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SubjectRe: [PATCH] retries in ext3_prepare_write() violate ordering requirements
Andrew,

answers on your questions.

>>in journal=ordered or journal=data mode retry in ext3_prepare_write()
>>breaks the requirements of journaling of data with respect to metadata.
>>The fix is to call commit_write to commit allocated zero blocks before
>>retry.
>>
>
>
> How was this problem detected? (ie: why was block_prepare_write() failing?)
>
> How was the patch tested?
>
> Was nobh-mode also tested?
with nobh block size can't be less than page size, so all the problems
with retries disappear and our code will be a no-op.

>>--- ./fs/ext3/inode.c.ext3pw 2006-11-08 17:44:14.000000000 +0300
>>+++ ./fs/ext3/inode.c 2006-11-08 17:48:59.000000000 +0300
>>@@ -1148,37 +1148,89 @@ static int do_journal_get_write_access(h
>> return ext3_journal_get_write_access(handle, bh);
>> }
>>
>>+/*
>>+ * The idea of this helper function is following:
>>+ * if prepare_write has allocated some blocks, but not all of them, the
>>+ * transaction must include the content of the newly allocated blocks.
>>+ * This content is expected to be set to zeroes by block_prepare_write().
>>+ * 2006/10/14 SAW
>>+ */
>>+static int ext3_prepare_failure(struct file *file, struct page *page,
>>+ unsigned from, unsigned to)
>>+{
>>+ struct address_space *mapping;
>>+ struct buffer_head *bh, *head, *next;
>>+ unsigned block_start, block_end;
>>+ unsigned blocksize;
>>+
>>+ mapping = page->mapping;
>>+ if (ext3_should_writeback_data(mapping->host)) {
>>+ /* optimization: no constraints about data */
>>+skip:
>>+ ext3_journal_stop(ext3_journal_current_handle());
>>+ return 0;
>
>
> Should this be `return ext3_journal_stop(...);'?
will fix and send an incremental patch to you.

>>+ }
>>+
>>+ head = page_buffers(page);
>>+ blocksize = head->b_size;
>>+ for ( bh = head, block_start = 0;
>>+ bh != head || !block_start;
>>+ block_start = block_end, bh = next)
>>+ {
>>+ next = bh->b_this_page;
>>+ block_end = block_start + blocksize;
>>+ if (block_end <= from)
>>+ continue;
>>+ if (block_start >= to) {
>>+ block_start = to;
>>+ break;
>>+ }
>>+ if (!buffer_mapped(bh))

<<<< /* prepare_write failed on this bh */

>>+ break;

<<<< lost here:
if (ext3_should_journal_data(inode)) {
ret = do_journal_get_write_access(XXX);
if (ret) {
journal_stop(handle);
return ret;
}
}

>
> What is the significance of buffer_mapped() here? Outside EOF or into a
> hole? If so, then block_start >= to, and we can't get here??
/*
* block_start here becomes the first block where the current iteration
* of prepare_write failed.
*/

>>+ }
>>+ if (block_start <= from)
>>+ goto skip;
>>+
>>+ /* commit allocated and zeroed buffers */
>>+ return mapping->a_ops->commit_write(file, page, from, block_start);
>>+}
>>+
>> static int ext3_prepare_write(struct file *file, struct page *page,
>> unsigned from, unsigned to)
>> {
>> struct inode *inode = page->mapping->host;
>>- int ret, needed_blocks = ext3_writepage_trans_blocks(inode);
>>+ int ret, ret2;
>>+ int needed_blocks = ext3_writepage_trans_blocks(inode);
>> handle_t *handle;
>> int retries = 0;
>>
>> retry:
>> handle = ext3_journal_start(inode, needed_blocks);
>>- if (IS_ERR(handle)) {
>>- ret = PTR_ERR(handle);
>>- goto out;
>>- }
>>+ if (IS_ERR(handle))
>>+ return PTR_ERR(handle);
>> if (test_opt(inode->i_sb, NOBH) && ext3_should_writeback_data(inode))
>> ret = nobh_prepare_write(page, from, to, ext3_get_block);
>> else
>> ret = block_prepare_write(page, from, to, ext3_get_block);
>> if (ret)
>>- goto prepare_write_failed;
>>+ goto failure;
>>
>> if (ext3_should_journal_data(inode)) {
>> ret = walk_page_buffers(handle, page_buffers(page),
>> from, to, NULL, do_journal_get_write_access);
>>+ if (ret)
>>+ /* fatal error, just put the handle and return */
>>+ journal_stop(handle);
>> }
>>-prepare_write_failed:
>>- if (ret)
>>- ext3_journal_stop(handle);
>>+ return ret;
>>+
>>+failure:
>>+ ret2 = ext3_prepare_failure(file, page, from, to);
>>+ if (ret2 < 0)
>>+ return ret2;
>> if (ret == -ENOSPC && ext3_should_retry_alloc(inode->i_sb, &retries))
>> goto retry;
>>-out:
>>+ /* retry number exceeded, or other error like -EDQUOT */
>> return ret;
>> }
>>

Kirill
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