mm_struct 中的 mm_count 和 mm_users ?

kouu

mm_struct 中的 mm_count 和 mm_users 有什么不同? 分別用于什么場(chǎng)合? 為什么有這樣的設(shè)計(jì)呢?
望高人指教呀~

scutan

原帖由 kouu 于 2009-3-24 21:22 發(fā)表
mm_struct 中的 mm_count 和 mm_users 有什么不同? 分別用于什么場(chǎng)合? 為什么有這樣的設(shè)計(jì)呢?
望高人指教呀~

ULK上面有解釋的：

The mm_users field stores the number of lightweight processes that share the mm_struct data structure (see the section "The clone( ), fork( ), and vfork( ) System Calls" in Chapter 3). The mm_count field is the main usage counter of the memory descriptor; all "users" in mm_users count as one unit in mm_count. Every time the mm_count field is decreased, the kernel checks whether it becomes zero; if so, the memory descriptor is deallocated because it is no longer in use.

We'll try to explain the difference between the use of mm_users and mm_count with an example. Consider a memory descriptor shared by two lightweight processes. Normally, its mm_users field stores the value 2, while its mm_count field stores the value 1 (both owner processes count as one).

If the memory descriptor is temporarily lent to a kernel thread (see the next section), the kernel increases the mm_count field. In this way, even if both lightweight processes die and the mm_users field becomes zero, the memory descriptor is not released until the kernel thread finishes using it because the mm_count field remains greater than zero.

If the kernel wants to be sure that the memory descriptor is not released in the middle of a lengthy operation, it might increase the mm_users field instead of mm_count (this is what the try_to_unuse( ) function does; see the section "Activating and Deactivating a Swap Area" in Chapter 17). The final result is the same because the increment of mm_users ensures that mm_count does not become zero even if all lightweight processes that own the memory descriptor die.

The mm_alloc( ) function is invoked to get a new memory descriptor. Because these descriptors are stored in a slab allocator cache, mm_alloc( ) calls kmem_cache_alloc( ), initializes the new memory descriptor, and sets the mm_count and mm_users field to 1.

Conversely, the mmput( ) function decreases the mm_users field of a memory descriptor. If that field becomes 0, the function releases the Local Descriptor Table, the memory region descriptors (see later in this chapter), and the Page Tables referenced by the memory descriptor, and then invokes mmdrop( ). The latter function decreases mm_count and, if it becomes zero, releases the mm_struct data structure.

kouu

多謝ls的回答。總算看明白了~
初始狀態(tài)下，這兩個(gè)值都是1。當(dāng) mm_users 減為 0 時(shí)，mm_count 會(huì)自減 1。也即是僅當(dāng) mm_users 減為 0 時(shí)，mm_count 才可能為 0。
mm_count 代表了對(duì) mm 本身的引用，而 mm_users 代表對(duì) mm 相關(guān)資源的引用，分了兩個(gè)層次。
可能存在這樣的情況，mm 已經(jīng)沒(méi)有人使用了（mm_users為0），所以其相關(guān)資源被釋放掉（如vma）；但是 mm 本身可能還在被人使用（比如被一個(gè)內(nèi)核線程，它不會(huì)使用vma），所以 mm 本身還不能被釋放；
這個(gè)理解應(yīng)該沒(méi)錯(cuò)吧？

embeddedlwp

mm_count是主使用計(jì)數(shù)，mm_users是共享mm_struct的進(jìn)程數(shù)，mm_users中的所有用戶在mm_count中只作為一個(gè)單位

cluter

mm_count類似于 以進(jìn)程為單位。  mm_users類似于以線程為單位。

omycle

恩。我當(dāng)初對(duì)這個(gè)也有疑問(wèn)。

Thanks above all.

embeddedlwp

5樓的很透徹