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Mon, 11 Sep 2023 05:59:36 -0700 (PDT) Received: from [10.84.144.104] ([203.208.167.146]) by smtp.gmail.com with ESMTPSA id o10-20020a170902778a00b001b53953f306sm6436614pll.178.2023.09.11.05.59.30 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 11 Sep 2023 05:59:36 -0700 (PDT) Message-ID: Date: Mon, 11 Sep 2023 20:59:27 +0800 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:102.0) Gecko/20100101 Thunderbird/102.15.0 Subject: Re: [PATCH v2 2/6] maple_tree: Introduce interfaces __mt_dup() and mtree_dup() To: "Liam R. Howlett" , Peng Zhang , corbet@lwn.net, akpm@linux-foundation.org, willy@infradead.org, brauner@kernel.org, surenb@google.com, michael.christie@oracle.com, peterz@infradead.org, mathieu.desnoyers@efficios.com, npiggin@gmail.com, avagin@gmail.com, linux-mm@kvack.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org References: <20230830125654.21257-1-zhangpeng.00@bytedance.com> <20230830125654.21257-3-zhangpeng.00@bytedance.com> <20230907201333.nyydilmlbbf2wzf7@revolver> From: Peng Zhang In-Reply-To: <20230907201333.nyydilmlbbf2wzf7@revolver> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspamd-Queue-Id: 3266040029 X-Rspam-User: X-Rspamd-Server: rspam05 X-Stat-Signature: r6tssk45b6dhs1phob9ot9i3yjoju9jc X-HE-Tag: 1694437177-636420 X-HE-Meta: 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 uLBBJ603 vHpLqbs/bxbpSOAj3HLx0duw08OHAeKTlxunpJbt4BlODBQ1mAEpJwUsnHQp5Ry4Y7zbyPx4oTx/rE3ewNSDQnfnQ2H9RkK09zzLzvumLiRuYuqMDWMj9lVKJl9BdAZ9jsYXk9R+Xt+PI/aRhCblypF7DN43wdprKJLN3C89uQOlouM/Iz/v2BErepqO6lTaDNFNWMcnCRr2QmZO0T2S9qooAi+6An2ZiK4jkEEXQ7PzqDQ9nNJRG8QN5UaB/M7HDGROtJdmaT+4TZdLVURi5U9J6WL5FtLWEjvUsrQkLk7P/gZ+/+IodcQw6XB75QZnjLMpADSHG18hOhlSB+/j+bfPtT6x9dXP0boLdTAgRTQdbcuo9pN71SyNdq2Nb4EOkAzZg9xNrUSNiPUhzzylXE2cl4CUCAtlHjYpej3urTwLGC3UPakc+wpNnwsBs+bgByeQrYXrgtGL6Bvhqydj19Td2TvRXfVYNqbWc X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: 在 2023/9/8 04:13, Liam R. Howlett 写道: > * Peng Zhang [230830 08:57]: >> Introduce interfaces __mt_dup() and mtree_dup(), which are used to >> duplicate a maple tree. Compared with traversing the source tree and >> reinserting entry by entry in the new tree, it has better performance. >> The difference between __mt_dup() and mtree_dup() is that mtree_dup() >> handles locks internally. > > __mt_dup() should be called mas_dup() to indicate the advanced interface > which requires users to handle their own locks. Changing to the mas_dup() interface may look like this: mas_dup(mas_old, mas_new) This still encounters the problem we discussed before. You expect both mas_old and mas_new to point to the first element after the function returns, but for_each_vma(vmi, mpnt) in dup_mmap() does not support this, and will skip the first element. Unless we have an iterator similar to "do {} while()", we have to reset mas_new. There is still additional overhead in making both mas_old and mas_new point to the first element, because mas will point to the last node after dfs order traversal. In fact, I think mtree_dup() and __mt_dup() are enough. They seem to match mtree_destroy() and __mt_destroy() very well. Underlines indicate that users need to handle the lock themselves. > >> >> Signed-off-by: Peng Zhang >> --- >> include/linux/maple_tree.h | 3 + >> lib/maple_tree.c | 265 +++++++++++++++++++++++++++++++++++++ >> 2 files changed, 268 insertions(+) >> >> diff --git a/include/linux/maple_tree.h b/include/linux/maple_tree.h >> index e41c70ac7744..44fe8a57ecbd 100644 >> --- a/include/linux/maple_tree.h >> +++ b/include/linux/maple_tree.h >> @@ -327,6 +327,9 @@ int mtree_store(struct maple_tree *mt, unsigned long index, >> void *entry, gfp_t gfp); >> void *mtree_erase(struct maple_tree *mt, unsigned long index); >> >> +int mtree_dup(struct