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Message-ID: <20230515180147.hgwk2vccsph7poxa@revolver>
Date: Mon, 15 May 2023 14:01:47 -0400
From: "Liam R. Howlett" <Liam.Howlett@...cle.com>
To: Peng Zhang <zhangpeng.00@...edance.com>
Cc: akpm@...ux-foundation.org, linux-mm@...ck.org,
linux-kernel@...r.kernel.org, maple-tree@...ts.infradead.org
Subject: Re: [PATCH 09/10] maple_tree: Rework mas_wr_slot_store() to be
cleaner and more efficient.
* Peng Zhang <zhangpeng.00@...edance.com> [230515 09:18]:
> The code of mas_wr_slot_store() is messy, make it clearer and concise,
> and add comments. In addition, get whether the two gaps are empty to
> avoid calling mas_update_gap() all the time.
Please drop the cases from the comments. These aren't that complicated
to need diagrams.
Case 1: Overwriting the range and over a part of the next range
Case 2: Overwriting a part of the range and over the entire next range
>
> Signed-off-by: Peng Zhang <zhangpeng.00@...edance.com>
> ---
> lib/maple_tree.c | 79 +++++++++++++++++++++++++++---------------------
> 1 file changed, 44 insertions(+), 35 deletions(-)
>
> diff --git a/lib/maple_tree.c b/lib/maple_tree.c
> index 538e49feafbe4..d558e7bcb6da8 100644
> --- a/lib/maple_tree.c
> +++ b/lib/maple_tree.c
> @@ -4190,53 +4190,62 @@ static inline bool mas_wr_node_store(struct ma_wr_state *wr_mas)
> * @wr_mas: the maple write state
> *
> * Return: True if stored, false otherwise
> + *
> + * Case 1:
> + * r_min r_max lmax
> + * +-------+-------+-------+
> + * original range: | |offset | end |
> + * +-----------------------+
> + * +-----------+
> + * overwrite: | |
> + * +-----------+
> + * index last
> + *
> + * Case 2:
> + * r_min r_max lmax
> + * +-------+-------+-------+
> + * original range: | |offest | end |
> + * +-------+---------------+
> + * +-----------+
> + * overwrite: | |
> + * +-----------+
> + * index last
> */
> static inline bool mas_wr_slot_store(struct ma_wr_state *wr_mas)
> {
> struct ma_state *mas = wr_mas->mas;
> - unsigned long lmax; /* Logical max. */
> unsigned char offset = mas->offset;
> + unsigned char offset_end = wr_mas->offset_end;
> + unsigned long lmax = wr_mas->end_piv; /* Logical max. */
> + bool gap = false;
>
> - if ((wr_mas->r_max > mas->last) && ((wr_mas->r_min != mas->index) ||
> - (offset != wr_mas->node_end)))
> - return false;
> -
> - if (offset == wr_mas->node_end - 1)
> - lmax = mas->max;
> - else
> - lmax = wr_mas->pivots[offset + 1];
> -
> - /* going to overwrite too many slots. */
> - if (lmax < mas->last)
> + if (offset_end - offset != 1)
> return false;
>
> - if (wr_mas->r_min == mas->index) {
> - /* overwriting two or more ranges with one. */
> - if (lmax == mas->last)
> - return false;
> -
> - /* Overwriting all of offset and a portion of offset + 1. */
> + if (mas->index == wr_mas->r_min && mas->last < lmax) {
> + /* Case 1 */
> + gap |= !mt_slot_locked(mas->tree, wr_mas->slots, offset);
> + gap |= !mt_slot_locked(mas->tree, wr_mas->slots, offset + 1);
> rcu_assign_pointer(wr_mas->slots[offset], wr_mas->entry);
> wr_mas->pivots[offset] = mas->last;
> - goto done;
> - }
> -
> - /* Doesn't end on the next range end. */
> - if (lmax != mas->last)
> + } else if (mas->index > wr_mas->r_min && mas->last == lmax) {
> + /* Case 2 */
> + gap |= !mt_slot_locked(mas->tree, wr_mas->slots, offset);
> + gap |= !mt_slot_locked(mas->tree, wr_mas->slots, offset + 1);
> + rcu_assign_pointer(wr_mas->slots[offset + 1], wr_mas->entry);
> + wr_mas->pivots[offset] = mas->index - 1;
These two lines need to be in opposite order to ensure a reader sees
either the value or the previous value. If you overwrite something with
a new value, it is possible that a reader looking for the next range
will get the value stored at offset (but not entry).
> + mas->offset++; /* Keep mas accurate. */
> + } else {
> return false;
> + }
>
> - /* Overwriting a portion of offset and all of offset + 1 */
> - if ((offset + 1 < mt_pivots[wr_mas->type]) &&
> - (wr_mas->entry || wr_mas->pivots[offset + 1]))
> - wr_mas->pivots[offset + 1] = mas->last;
> -
> - rcu_assign_pointer(wr_mas->slots[offset + 1], wr_mas->entry);
> - wr_mas->pivots[offset] = mas->index - 1;
> - mas->offset++; /* Keep mas accurate. */
> -
> -done:
> trace_ma_write(__func__, mas, 0, wr_mas->entry);
> - mas_update_gap(mas);
> + /*
> + * Only update gap when the new entry is empty or there is an empty
> + * entry in the original two ranges.
> + */
> + if (!wr_mas->entry || gap)
> + mas_update_gap(mas);
> return true;
> }
>
> @@ -4418,7 +4427,7 @@ static inline void mas_wr_modify(struct ma_wr_state *wr_mas)
> if (new_end == wr_mas->node_end + 1 && mas_wr_append(wr_mas))
> return;
>
> - if ((wr_mas->offset_end - mas->offset <= 1) && mas_wr_slot_store(wr_mas))
> + if (new_end == wr_mas->node_end && mas_wr_slot_store(wr_mas))
> return;
> else if (mas_wr_node_store(wr_mas))
> return;
> --
> 2.20.1
>
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