On 2025/6/20 22:18, Jan Kara wrote: > 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? > Yeah, this solution looks nice to me, it should works! Thank you for the suggestion. Best regards, Yi.