[RFC] 2.4 truncate locking

William Lee Irwin III (wli@holomorphy.com)
Wed, 17 Apr 2002 08:09:12 -0700


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
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