Hi Matthew,
On 2019/2/26 13:14, Gao Xiang wrote:
> From: Matthew Wilcox <willy@xxxxxxxxxxxxx>
>
> 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@xxxxxxxxxxxxx>
> Reviewed-by: Josef Bacik <jbacik@xxxxxx>
> [ take the minimum subset of tagged pointer support only. ]
> Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> Signed-off-by: Gao Xiang <gaoxiang25@xxxxxxxxxx>
> ---
> 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)
>
