[PATCH v2 for-4.19 1/2] xarray: Replace exceptional entries

[Gao Xiang]

ping? Is these two patches can be merged into linux-4.19? :'(

Thanks,
Gao Xiang

On 2019/2/26 20:43, Gao Xiang wrote:
> Hi Matthew,
> 
> On 2019/2/26 13:14, Gao Xiang wrote:
>> From: Matthew Wilcox <willy at infradead.org>
>>
>> commit 3159f943aafdbacb2f94c38fdaadabf2bbde2a14 upstream.
>>
>> Introduce xarray value entries and tagged pointers to replace radix
>> tree exceptional entries.  This is a slight change in encoding to allow
>> the use of an extra bit (we can now store BITS_PER_LONG - 1 bits in a
>> value entry).  It is also a change in emphasis; exceptional entries are
>> intimidating and different.  As the comment explains, you can choose
>> to store values or pointers in the xarray and they are both first-class
>> citizens.
>>
>> Signed-off-by: Matthew Wilcox <willy at infradead.org>
>> Reviewed-by: Josef Bacik <jbacik at fb.com>
>> [ take the minimum subset of tagged pointer support only. ]
>> Cc: Matthew Wilcox <willy at infradead.org>
>> Signed-off-by: Gao Xiang <gaoxiang25 at huawei.com>
>> ---
>> change log v2:
>>  - fix tagged pointer apis to make it work properly pointed out by Matthew;
> 
> Is this version ok? Could you kindly give an "Acked-by" tag for this patch?
> 
> The following patch is important for erofs, I'd like to backport it ASAP...
>  staging: erofs: fix race when the managed cache is enabled
> 
> It will be of great help. Thanks in advance!
> 
> Thanks,
> Gao Xiang
> 
>>
>>  drivers/staging/erofs/utils.c | 18 +++++----------
>>  include/linux/xarray.h        | 52 +++++++++++++++++++++++++++++++++++++++++++
>>  2 files changed, 58 insertions(+), 12 deletions(-)
>>
>> diff --git a/drivers/staging/erofs/utils.c b/drivers/staging/erofs/utils.c
>> index 595cf90af9bb..bdee9bd09f11 100644
>> --- a/drivers/staging/erofs/utils.c
>> +++ b/drivers/staging/erofs/utils.c
>> @@ -35,7 +35,6 @@ static atomic_long_t erofs_global_shrink_cnt;
>>  
>>  #ifdef CONFIG_EROFS_FS_ZIP
>>  
>> -/* radix_tree and the future XArray both don't use tagptr_t yet */
>>  struct erofs_workgroup *erofs_find_workgroup(
>>  	struct super_block *sb, pgoff_t index, bool *tag)
>>  {
>> @@ -47,9 +46,8 @@ struct erofs_workgroup *erofs_find_workgroup(
>>  	rcu_read_lock();
>>  	grp = radix_tree_lookup(&sbi->workstn_tree, index);
>>  	if (grp != NULL) {
>> -		*tag = radix_tree_exceptional_entry(grp);
>> -		grp = (void *)((unsigned long)grp &
>> -			~RADIX_TREE_EXCEPTIONAL_ENTRY);
>> +		*tag = xa_pointer_tag(grp);
>> +		grp = xa_untag_pointer(grp);
>>  
>>  		if (erofs_workgroup_get(grp, &oldcount)) {
>>  			/* prefer to relax rcu read side */
>> @@ -83,9 +81,7 @@ int erofs_register_workgroup(struct super_block *sb,
>>  	sbi = EROFS_SB(sb);
>>  	erofs_workstn_lock(sbi);
>>  
>> -	if (tag)
>> -		grp = (void *)((unsigned long)grp |
>> -			1UL << RADIX_TREE_EXCEPTIONAL_SHIFT);
>> +	grp = xa_tag_pointer(grp, tag);
>>  
>>  	err = radix_tree_insert(&sbi->workstn_tree,
>>  		grp->index, grp);
>> @@ -131,9 +127,7 @@ unsigned long erofs_shrink_workstation(struct erofs_sb_info *sbi,
>>  
>>  	for (i = 0; i < found; ++i) {
>>  		int cnt;
>> -		struct erofs_workgroup *grp = (void *)
>> -			((unsigned long)batch[i] &
>> -				~RADIX_TREE_EXCEPTIONAL_ENTRY);
>> +		struct erofs_workgroup *grp = xa_untag_pointer(batch[i]);
>>  
>>  		first_index = grp->index + 1;
>>  
>> @@ -150,8 +144,8 @@ unsigned long erofs_shrink_workstation(struct erofs_sb_info *sbi,
>>  #endif
>>  			continue;
>>  
>> -		if (radix_tree_delete(&sbi->workstn_tree,
>> -			grp->index) != grp) {
>> +		if (xa_untag_pointer(radix_tree_delete(&sbi->workstn_tree,
>> +			grp->index)) != grp) {
>>  #ifdef EROFS_FS_HAS_MANAGED_CACHE
>>  skip:
>>  			erofs_workgroup_unfreeze(grp, 1);
>> diff --git a/include/linux/xarray.h b/include/linux/xarray.h
>> index 2dfc8006fe64..51ae9779d08b 100644
>> --- a/include/linux/xarray.h
>> +++ b/include/linux/xarray.h
>> @@ -9,6 +9,58 @@
>>  
>>  #include <linux/spinlock.h>
>>  
>> +/**
>> + * xa_tag_pointer() - Create an XArray entry for a tagged pointer.
>> + * @p: Plain pointer.
>> + * @tag: Tag value (0 or 1).
>> + *
>> + * If the user of the XArray prefers, they can tag their pointers instead
>> + * of storing value entries.  Two tags are available (0 and 1).
>> + * These are distinct from the xa_mark_t as they are not replicated up
>> + * through the array and cannot be searched for.
>> + *
>> + * Context: Any context.
>> + * Return: An XArray entry.
>> + */
>> +static inline void *xa_tag_pointer(void *p, unsigned long tag)
>> +{
>> +	if (__builtin_constant_p(tag))
>> +		BUILD_BUG_ON(tag > 1);
>> +	else
>> +		BUG_ON(tag > 1);
>> +	return (void *)((unsigned long)p | (tag << 1));
>> +}
>> +
>> +/**
>> + * xa_untag_pointer() - Turn an XArray entry into a plain pointer.
>> + * @entry: XArray entry.
>> + *
>> + * If you have stored a tagged pointer in the XArray, call this function
>> + * to get the untagged version of the pointer.
>> + *
>> + * Context: Any context.
>> + * Return: A pointer.
>> + */
>> +static inline void *xa_untag_pointer(void *entry)
>> +{
>> +	return (void *)((unsigned long)entry & ~3UL);
>> +}
>> +
>> +/**
>> + * xa_pointer_tag() - Get the tag stored in an XArray entry.
>> + * @entry: XArray entry.
>> + *
>> + * If you have stored a tagged pointer in the XArray, call this function
>> + * to get the tag of that pointer.
>> + *
>> + * Context: Any context.
>> + * Return: A tag.
>> + */
>> +static inline unsigned int xa_pointer_tag(void *entry)
>> +{
>> +	return ((unsigned long)entry & 3UL) >> 1;
>> +}
>> +
>>  #define xa_trylock(xa)		spin_trylock(&(xa)->xa_lock)
>>  #define xa_lock(xa)		spin_lock(&(xa)->xa_lock)
>>  #define xa_unlock(xa)		spin_unlock(&(xa)->xa_lock)
>>