Re: [RFC PATCH v2 2/8] ovl: implement ->writepages operation

[Chengguang Xu <cgxu519@mykernel.net>]

 ---- 在 星期四, 2020-11-05 21:55:06 Jan Kara <jack@suse.cz> 撰写 ----
 > On Wed 04-11-20 20:18:16, Chengguang Xu wrote:
 > >  ---- 在 星期二, 2020-11-03 01:17:41 Jan Kara <jack@suse.cz> 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@mykernel.net>
 > >  > > ---
 > >  > >  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...
 > 

Yeah, I agree with you, I'll modify in next version.


Thanks,
Chengguang