Re: Delayed allocation and page_lock vs transaction start ordering

Aneesh Kumar K.V
2008-05-28T09:44:35Z

On Wed, May 28, 2008 at 11:33:24AM +0200, Jan Kara wrote:
> On Tue 27-05-08 20:41:28, Aneesh Kumar K.V wrote:
> > On Tue, May 27, 2008 at 02:43:12PM +0200, Jan Kara wrote:
> > > On Mon 26-05-08 23:30:43, Aneesh Kumar K.V wrote:
> > >
> > > > I have got another question now related to page_mkwrite. AFAIU writepage
> > > > writeout dirty buffer_heads. It also looks at whether the pages are
> > > > dirty or not. In the page_mkwrite callback both are not true. ie we call
> > > > set_page_dirty from do_wp_page after calling page_mkwrite. I haven't
> > > > verified whether the above is correct or not. Just thinking reading the
> > > > code.
> > > Writepage call itself doesn't look at whether the page is dirty or not -
> > > that flag is already cleared when writepage is called. You are right that
> > > the page is marked dirty only after page_mkwrite is called - the meaning of
> > > page_mkwrite() call is roughly "someone wants to do the first write to this
> > > page via mmap, prepare filesystem for that". But we don't really care
> > > whether the page is dirty or not - we know it carries correct data (it is
> > > uptodate) and so we can write it if we want (and need).
> > >
> >
> > I am looking at __block_write_full_page and we have
> >
> > if (!buffer_mapped(bh) && buffer_dirty(bh)) {
> > WARN_ON(bh->b_size != blocksize);
> > err = get_block(inode, block, bh, 1);
> > if (err)
> >
> > ie, we do get_block only if the buffer_head is dirty. So I am bit
> > doubtful whether we are actually allocating blocks via page_mkwrite.
> Good catch, we should mark unmapped buffers dirty before calling writepage.
> Actually, if the page didn't have any buffers, block_write_full_page() will
> create them all dirty so that's probably why I didn't hit it in my testing
> but it's definitely safer to mark them dirty explicitely. Thanks.

looking at create_empty_buffers we do that only if page is marked as
dirty. In the case of page_mkwrite the page is also not marked dirty
when we call the call back right ?

> It is enough to change ext4_bh_mapped() to something like:
> static int ext4_bh_prepare_fill(handle_t *handle, struct buffer_head *bh)
> {
> if (!buffer_mapped(bh)) {
> /*
> * Mark buffer as dirty so that block_write_full_page()
> * writes it
> */
> set_buffer_dirty(bh);
> return 1;
> }
> return 0;
> }
>
> Should I send you an updated patch with this change and the changes we spoke
> about yesterday, or just an incremental changes which you will fold yourself
> into the big one?
>

This will mark only the first unmapped buffer_head as dirty. What about
the rest of the buffer_heads in the page that are unmapped ?

I am looking at pushing the ext4_page_mkwrite before rest of the
changes. That is needed to handle ENOSPC when mmap write to files with
holes.

-aneesh