a few more; you are too fast.
and thanks for doing this.
On Mon, 13 Mar 2006 10:53:16 +1100 NeilBrown wrote:
> @@ -443,14 +447,81 @@ otherwise noted.
> The Address Space Object
> ========================
>
> +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
s/vm/VM/
> +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
prefer s/lru/LRU/
> +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
its
> +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
s/though/through/ ??
> +complete. While waiting ->sync_page (if defined) will be called on
> +each page that is found to require writeback
full stop.
> +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
prefer s/mm/MM/ (or use VM as above)
> +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.
finer
> +
> +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.
> - 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
reasons
> + 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
any definition of appropriate?
> + 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
drop the hyphens
> + 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
If WBC_SYNC_ALL <what> ?
> + writeback_control will specify a range of pages that must be
> + written out. If WBC_SYNC_NONE, then a nr_to_write is given
If WBC_SYNC_NONE <what> ?
> + 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.
>
> - 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
is about to be written.
> + 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
housekeeping.
> + 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.
>
> - direct_IO: called by the VM for direct I/O writes and reads.
> + physical block number. This method is used by for the FIBMAP
by for ??
> + 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
must have a
> + 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
corresponds to
> + 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
its
> + 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
transfer
> + application's address space.
>
HTH.
---
~Randy
You can't do anything without having to do something else first.
-- Belefant's Law
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