maple_tree *mt, struct maple_tree *new, gfp_t gfp); >> +int __mt_dup(struct maple_tree *mt, struct maple_tree *new, gfp_t gfp); >> + >> void mtree_destroy(struct maple_tree *mt); >> void __mt_destroy(struct maple_tree *mt); >> >> diff --git a/lib/maple_tree.c b/lib/maple_tree.c >> index ef234cf02e3e..8f841682269c 100644 >> --- a/lib/maple_tree.c >> +++ b/lib/maple_tree.c >> @@ -6370,6 +6370,271 @@ void *mtree_erase(struct maple_tree *mt, unsigned long index) >> } >> EXPORT_SYMBOL(mtree_erase); >> >> +/* >> + * mas_dup_free() - Free a half-constructed tree. > > Maybe "Free an incomplete duplication of a tree" ? > >> + * @mas: Points to the last node of the half-constructed tree. > > Your use of "Points to" seems to indicate someone knows you are talking > about a "maple state that has a node pointing to". Can this be made > more clear? > @mas: The maple state of a incomplete tree. > > Then add a note that @mas->node points to the last successfully > allocated node? > > Or something along those lines. > >> + * >> + * This function frees all nodes starting from @mas->node in the reverse order >> + * of mas_dup_build(). There is no need to hold the source tree lock at this >> + * time. >> + */ >> +static void mas_dup_free(struct ma_state *mas) >> +{ >> + struct maple_node *node; >> + enum maple_type type; >> + void __rcu **slots; >> + unsigned char count, i; >> + >> + /* Maybe the first node allocation failed. */ >> + if (!mas->node) >> + return; >> + >> + while (!mte_is_root(mas->node)) { >> + mas_ascend(mas); >> + >> + if (mas->offset) { >> + mas->offset--; >> + do { >> + mas_descend(mas); >> + mas->offset = mas_data_end(mas); >> + } while (!mte_is_leaf(mas->node)); > > Can you blindly descend and check !mte_is_leaf()? What happens when the > tree duplication fails at random internal nodes? Maybe I missed how > this cannot happen? > >> + >> + mas_ascend(mas); >> + } >> + >> + node = mte_to_node(mas->node); >> + type = mte_node_type(mas->node); >> + slots = (void **)ma_slots(node, type); >> + count = mas_data_end(mas) + 1; >> + for (i = 0; i < count; i++) >> + ((unsigned long *)slots)[i] &= ~MAPLE_NODE_MASK; >> + >> + mt_free_bulk(count, slots); >> + } > > >> + >> + node = mte_to_node(mas->node); >> + mt_free_one(node); >> +} >> + >> +/* >> + * mas_copy_node() - Copy a maple node and allocate child nodes. > > if required. "..and allocate child nodes if required." > >> + * @mas: Points to the source node. >> + * @new_mas: Points to the new node. >> + * @parent: The parent node of the new node. >> + * @gfp: The GFP_FLAGS to use for allocations. >> + * >> + * Copy @mas->node to @new_mas->node, set @parent to be the parent of >> + * @new_mas->node and allocate new child nodes for @new_mas->node. >> + * If memory allocation fails, @mas is set to -ENOMEM. >> + */ >> +static inline void mas_copy_node(struct ma_state *mas, struct ma_state *new_mas, >> + struct maple_node *parent, gfp_t gfp) >> +{ >> + struct maple_node *node = mte_to_node(mas->node); >> + struct maple_node *new_node = mte_to_node(new_mas->node); >> + enum maple_type type; >> + unsigned long val; >> + unsigned char request, count, i; >> + void __rcu **slots; >> + void __rcu **new_slots; >> + >> + /* Copy the node completely. */ >> + memcpy(new_node, node, sizeof(struct maple_node)); >> + >> + /* Update the parent node pointer. */ >> + if (unlikely(ma_is_root(node))) >> + val = MA_ROOT_PARENT; >> + else >> + val = (unsigned long)node->parent & MAPLE_NODE_MASK; > > If you treat the root as special and outside the loop, then you can > avoid the check for root for every non-root node. For root, you just > need to copy and do this special parent thing before the main loop in > mas_dup_build(). This will avoid an extra branch for each VMA over 14, > so that would add up to a lot of instructions. > >> + >> + new_node->parent = ma_parent_ptr(val | (unsigned long)parent); >> + >> + if (mte_is_leaf(mas->node)) >> + return; > > You are checking here and in mas_dup_build() for the leaf, splitting the > function into parent assignment and allocate would allow you to check > once. Copy could be moved to the main loop or with the parent setting, > depending on how you handle the root suggestion above. > >> + >> + /* Allocate memory for child nodes. */ >> + type = mte_node_type(mas->node); >> + new_slots = ma_slots(new_node, type); >> + request = mas_data_end(mas) + 1; >> + count = mt_alloc_bulk(gfp, request, new_slots); >> + if (unlikely(count < request)) { >> + if (count) >> + mt_free_bulk(count, new_slots); > > The new_slots will still contain the addresses of the freed nodes. > Don't you need to clear it here to avoid a double free? Is there a > test case for this in your testing? Again, I may have missed how this > is not possible.. > >> + mas_set_err(mas, -ENOMEM); >> + return; >> + } >> + >> + /* Restore node type information in slots. */ >> + slots = ma_slots(node, type); >> + for (i = 0; i < count; i++) >> + ((unsigned long *)new_slots)[i] |= >> + ((unsigned long)mt_slot_locked(mas->tree, slots, i) & >> + MAPLE_NODE_MASK); > > Can you expand this to multiple lines to make it more clear what is > going on? > >> +} >> + >> +/* >> + * mas_dup_build() - Build a new maple tree from a source tree >> + * @mas: The maple state of source tree. >> + * @new_mas: The maple state of new tree. >> + * @gfp: The GFP_FLAGS to use for allocations. >> + * >> + * This function builds a new tree in DFS preorder. If the memory allocation >> + * fails, the error code -ENOMEM will be set in @mas, and @new_mas points to the >> + * last node. mas_dup_free() will free the half-constructed tree. >> + * >> + * Note that the attributes of the two trees must be exactly the same, and the >> + * new tree must be empty, otherwise -EINVAL will be returned. >> + */ >> +static inline void mas_dup_build(struct ma_state *mas, struct ma_state *new_mas, >> + gfp_t gfp) >> +{ >> + struct maple_node *node, *parent; > > Could parent be struct maple_pnode? > >> + struct maple_enode *root; >> + enum maple_type type; >> + >> + if (unlikely(mt_attr(mas->tree) != mt_attr(new_mas->tree)) || >> + unlikely(!mtree_empty(new_mas->tree))) { >> + mas_set_err(mas, -EINVAL); >> + return; >> + } >> + >> + mas_start(mas); >> + if (mas_is_ptr(mas) || mas_is_none(mas)) { >> + /* >> + * The attributes of the two trees must be the same before this. >> + * The following assignment makes them the same height. >> + */ >> + new_mas->tree->ma_flags = mas->tree->ma_flags; >> + rcu_assign_pointer(new_mas->tree->ma_root, mas->tree->ma_root); >> + return; >> + } >> + >> + node = mt_alloc_one(gfp); >> + if (!node) { >> + new_mas->node = NULL; > > We don't have checks around for node == NULL, MAS_NONE would be a safer > choice. It is unlikely that someone would dup the tree and fail then > call something else, but I avoid setting node to NULL. > >> + mas_set_err(mas, -ENOMEM); >> + return; >> + } >> + >> + type = mte_node_type(mas->node); >> + root = mt_mk_node(node, type); >> + new_mas->node = root; >> + new_mas->min = 0; >> + new_mas->max = ULONG_MAX; >> + parent = ma_mnode_ptr(new_mas->tree); >> + >> + while (1) { >> + mas_copy_node(mas, new_mas, parent, gfp); >> + >> + if (unlikely(mas_is_err(mas))) >> + return; >> + >> + /* Once we reach a leaf, we need to ascend, or end the loop. */ >> + if (mte_is_leaf(mas->node)) { >> + if (mas->max == ULONG_MAX) { >> + new_mas->tree->ma_flags = mas->tree->ma_flags; >> + rcu_assign_pointer(new_mas->tree->ma_root, >> + mte_mk_root(root)); >> + break; > > If you move this to the end of the function, you can replace the