Re: [PATCH 0/9] ext4: Stop using ext4_writepage() for writeout of ordered data

Ritesh Harjani (IBM)
2022-12-01T11:42:18Z

On 22/11/30 05:35PM, Jan Kara wrote:
> Hello,
>
> this patch series modifies ext4 so that we stop using ext4_writepage() for
> writeout of ordered data during transaction commit (through
> generic_writepages() from jbd2_journal_submit_inode_data_buffers()). Instead we
> directly call ext4_writepages() from the
> ext4_journal_submit_inode_data_buffers().

Hello Jan,

Do you think we should add a WARN_ON_ONCE() or something in
ext4_do_writepages() function where we might try to start a transaction
at [J]. Since we can now enter into ext4_do_writepages() from two places:
1. writeback
2. jbd2_journal_commit_transaction()
mpage_submit_page
mpage_prepare_extent_to_map
ext4_do_writepages
ext4_normal_submit_inode_data_buffers
ext4_journal_submit_inode_data_buffers
journal_submit_data_buffers
jbd2_journal_commit_transaction
kjournald2

So IIUC, we will call mpage_submit_page() in the first call to
mpage_prepare_extent_to_map() [1] itself. That may set mpd->scanned_until_end = 1
at the end of it. So then we should never enter into the while loop where we
start a journal txn.

But can it ever happen that we ever into this while loop for writing out
pages for journal_submit_data_buffers()?

ext4_do_writepages()
{
<...>
mpd->do_map = 0;
mpd->scanned_until_end = 0;
mpd->io_submit.io_end = ext4_init_io_end(inode, GFP_KERNEL);
<...>
[1] ret = mpage_prepare_extent_to_map(mpd);
/* Unlock pages we didn't use */
mpage_release_unused_pages(mpd, false);
/* Submit prepared bio */
ext4_io_submit(&mpd->io_submit);
ext4_put_io_end_defer(mpd->io_submit.io_end);
mpd->io_submit.io_end = NULL;
<...>
while (!mpd->scanned_until_end && wbc->nr_to_write > 0) {
/* For each extent of pages we use new io_end */
mpd->io_submit.io_end = ext4_init_io_end(inode, GFP_KERNEL);
<....>
BUG_ON(ext4_should_journal_data(inode));
needed_blocks = ext4_da_writepages_trans_blocks(inode);

/* start a new transaction */
[J] handle = ext4_journal_start_with_reserve(inode,
EXT4_HT_WRITE_PAGE, needed_blocks, rsv_blocks);
<...>
mpd->do_map = 1;

trace_ext4_da_write_pages(inode, mpd->first_page, wbc);
[2] ret = mpage_prepare_extent_to_map(mpd);
if (!ret && mpd->map.m_len)
ret = mpage_map_and_submit_extent(handle, mpd,
&give_up_on_write);
<...>
}
<...>
}

-ritesh