Re: Delayed allocation and page_lock vs transaction start ordering

Jan Kara
2008-05-28T09:33:25Z

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.
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?

Honza
--
Jan Kara <jack@suse.cz>
SUSE Labs, CR