On Tue, 2009-02-03 at 09:36 +0900, Hisashi Hifumi wrote:
> At 23:12 09/02/02, Chris Mason wrote:
> >On Mon, 2009-02-02 at 20:00 +0900, Hisashi Hifumi wrote:
> >> Hi Chris.
> >>
> >> I think it is needed to call mark_inode_dirty() when file size expands
> >> in order to flush metadata updates to HDD through sync() syscall or
> >> background_writeout().
> >>
> >
> >Thanks for reading through this code and sending the patch.
> >
> >I find the I_DIRTY flags one of the more confusing parts of the generic
> >fs writeback cdoe. But, I think what happens is the
> >btrfs_set_page_dirty function calls __set_page_dirty_nobuffers() which
> >does:
> >
> >if (mapping->host) {
> > /* !PageAnon && !swapper_space */
> > __mark_inode_dirty(mapping->host, I_DIRTY_PAGES);
> >}
> >
> >This should be enough to make sure the btrfs inodes are processed by
> >background writeout and sync(). Please let me know if I'm misreading
> >things.
>
> Surely, as you pointed out, btrfs_set_page_dirty calls
> if (mapping->host) {
> /* !PageAnon && !swapper_space */
> __mark_inode_dirty(mapping->host, I_DIRTY_PAGES);
> }
> through _set_page_dirty_nobuffers.
> But I_DIRTY_PAGES is not sufficient.
> To flush metadata update to HDD through sync(), I_DIRTY_SYNC or
> I_DIRTY_DATASYNC flag is needed. see __sync_single_inode.
Since btrfs uses a dirty_inode callback, our inodes are never really
dirty. The btree metadata always has the same information as the in-core
inode does.
The extra transaction commit steps taken at sync time are enough to get
all the relevant metadata on disk.
So, I think what happens is that I_DIRTY_PAGES is enough to get the data
pages on disk and the transaction commit gets the metadata on disk.
-chris
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