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Date: Tue, 29 Sep 2020 13:48:25 +0200
From: Jan Kara <jack@...e.cz>
To: Mauricio Faria de Oliveira <mfo@...onical.com>
Cc: Jan Kara <jack@...e.cz>, linux-ext4@...r.kernel.org,
dann frazier <dann.frazier@...onical.com>
Subject: Re: [RFC PATCH v4 3/4] ext4: data=journal: fixes for
ext4_page_mkwrite()
On Mon 28-09-20 16:41:02, Mauricio Faria de Oliveira wrote:
> These are two fixes for data journalling required by
> the next patch, discovered while testing it.
>
> First, the optimization to return early if all buffers
> are mapped is not appropriate for the next patch:
>
> The inode _must_ be added to the transaction's list in
> data=journal mode (so to write-protect pages on commit)
> thus we cannot return early there.
>
> Second, once that optimization to reduce transactions
> was disabled for data=journal mode, more transactions
> happened, and occasionally hit this warning message:
> 'JBD2: Spotted dirty metadata buffer'.
>
> Reason is, block_page_mkwrite() will set_buffer_dirty()
> before do_journal_get_write_access() that is there to
> prevent it. This issue was masked by the optimization.
>
> So, on data=journal use __block_write_begin() instead.
> This also requires page locking and len recalculation.
> (see block_page_mkwrite() for implementation details.)
>
> Finally, as Jan noted there is little sharing between
> data=journal and other modes in ext4_page_mkwrite().
>
> However, a prototype of ext4_journalled_page_mkwrite()
> showed there still would be lots of duplicated lines
> (tens of) that didn't seem worth it.
>
> Thus this patch ends up with an ugly goto to skip all
> non-data journalling code (to avoid long indentations,
> but that can be changed..) in the beginning, and just
> a conditional in the transaction section.
>
> Well, we skip a common part to data journalling which
> is the page truncated check, but we do it again after
> ext4_journal_start() when we re-acquire the page lock
> (so not to acquire the page lock twice needlessly for
> data journalling.)
>
> Signed-off-by: Mauricio Faria de Oliveira <mfo@...onical.com>
> Suggested-by: Jan Kara <jack@...e.cz>
The patch looks good to me. You can add:
Reviewed-by: Jan Kara <jack@...e.cz>
Honza
> ---
> fs/ext4/inode.c | 51 ++++++++++++++++++++++++++++++++++++++++++-------
> 1 file changed, 44 insertions(+), 7 deletions(-)
>
> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> index bf596467c234..ac153e340a6f 100644
> --- a/fs/ext4/inode.c
> +++ b/fs/ext4/inode.c
> @@ -5977,9 +5977,17 @@ vm_fault_t ext4_page_mkwrite(struct vm_fault *vmf)
> if (err)
> goto out_ret;
>
> + /*
> + * On data journalling we skip straight to the transaction handle:
> + * there's no delalloc; page truncated will be checked later; the
> + * early return w/ all buffers mapped (calculates size/len) can't
> + * be used; and there's no dioread_nolock, so only ext4_get_block.
> + */
> + if (ext4_should_journal_data(inode))
> + goto retry_alloc;
> +
> /* Delalloc case is easy... */
> if (test_opt(inode->i_sb, DELALLOC) &&
> - !ext4_should_journal_data(inode) &&
> !ext4_nonda_switch(inode->i_sb)) {
> do {
> err = block_page_mkwrite(vma, vmf,
> @@ -6005,6 +6013,9 @@ vm_fault_t ext4_page_mkwrite(struct vm_fault *vmf)
> /*
> * Return if we have all the buffers mapped. This avoids the need to do
> * journal_start/journal_stop which can block and take a long time
> + *
> + * This cannot be done for data journalling, as we have to add the
> + * inode to the transaction's list to writeprotect pages on commit.
> */
> if (page_has_buffers(page)) {
> if (!ext4_walk_page_buffers(NULL, page_buffers(page),
> @@ -6029,16 +6040,42 @@ vm_fault_t ext4_page_mkwrite(struct vm_fault *vmf)
> ret = VM_FAULT_SIGBUS;
> goto out;
> }
> - err = block_page_mkwrite(vma, vmf, get_block);
> - if (!err && ext4_should_journal_data(inode)) {
> - if (ext4_walk_page_buffers(handle, page_buffers(page), 0,
> - PAGE_SIZE, NULL, do_journal_get_write_access)) {
> + /*
> + * Data journalling can't use block_page_mkwrite() because it
> + * will set_buffer_dirty() before do_journal_get_write_access()
> + * thus might hit warning messages for dirty metadata buffers.
> + */
> + if (!ext4_should_journal_data(inode)) {
> + err = block_page_mkwrite(vma, vmf, get_block);
> + } else {
> + lock_page(page);
> + size = i_size_read(inode);
> + /* Page got truncated from under us? */
> + if (page->mapping != mapping || page_offset(page) > size) {
> unlock_page(page);
> - ret = VM_FAULT_SIGBUS;
> + ret = VM_FAULT_NOPAGE;
> ext4_journal_stop(handle);
> goto out;
> }
> - ext4_set_inode_state(inode, EXT4_STATE_JDATA);
> +
> + if (page->index == size >> PAGE_SHIFT)
> + len = size & ~PAGE_MASK;
> + else
> + len = PAGE_SIZE;
> +
> + err = __block_write_begin(page, 0, len, ext4_get_block);
> + if (!err) {
> + if (ext4_walk_page_buffers(handle, page_buffers(page),
> + 0, len, NULL, do_journal_get_write_access)) {
> + unlock_page(page);
> + ret = VM_FAULT_SIGBUS;
> + ext4_journal_stop(handle);
> + goto out;
> + }
> + ext4_set_inode_state(inode, EXT4_STATE_JDATA);
> + } else {
> + unlock_page(page);
> + }
> }
> ext4_journal_stop(handle);
> if (err == -ENOSPC && ext4_should_retry_alloc(inode->i_sb, &retries))
> --
> 2.17.1
>
--
Jan Kara <jack@...e.com>
SUSE Labs, CR
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