Mailing List Archive

Flesh out the description of the address_space operations.

Signed-off-by: Neil Brown <neilb@suse.de>

### Diffstat output
./Documentation/filesystems/vfs.txt | 216 ++++++++++++++++++++++++++++++++----
1 file changed, 194 insertions(+), 22 deletions(-)

diff ./Documentation/filesystems/vfs.txt~current~ ./Documentation/filesystems/vfs.txt
--- ./Documentation/filesystems/vfs.txt~current~ 2006-03-09 17:29:35.000000000 +1100
+++ ./Documentation/filesystems/vfs.txt 2006-03-13 10:42:31.000000000 +1100
@@ -230,10 +230,14 @@ only called from a process context (i.e.
or bottom half).

alloc_inode: this method is called by inode_alloc() to allocate memory
- for struct inode and initialize it.
+ for struct inode and initialize it. If this function is not
+ defined, a simple 'struct inode' is allocated. Normally
+ alloc_inode will be used to allocate a larger structure which
+ contains a 'struct inode' embedded within it.

destroy_inode: this method is called by destroy_inode() to release
- resources allocated for struct inode.
+ resources allocated for struct inode. It is only required of
+ ->alloc_inode was defined and simply does a deallocate.

read_inode: this method is called to read a specific inode from the
mounted filesystem. The i_ino member in the struct inode is
@@ -443,14 +447,81 @@ otherwise noted.
The Address Space Object
========================

-The address space object is used to identify pages in the page cache.
+The address space object is used to group and manage pages in the page
+cache. It can be used to keep track of the pages in a file (or
+anything else) and also track the mapping of sections of the file into
+process address spaces.
+
+There are a number of distinct yet related services that an
+address-space can provide. These include communicating memory
+pressure, page lookup by address, and keeping track of pages tagged as
+Dirty or Writeback.
+
+The first can be used independantly to the others. The vm can try to
+either write dirty pages in order to clean them, or release clean
+pages in order to reuse them. To do this it can call the ->writepage
+method on dirty pages, and ->releasepage on clean pages with
+PagePrivate set. Clean pages without PagePrivate and with no external
+references will be released without notice being given to the
+address_space.
+
+To achieve this functionality, pages need to be placed on an lru with
+lru_cache_add and mark_page_active needs to be called whenever the
+page is used.
+
+Pages are normally kept in a radix tree index by ->index. This tree
+maintains information about the PG_Dirty and PG_Writeback status of
+each page, so that pages with either of these flags can be found
+quickly.
+
+The Dirty tag is primarily used by mpage_writepages - the default
+->writepages method. It uses the tag to find dirty pages to call
+->writepage on. If mpage_writepages is not used (i.e. the address
+provides it's own ->writepages) , the PAGECACHE_TAG_DIRTY tag is
+almost unused. write_inode_now and sync_inode do use it (through
+__sync_single_inode) to check if ->writepages has been successful in
+writing out the whole address_space.
+
+The Writeback tag is used by filemap*wait* and sync_page* functions,
+though wait_on_page_writeback_range, to wait for all writeback to
+complete. While waiting ->sync_page (if defined) will be called on
+each page that is found to require writeback
+
+An address_space handler may attach extra information to a page,
+typically using the 'private' field in the 'struct page'. If such
+information is attached, the PG_Private flag should be set. This will
+cause various mm routines to make extra calls into the address_space
+handler to deal with that data.
+
+An address space acts as an intermediate between storage and
+application. Data is read into the address space a whole page at a
+time, and provided to the application either by copying of the page,
+or by memory-mapping the page.
+Data is written into the address space by the application, and then
+written-back to storage typically in whole pages, however the
+address_space has finner control of write sizes.
+
+The read process essentially only requires 'readpage'. The write
+process is more complicated and uses prepare_write/commit_write or
+set_page_dirty to write data into the address_space, and writepage,
+sync_page, and writepages to writeback data to storage.
+
+Adding and removing pages to/from an address_space is protected by the
+inode's i_mutex.
+
+When data is written to a page, the PG_Dirty flag should be set. It
+typically remains set until writepage asks for it to be written. This
+should clear PG_Dirty and set PG_Writeback. It can be actually
+written at any point after PG_Dirty is clear. Once it is known to be
+safe, PG_Writeback is cleared.

+Writeback makes use of a writeback_control structure...

struct address_space_operations
-------------------------------

This describes how the VFS can manipulate mapping of a file to page cache in
-your filesystem. As of kernel 2.6.13, the following members are defined:
+your filesystem. As of kernel 2.6.16, the following members are defined:

struct address_space_operations {
int (*writepage)(struct page *page, struct writeback_control *wbc);
@@ -469,47 +540,148 @@ struct address_space_operations {
loff_t offset, unsigned long nr_segs);
struct page* (*get_xip_page)(struct address_space *, sector_t,
int);
+ /* migrate the contents of a page to the specified target */
+ int (*migratepage) (struct page *, struct page *);
};

- writepage: called by the VM write a dirty page to backing store.
+ writepage: called by the VM to write a dirty page to backing store.
+ This may happen for data integrity reason (i.e. 'sync'), or
+ to free up memory (flush). The difference can be seen in
+ wbc->sync_mode.
+ The PG_Dirty flag has been cleared and PageLocked is true.
+ writepage should start writeout, should set PG_Writeback,
+ and should make sure the page is Unlocked, either synchronously
+ or asynchronously when the write operation completes.
+
+ If wbc->sync_mode is WB_SYNC_NONE, ->writepage doesn't have to
+ try too hard if there are problems, and may choose to write out a
+ different page from the mapping if that would be more
+ appropriate. If it chooses not to start writeout, it should
+ return AOP_WRITEPAGE_ACTIVATE so that the VM will not keep
+ calling ->writepage on that page.
+
+ See the file "Locking" for more details.

readpage: called by the VM to read a page from backing store.
+ The page will be Locked when readpage is called, and should be
+ unlocked and marked uptodate once the read completes.
+ If ->readpage discovers that it needs to unlock the page for
+ some reason, it can do so, and then return AOP_TRUNCATED_PAGE.
+ In this case, the page will be re-located, re-locked and if
+ that all succeeds, ->readpage will be called again.

sync_page: called by the VM to notify the backing store to perform all
queued I/O operations for a page. I/O operations for other pages
- associated with this address_space object may also be performed.
+ associated with this address_space object may also be
+ performed.
+ This function is optional and is called only for pages with
+ PG_Writeback set while waiting for the writeback to complete.

writepages: called by the VM to write out pages associated with the
- address_space object.
+ address_space object. If WBC_SYNC_ALL, then the
+ writeback_control will specify a range of pages that must be
+ written out. If WBC_SYNC_NONE, then a nr_to_write is given
+ and that many pages should be written if possible.
+ If no ->writepages is given, then mpage_writepages is used
+ instead. This will choose pages from the addresspace that are
+ tagged as DIRTY and will pass them to ->writepage.

set_page_dirty: called by the VM to set a page dirty.
+ This is particularly needed if an address space attaches
+ private data to a page, and that data needs to be updated when
+ a page is dirtied. This is called, for example, when a memory
+ mapped page gets modified.
+ If defined, it should set the PageDirty flag, and the
+ PAGECACHE_TAG_DIRTY tag in the radix tree.

readpages: called by the VM to read pages associated with the address_space
- object.
+ object. This is essentailly just a vector version of
+ readpage. Instead of just one page, several pages are
+ requested.
+ readpages is only used for readahead, so read errors are
+ ignored. If anything goes wrong, feel free to give up.

prepare_write: called by the generic write path in VM to set up a write
- request for a page.
-
- commit_write: called by the generic write path in VM to write page to
- its backing store.
+ request for a page. This indicates to the address space that
+ the given range of bytes are about to be written. The
+ address_space should check that the write will be able to
+ complete, by allocating space if necessary and doing any other
+ internal house keeping. If the write will update parts some
+ some basic-blocks on storage, then those blocks should be
+ pre-read (if they haven't been read already) so that the
+ update will not leave half-blocks that need to be written out.
+ The page will be locked. If prepare_write wants to unlock the
+ page it, like readpage, may do so and return
+ AOP_TRUNCATED_PAGE.
+ In this case the prepare_write will be retried one the lock is
+ regained.
+
+ commit_write: If prepare_write succeeds, new data will be copied
+ into the page and then commit_write will be called. It will
+ typically update the size of the file (if appropriate) and
+ mark the inode as dirty, and do any other related housekeeping
+ operations. It should avoid returning an error if possible -
+ errors should have been handled by prepare_write.

bmap: called by the VFS to map a logical block offset within object to
- physical block number. This method is use by for the legacy FIBMAP
- ioctl. Other uses are discouraged.
-
- invalidatepage: called by the VM on truncate to disassociate a page from its
- address_space mapping.
-
- releasepage: called by the VFS to release filesystem specific metadata from
- a page.
-
- direct_IO: called by the VM for direct I/O writes and reads.
+ physical block number. This method is used by for the FIBMAP
+ ioctl and for working with swap-files. To be able to swap to
+ a file, the file must have as stable mapping to a block
+ device. The swap system does not go through the filesystem
+ but instead uses bmap to find out where the blocks in the file
+ are and uses those addresses directly.
+
+
+ invalidatepage: If a page has PagePrivate set, then invalidatepage
+ will be called when part or all of the page is to be removed
+ from the address space. This generally corresponds either a
+ truncation or a complete invalidation of the address space
+ (in the latter case 'offset' will always be 0).
+ Any private data associated with the page should be updated
+ to reflect this truncation. If offset is 0, then
+ the private data should be released, because the page
+ must be able to be completely discarded. This may be done by
+ calling the ->releasepage function, but in this case the
+ release MUST succeed.
+
+ releasepage: releasepage is called on PagePrivate pages to indicate
+ that the page should be freed if possible. ->releasepage
+ should remove any private data from the page and clear the
+ PagePrivate flag. It may also remove the page from the
+ address_space. If this fails for some reason, it may indicate
+ failure with a 0 return value.
+ This is used in two distinct though related cases. The first
+ is when the VM finds a clean page with no active users and
+ wants to make it a free page. If ->releasepage succeeds, the
+ page will be removed from the address_space and become free.
+
+ The second case if when a request has been made to invalidate
+ some or all pages in an address_space. This can happen
+ through the fadvice(POSIX_FADV_DONTNEED) system call or by the
+ filesystem explicitly requesting it as nfs and 9fs do (when
+ they believe the cache may be out of date with storage) by
+ calling invalidate_inode_pages2().
+ If the filesystem makes such a call, and needs to be certain
+ that all pages are invalidated, then it's releasepage will
+ need to ensure this. Possibly it can clear the PageUptodate
+ bit if it cannot free private data yet.
+
+ direct_IO: called by the generic read/write routines to perform
+ direct_IO - that is IO requests which bypass the page cache
+ and tranfer data directly between the storage and the
+ application's address space.

get_xip_page: called by the VM to translate a block number to a page.
The page is valid until the corresponding filesystem is unmounted.
Filesystems that want to use execute-in-place (XIP) need to implement
it. An example implementation can be found in fs/ext2/xip.c.

+ migrate_page: This is used to compact the physical memory usage.
+ If the VM wants to relocate a page (maybe off a memory card
+ that is signalling imminent failure) it will pass a new page
+ and an old page to this function. migrate_page should
+ transfer any private data across and update any references
+ that it has to the page.

The File Object
===============
-
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