax_superblock __read_mostly;
 
-int dax_read_lock(void)
+void dax_read_lock(struct dax_device *dax_dev)
 {
-	return srcu_read_lock(&dax_srcu);
+	percpu_down_read(&dax_dev->rwsem);
 }
 EXPORT_SYMBOL_GPL(dax_read_lock);
 
-void dax_read_unlock(int id)
+void dax_read_unlock(struct dax_device *dax_dev)
 {
-	srcu_read_unlock(&dax_srcu, id);
+	percpu_up_read(&dax_dev->rwsem);
 }
 EXPORT_SYMBOL_GPL(dax_read_unlock);
 
+void dax_write_lock(struct dax_device *dax_dev)
+{
+	percpu_down_write(&dax_dev->rwsem);
+}
+EXPORT_SYMBOL_GPL(dax_write_lock);
+
+void dax_write_unlock(struct dax_device *dax_dev)
+{
+	percpu_up_write(&dax_dev->rwsem);
+}
+EXPORT_SYMBOL_GPL(dax_write_unlock);
+
 #if defined(CONFIG_BLOCK) && defined(CONFIG_FS_DAX)
 #include <linux/blkdev.h>
 
@@ -75,7 +87,7 @@  struct dax_device *fs_dax_get_by_bdev(struct block_device *bdev, u64 *start_off)
 {
 	struct dax_device *dax_dev;
 	u64 part_size;
-	int id;
+	bool not_found;
 
 	if (!blk_queue_dax(bdev->bd_disk->queue))
 		return NULL;
@@ -87,11 +99,14 @@  struct dax_device *fs_dax_get_by_bdev(struct block_device *bdev, u64 *start_off)
 		return NULL;
 	}
 
-	id = dax_read_lock();
 	dax_dev = xa_load(&dax_hosts, (unsigned long)bdev->bd_disk);
-	if (!dax_dev || !dax_alive(dax_dev) || !igrab(&dax_dev->inode))
-		dax_dev = NULL;
-	dax_read_unlock(id);
+	if (dax_dev) {
+		dax_read_lock(dax_dev);
+		not_found = !dax_alive(dax_dev) || !igrab(&dax_dev->inode);
+		dax_read_unlock(dax_dev);
+		if (not_found)
+			dax_dev = NULL;
+	}
 
 	return dax_dev;
 }
@@ -222,7 +237,7 @@  EXPORT_SYMBOL_GPL(__set_dax_synchronous);
 
 bool dax_alive(struct dax_device *dax_dev)
 {
-	lockdep_assert_held(&dax_srcu);
+	lockdep_assert_held(&dax_dev->rwsem);
 	return test_bit(DAXDEV_ALIVE, &dax_dev->flags);
 }
 EXPORT_SYMBOL_GPL(dax_alive);
@@ -237,9 +252,9 @@  void kill_dax(struct dax_device *dax_dev)
 {
 	if (!dax_dev)
 		return;
-
+	dax_write_lock(dax_dev);
 	clear_bit(DAXDEV_ALIVE, &dax_dev->flags);
-	synchronize_srcu(&dax_srcu);
+	dax_write_unlock(dax_dev);
 }
 EXPORT_SYMBOL_GPL(kill_dax);
 
@@ -366,6 +381,7 @@  struct dax_device *alloc_dax(void *private, const struct dax_operations *ops,
 
 	dax_dev->ops = ops;
 	dax_dev->private = private;
+	percpu_init_rwsem(&dax_dev->rwsem);
 	if (flags & DAXDEV_F_SYNC)
 		set_dax_synchronous(dax_dev);
 
diff --git a/drivers/md/dm-writecache.c b/drivers/md/dm-writecache.c
index 4f31591d2d25..ebe1ec345d1d 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(wc->ssd_dev->dax_dev);
 
 	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->ssd_dev->dax_dev);
 
 	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(wc->ssd_dev->dax_dev);
 err1:
 	return r;
 }
diff --git a/fs/dax.c b/fs/dax.c
index e0eecd8e3a8f..1f46810d4b68 100644
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -717,20 +717,20 @@  static pgoff_t dax_iomap_pgoff(const struct iomap *iomap, loff_t pos)
 static int copy_cow_page_dax(struct vm_fault *vmf, const struct iomap_iter *iter)
 {
 	pgoff_t pgoff = dax_iomap_pgoff(&iter->iomap, iter->pos);
+	struct dax_device *dax_dev = iter->iomap.dax_dev;
 	void *vto, *kaddr;
 	long rc;
-	int id;
 
-	id = dax_read_lock();
-	rc = dax_direct_access(iter->iomap.dax_dev, pgoff, 1, &kaddr, NULL);
+	dax_read_lock(dax_dev);
+	rc = dax_direct_access(dax_dev, pgoff, 1, &kaddr, NULL);
 	if (rc < 0) {
-		dax_read_unlock(id);
+		dax_read_unlock(dax_dev);
 		return rc;
 	}
 	vto = kmap_atomic(vmf->cow_page);
 	copy_user_page(vto, kaddr, vmf->address, vmf->cow_page);
 	kunmap_atomic(vto);
-	dax_read_unlock(id);
+	dax_read_unlock(dax_dev);
 	return 0;
 }
 
