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ext3: Update outdated comment before ext3_ordered_writepage()

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The comment is heavily outdated. The recursion into the filesystem isn't
possible because we use GFP_NOFS for our allocations, the issue about
block_write_full_page() dirtying tail page is long resolved as well
(that function doesn't dirty buffers at all), and finally we don't start
a transaction if all blocks are already allocated and mapped.

Signed-off-by: Jan Kara <jack@suse.cz>
This commit is contained in:
Jan Kara 2014-02-28 09:31:10 +01:00
parent 99128addc9
commit d680104f3d
1 changed files with 4 additions and 43 deletions
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47
fs/ext3/inode.c
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@ -1559,56 +1559,17 @@ static int buffer_unmapped(handle_t *handle, struct buffer_head *bh)
}
/*
* Note that we always start a transaction even if we're not journalling
* data. This is to preserve ordering: any hole instantiation within
* __block_write_full_page -> ext3_get_block() should be journalled
* along with the data so we don't crash and then get metadata which
* Note that whenever we need to map blocks we start a transaction even if
* we're not journalling data. This is to preserve ordering: any hole
* instantiation within __block_write_full_page -> ext3_get_block() should be
* journalled along with the data so we don't crash and then get metadata which
* refers to old data.
*
* In all journalling modes block_write_full_page() will start the I/O.
*
* Problem:
*
* ext3_writepage() -> kmalloc() -> __alloc_pages() -> page_launder() ->
* ext3_writepage()
*
* Similar for:
*
* ext3_file_write() -> generic_file_write() -> __alloc_pages() -> ...
*
* Same applies to ext3_get_block(). We will deadlock on various things like
* lock_journal and i_truncate_mutex.
*
* Setting PF_MEMALLOC here doesn't work - too many internal memory
* allocations fail.
*
* 16May01: If we're reentered then journal_current_handle() will be
* non-zero. We simply *return*.
*
* 1 July 2001: @@@ FIXME:
* In journalled data mode, a data buffer may be metadata against the
* current transaction. But the same file is part of a shared mapping
* and someone does a writepage() on it.
*
* We will move the buffer onto the async_data list, but *after* it has
* been dirtied. So there's a small window where we have dirty data on
* BJ_Metadata.
*
* Note that this only applies to the last partial page in the file. The
* bit which block_write_full_page() uses prepare/commit for. (That's
* broken code anyway: it's wrong for msync()).
*
* It's a rare case: affects the final partial page, for journalled data
* where the file is subject to bith write() and writepage() in the same
* transction. To fix it we'll need a custom block_write_full_page().
* We'll probably need that anyway for journalling writepage() output.
*
* We don't honour synchronous mounts for writepage(). That would be
* disastrous. Any write() or metadata operation will sync the fs for
* us.
*
* AKPM2: if all the page's buffers are mapped to disk and !data=journal,
* we don't need to open a transaction here.
*/
static int ext3_ordered_writepage(struct page *page,
struct writeback_control *wbc)