locking rules:
All except set_page_dirty may block
- BKL PageLocked(page)
+ BKL PageLocked(page) i_sem
writepage: no yes, unlocks (see below)
readpage: no yes, unlocks
sync_page: no maybe
writepages: no
set_page_dirty no no
readpages: no
-prepare_write: no yes
-commit_write: no yes
+prepare_write: no yes yes
+commit_write: no yes yes
+write_begin: no locks the page yes
+write_end: no yes, unlocks yes
+perform_write: no n/a yes
bmap: yes
invalidatepage: no yes
releasepage: no yes