@@ -1009,10 +1009,10 @@  static int dax_iomap_pfn(const struct iomap *iomap, loff_t pos, size_t size,
 			 pfn_t *pfnp)
 {
 	pgoff_t pgoff = dax_iomap_pgoff(iomap, pos);
-	int id, rc;
+	int rc;
 	long length;
 
-	id = dax_read_lock();
+	dax_read_lock(iomap->dax_dev);
 	length = dax_direct_access(iomap->dax_dev, pgoff, PHYS_PFN(size),
 				   NULL, pfnp);
 	if (length < 0) {
@@ -1029,7 +1029,7 @@  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(iomap->dax_dev);
 	return rc;
 }
 
@@ -1135,6 +1135,7 @@  static s64 dax_zero_iter(struct iomap_iter *iter, bool *did_zero)
 {
 	const struct iomap *iomap = &iter->iomap;
 	const struct iomap *srcmap = iomap_iter_srcmap(iter);
+	struct dax_device *dax_dev = iomap->dax_dev;
 	loff_t pos = iter->pos;
 	u64 length = iomap_length(iter);
 	s64 written = 0;
@@ -1148,14 +1149,13 @@  static s64 dax_zero_iter(struct iomap_iter *iter, bool *did_zero)
 		unsigned size = min_t(u64, PAGE_SIZE - offset, length);
 		pgoff_t pgoff = dax_iomap_pgoff(iomap, pos);
 		long rc;
-		int id;
 
-		id = dax_read_lock();
+		dax_read_lock(dax_dev);
 		if (IS_ALIGNED(pos, PAGE_SIZE) && size == PAGE_SIZE)
-			rc = dax_zero_page_range(iomap->dax_dev, pgoff, 1);
+			rc = dax_zero_page_range(dax_dev, pgoff, 1);
 		else
-			rc = dax_memzero(iomap->dax_dev, pgoff, offset, size);
-		dax_read_unlock(id);
+			rc = dax_memzero(dax_dev, pgoff, offset, size);
+		dax_read_unlock(dax_dev);
 
 		if (rc < 0)
 			return rc;
@@ -1209,7 +1209,6 @@  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));
@@ -1234,7 +1233,7 @@  static loff_t dax_iomap_iter(const struct iomap_iter *iomi,
 					      (end - 1) >> PAGE_SHIFT);
 	}
 
-	id = dax_read_lock();
+	dax_read_lock(dax_dev);
 	while (pos < end) {
 		unsigned offset = pos & (PAGE_SIZE - 1);
 		const size_t size = ALIGN(length + offset, PAGE_SIZE);
@@ -1281,7 +1280,7 @@  static loff_t dax_iomap_iter(const struct iomap_iter *iomi,
 		if (xfer < map_len)
 			break;
 	}
-	dax_read_unlock(id);
+	dax_read_unlock(dax_dev);
 
 	return done ? done : ret;
 }
diff --git a/fs/fuse/dax.c b/fs/fuse/dax.c
index 713818d74de6..00a419acab79 100644
--- a/fs/fuse/dax.c
+++ b/fs/fuse/dax.c
@@ -1231,7 +1231,7 @@  static int fuse_dax_mem_range_init(struct fuse_conn_dax *fcd)
 {
 	long nr_pages, nr_ranges;
 	struct fuse_dax_mapping *range;
-	int ret, id;
+	int ret;
 	size_t dax_size = -1;
 	unsigned long i;
 
@@ -1240,10 +1240,10 @@  static int fuse_dax_mem_range_init(struct fuse_conn_dax *fcd)
 	INIT_LIST_HEAD(&fcd->busy_ranges);
 	INIT_DELAYED_WORK(&fcd->free_work, fuse_dax_free_mem_worker);
 
-	id = dax_read_lock();
+	dax_read_lock(fcd->dev);
 	nr_pages = dax_direct_access(fcd->dev, 0, PHYS_PFN(dax_size), NULL,
 				     NULL);
-	dax_read_unlock(id);
+	dax_read_unlock(fcd->dev);
 	if (nr_pages < 0) {
 		pr_debug("dax_direct_access() returned %ld\n", nr_pages);
 		return nr_pages;
diff --git a/include/linux/dax.h b/include/linux/dax.h
index 87ae4c9b1d65..a146bfb80804 100644
--- a/include/linux/dax.h
+++ b/include/linux/dax.h
@@ -178,15 +178,24 @@  int dax_truncate_page(struct inode *inode, loff_t pos, bool *did_zero,
 		const struct iomap_ops *ops);
 
 #if IS_ENABLED(CONFIG_DAX)
-int dax_read_lock(void);
-void dax_read_unlock(int id);
+void dax_read_lock(struct dax_device *dax_dev);
+void dax_read_unlock(struct dax_device *dax_dev);
+void dax_write_lock(struct dax_device *dax_dev);
+void dax_write_unlock(struct dax_device *dax_dev);
 #else
-static inline int dax_read_lock(void)
+static inline void dax_read_lock(struct dax_device *dax_dev)
+{
+}
+
+static inline void dax_read_unlock(struct dax_device *dax_dev)
+{
+}
+
+static inline void dax_write_lock(struct dax_device *dax_dev)
 {
-	return 0;
 }
 
-static inline void dax_read_unlock(int id)
+static inline void dax_write_unlock(struct dax_device *dax_dev)
 {
 }
 #endif /* CONFIG_DAX */