Memory managment locks

Martin Maletinsky
2001-09-10T13:12:32Z

Hi,

I am writing a kernel thread, that should check if a process' page (specified by a virtual address and the pointer to a task structure) is present in physical memory, and
if this is the case pin it in memory (i.e. prevent it from being swapped out). I plan to pin the page by incrementing it's usage count (i.e. the count field of the
corresponding page descriptor) - this is the way map_user_kiobuf() pins pages in memory. I have some questions about semaphores and spinlocks to be used, when accessing a
process' mm structure and page tables:

(1) To parse a process' page tables I need to hold the page_table_lock in the process' mm_struct structure (according to A.Rubini's device driver book,
http://www.xml.com/ldd/chapter/book/ch13.html). I still need to keep the lock held while incrementing the page's usage count (according to various comments throughout the
kernel sources, page_table_lock prevents kswapd() from swapping out pages of the process). Question: Are there other spinlocks or semaphores to be held during the
operations I mentioned?

(2) When is the semaphore mmap_sem in the mm_struct structure to be held? When is the alloc_lock spinlock in the task_struct structure to be held?

(3) When multiple locks / semaphores have to be acquired, is there any rule concerning the order in which they should be acquired to prevent dead locks?

(4) Why isn't page_table_lock a read/write spinlock (is there any reason to prevent several threads from scanning a process' page table simultaneously)?

Thanks in advance for any help
Martin

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Martin Maletinsky phone: +41 (0)1 445 16 05
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