lkml.org 
[lkml]   [2021]   [Oct]   [20]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
SubjectRe: [PATCH v7 1/8] dax: Use rwsem for dax_{read,write}_lock()
From
Date


在 2021/10/15 1:48, Darrick J. Wong 写道:
> 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@fujitsu.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?

Yes, I understood now. I'll change it to a per-device
percpu_rw_semaphore. The global rw_sem is not so good.


--
Thanks,
Ruan

>
> --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
>>
>>
>>


\
 
 \ /
  Last update: 2021-10-20 07:21    [W:2.131 / U:6.520 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site