2.4 truncate locking

I did some research on how truncate_inode_pages()'s locking works. Please feel free to clarify and/or correct my notes on the subject, which I'd like to turn into a docpatch soon.

Thanks to Al Viro, Rik van Riel, Arjan van de Ven, and Christoph Hellwig for their help to clarify these rules.

Cheers,
Bill

(1) exclusion from freeing of the inode

	inode->i_count for holding a reference count to protect against
	prematurely freeing the inode.

(2) exclusion from simultaneous manipulation of the inode page lists

	pagecache_lock protects page->list and the inode list heads.

(3) exclusion from creation of memory mappings to the affected region

inode->i_shared_lock and inode->i_mmap_lock serialize mmap operations, but are not held by truncate_inode_pages() and so proceed simultaneously. No pages are allocated by this operation to refer to the inode, as this is done lazily at fault-time. See (8) for fault-time exclusion.

(4) exclusion from writes to the affected region

	inode->i_sem

(5) exclusion from reads to the affected region

	reads beyond inode->i_size do not perform pagecache lookup
	pagecache_lock serializes pagecache access while truncation
	simultaneously proceeds

(6) exclusion from pageout of the affected region

pagemap_lru_lock

(7) exclusion from multiple simultaneous truncate operations

	inode->i_sem

(8) exclusion from memory-mapped reads or writes of the affected region

either

(1) unmapping the pages from all processes referencing the pages to obtain unique references to them all by means of vmtruncate() / zap_page_range()
*or*
(2) a unique reference to the inode itself (vma's mapping a file have ->vm_file which holds a reference to the inode)

(9) exclusion from simultaneous manipulation of page->mapping

pagemap_lru_lock

(10) exclusion from simultaneous manipulation of pages by buffer cache

	PG_locked
		and
	unique reference to page by page->buffers == NULL

This is incorrect. It appears to be a combination of PG_locked and pagecache_lock, where both are required for writing, and only one of the two for reading.

Cheers,
Bill