On Wed 04-11-20 20:18:16, Chengguang Xu wrote:
> ---- 在 星期二, 2020-11-03 01:17:41 Jan Kara <jack@xxxxxxx> 撰写 ----
> > On Sun 25-10-20 11:41:11, Chengguang Xu wrote:
> > > Implement overlayfs' ->writepages operation so that
> > > we can sync dirty data/metadata to upper filesystem.
> > >
> > > Signed-off-by: Chengguang Xu <cgxu519@xxxxxxxxxxxx>
> > > ---
> > > fs/overlayfs/inode.c | 26 ++++++++++++++++++++++++++
> > > 1 file changed, 26 insertions(+)
> > >
> > > diff --git a/fs/overlayfs/inode.c b/fs/overlayfs/inode.c
> > > index b584dca845ba..f27fc5be34df 100644
> > > --- a/fs/overlayfs/inode.c
> > > +++ b/fs/overlayfs/inode.c
> > > @@ -11,6 +11,7 @@
> > > #include <linux/posix_acl.h>
> > > #include <linux/ratelimit.h>
> > > #include <linux/fiemap.h>
> > > +#include <linux/writeback.h>
> > > #include "overlayfs.h"
> > >
> > >
> > > @@ -516,7 +517,32 @@ static const struct inode_operations ovl_special_inode_operations = {
> > > .update_time = ovl_update_time,
> > > };
> > >
> > > +static int ovl_writepages(struct address_space *mapping,
> > > + struct writeback_control *wbc)
> > > +{
> > > + struct inode *upper_inode = ovl_inode_upper(mapping->host);
> > > + struct ovl_fs *ofs = mapping->host->i_sb->s_fs_info;
> > > + struct writeback_control tmp_wbc = *wbc;
> > > +
> > > + if (!ovl_should_sync(ofs))
> > > + return 0;
> > > +
> > > + /*
> > > + * for sync(2) writeback, it has a separate external IO
> > > + * completion path by checking PAGECACHE_TAG_WRITEBACK
> > > + * in pagecache, we have to set for_sync to 0 in thie case,
> > > + * let writeback waits completion after syncing individual
> > > + * dirty inode, because we haven't implemented overlayfs'
> > > + * own pagecache yet.
> > > + */
> > > + if (wbc->for_sync && (wbc->sync_mode == WB_SYNC_ALL))
> > > + tmp_wbc.for_sync = 0;
> >
> > This looks really hacky as it closely depends on the internal details of
> > writeback implementation. I'd be more open to say export wait_sb_inodes()
> > for overlayfs use... Because that's what I gather you need in your
> > overlayfs ->syncfs() implementation.
> >
>
> Does that mean we gather synced overlay's inode into a new waiting list(overlay's) and
> do the waiting behavior in overlay's ->syncfs() ?
My idea was that you'd just use the standard writeback logic which ends up
gathering upper_sb inodes in the upper_sb->s_inodes_wb and then wait for
them in overlay's ->syncfs(). Maybe we'll end up waiting for more inodes
than strictly necessary but it shouldn't be too bad I'd say...
Honza
--
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR
