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| -rw-r--r-- | Documentation/filesystems/vfs.txt | 217 |
1 files changed, 195 insertions, 22 deletions
diff --git a/Documentation/filesystems/vfs.txt b/Documentation/filesystems/vfs.txt index e56e842847d..0fcbd74efd2 100644 --- a/Documentation/filesystems/vfs.txt +++ b/Documentation/filesystems/vfs.txt @@ -230,10 +230,15 @@ only called from a process context (i.e. not from an interrupt handler 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 if + ->alloc_inode was defined and simply undoes anything done by + ->alloc_inode. 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 +448,81 @@ 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 +541,148 @@ 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. + 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 essentially 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 of + any basic-blocks on storage, then those blocks should be + pre-read (if they haven't been read already) so that the + updated blocks can be written out properly. + 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 =============== |