same > block above with a goto. That will avoid breaking the line up. > >> + } >> + >> + do { >> + /* >> + * Must not at the root node, because we've >> + * already end the loop when we reach the last >> + * leaf. >> + */ > > I'm not sure what the comment above is trying to say. Do you mean "This > won't reach the root node because the loop will break when the last leaf > is hit"? I don't think that is accurate.. it will hit the root node but > not the end of the root node, right? Anyways, the comment isn't clear > so please have a look. > >> + mas_ascend(mas); >> + mas_ascend(new_mas); >> + } while (mas->offset == mas_data_end(mas)); >> + >> + mas->offset++; >> + new_mas->offset++; >> + } >> + >> + mas_descend(mas); >> + parent = mte_to_node(new_mas->node); >> + mas_descend(new_mas); >> + mas->offset = 0; >> + new_mas->offset = 0; >> + } >> +} >> + >> +/** >> + * __mt_dup(): Duplicate a maple tree >> + * @mt: The source maple tree >> + * @new: The new maple tree >> + * @gfp: The GFP_FLAGS to use for allocations >> + * >> + * This function duplicates a maple tree using a faster method than traversing >> + * the source tree and inserting entries into the new tree one by one. > > Can you make this comment more about what your code does instead of the > "one by one" description? > >> + * The user needs to ensure that the attributes of the source tree and the new >> + * tree are the same, and the new tree needs to be an empty tree, otherwise >> + * -EINVAL will be returned. >> + * Note that the user needs to manually lock the source tree and the new tree. >> + * >> + * Return: 0 on success, -ENOMEM if memory could not be allocated, -EINVAL If >> + * the attributes of the two trees are different or the new tree is not an empty >> + * tree. >> + */ >> +int __mt_dup(struct maple_tree *mt, struct maple_tree *new, gfp_t gfp) >> +{ >> + int ret = 0; >> + MA_STATE(mas, mt, 0, 0); >> + MA_STATE(new_mas, new, 0, 0); >> + >> + mas_dup_build(&mas, &new_mas, gfp); >> + >> + if (unlikely(mas_is_err(&mas))) { >> + ret = xa_err(mas.node); >> + if (ret == -ENOMEM) >> + mas_dup_free(&new_mas); >> + } >> + >> + return ret; >> +} >> +EXPORT_SYMBOL(__mt_dup); >> + >> +/** >> + * mtree_dup(): Duplicate a maple tree >> + * @mt: The source maple tree >> + * @new: The new maple tree >> + * @gfp: The GFP_FLAGS to use for allocations >> + * >> + * This function duplicates a maple tree using a faster method than traversing >> + * the source tree and inserting entries into the new tree one by one. > > Again, it's more interesting to state it uses the DFS preorder copy. > > It is also worth mentioning the superior allocation behaviour since that > is a desirable trait for many. In fact, you should add the allocation > behaviour in your cover letter. > >> + * The user needs to ensure that the attributes of the source tree and the new >> + * tree are the same, and the new tree needs to be an empty tree, otherwise >> + * -EINVAL will be returned. >> + * >> + * Return: 0 on success, -ENOMEM if memory could not be allocated, -EINVAL If >> + * the attributes of the two trees are different or the new tree is not an empty >> + * tree. >> + */ >> +int mtree_dup(struct maple_tree *mt, struct maple_tree *new, gfp_t gfp) >> +{ >> + int ret = 0; >> + MA_STATE(mas, mt, 0, 0); >> + MA_STATE(new_mas, new, 0, 0); >> + >> + mas_lock(&new_mas); >> + mas_lock(&mas); >> + >> + mas_dup_build(&mas, &new_mas, gfp); >> + mas_unlock(&mas); >> + >> + if (unlikely(mas_is_err(&mas))) { >> + ret = xa_err(mas.node); >> + if (ret == -ENOMEM) >> + mas_dup_free(&new_mas); >> + } >> + >> + mas_unlock(&new_mas); >> + >> + return ret; >> +} >> +EXPORT_SYMBOL(mtree_dup); >> + >> /** >> * __mt_destroy() - Walk and free all nodes of a locked maple tree. >> * @mt: The maple tree >> -- >> 2.20.1 >>