On Fri 20-06-25 13:00:32, Zhang Yi wrote:
> On 2025/6/20 0:33, Jan Kara wrote:
> > On Wed 11-06-25 19:16:22, Zhang Yi wrote:
> >> From: Zhang Yi <yi.zhang@xxxxxxxxxx>
> >>
> >> After large folios are supported on ext4, writing back a sufficiently
> >> large and discontinuous folio may consume a significant number of
> >> journal credits, placing considerable strain on the journal. For
> >> example, in a 20GB filesystem with 1K block size and 1MB journal size,
> >> writing back a 2MB folio could require thousands of credits in the
> >> worst-case scenario (when each block is discontinuous and distributed
> >> across different block groups), potentially exceeding the journal size.
> >> This issue can also occur in ext4_write_begin() and ext4_page_mkwrite()
> >> when delalloc is not enabled.
> >>
> >> Fix this by ensuring that there are sufficient journal credits before
> >> allocating an extent in mpage_map_one_extent() and _ext4_get_block(). If
> >> there are not enough credits, return -EAGAIN, exit the current mapping
> >> loop, restart a new handle and a new transaction, and allocating blocks
> >> on this folio again in the next iteration.
> >>
> >> Suggested-by: Jan Kara <jack@xxxxxxx>
> >> Signed-off-by: Zhang Yi <yi.zhang@xxxxxxxxxx>
> >
> > ...
> >
> >> static int _ext4_get_block(struct inode *inode, sector_t iblock,
> >> struct buffer_head *bh, int flags)
> >> {
> >> struct ext4_map_blocks map;
> >> + handle_t *handle = ext4_journal_current_handle();
> >> int ret = 0;
> >>
> >> if (ext4_has_inline_data(inode))
> >> return -ERANGE;
> >>
> >> + /* Make sure transaction has enough credits for this extent */
> >> + if (flags & EXT4_GET_BLOCKS_CREATE) {
> >> + ret = ext4_journal_ensure_extent_credits(handle, inode);
> >> + if (ret)
> >> + return ret;
> >> + }
> >> +
> >> map.m_lblk = iblock;
> >> map.m_len = bh->b_size >> inode->i_blkbits;
> >>
> >> - ret = ext4_map_blocks(ext4_journal_current_handle(), inode, &map,
> >> - flags);
> >> + ret = ext4_map_blocks(handle, inode, &map, flags);
> >
> > Good spotting with ext4_page_mkwrite() and ext4_write_begin() also needing
> > this treatment! But rather then hiding the transaction extension in
> > _ext4_get_block() I'd do this in ext4_block_write_begin() where it is much
> > more obvious (and also it is much more obvious who needs to be prepared for
> > handling EAGAIN error). Otherwise the patch looks good!
> >
>
> Yes, I completely agree with you. However, unfortunately, do this in
> ext4_block_write_begin() only works for ext4_write_begin().
> ext4_page_mkwrite() does not call ext4_block_write_begin() to allocate
> blocks, it call the vfs helper __block_write_begin_int() instead.
>
> vm_fault_t ext4_page_mkwrite(struct vm_fault *vmf)
> {
> ...
> if (!ext4_should_journal_data(inode)) {
> err = block_page_mkwrite(vma, vmf, get_block);
> ...
> }
>
>
> So...
Right, I forgot about the nodelalloc case. But since we do most of things
by hand for data=journal mode, perhaps we could lift some code from
data=journal mode and reuse it for nodelalloc as well like:
folio_lock(folio);
size = i_size_read(inode);
/* Page got truncated from under us? */
if (folio->mapping != mapping || folio_pos(folio) > size) {
ret = VM_FAULT_NOPAGE;
goto out_error;
}
len = folio_size(folio);
if (folio_pos(folio) + len > size)
len = size - folio_pos(folio);
err = ext4_block_write_begin(handle, folio, 0, len,
get_block);
if (err)
goto out_error;
if (!ext4_should_journal_data(inode))
block_commit_write(folio, 0, len);
folio_mark_dirty(folio);
} else {
if (ext4_journal_folio_buffers(handle, folio, len)) {
ret = VM_FAULT_SIGBUS;
goto out_error;
}
}
ext4_journal_stop(handle);
folio_wait_stable(folio);
We get an additional bonus for not waiting for page writeback with
transaction handle held (which is a potential deadlock vector). What do you
think?
Honza
--
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR
