Re: [RFC PATCH v2 5/8] ovl: mark overlayfs' inode dirty on shared writable mmap

[Chengguang Xu <cgxu519@mykernel.net>]

 ---- 在 星期二, 2020-11-03 01:30:52 Jan Kara <jack@suse.cz> 撰写 ----
 > On Sun 25-10-20 11:41:14, Chengguang Xu wrote:
 > > Overlayfs cannot be notified when mmapped area gets dirty,
 > > so we need to proactively mark inode dirty in ->mmap operation.
 > > 
 > > Signed-off-by: Chengguang Xu <cgxu519@mykernel.net>
 > > ---
 > >  fs/overlayfs/file.c | 4 ++++
 > >  1 file changed, 4 insertions(+)
 > > 
 > > diff --git a/fs/overlayfs/file.c b/fs/overlayfs/file.c
 > > index efccb7c1f9bc..cd6fcdfd81a9 100644
 > > --- a/fs/overlayfs/file.c
 > > +++ b/fs/overlayfs/file.c
 > > @@ -486,6 +486,10 @@ static int ovl_mmap(struct file *file, struct vm_area_struct *vma)
 > >          /* Drop reference count from new vm_file value */
 > >          fput(realfile);
 > >      } else {
 > > +        if (vma->vm_flags & (VM_SHARED|VM_MAYSHARE) &&
 > > +            vma->vm_flags & (VM_WRITE|VM_MAYWRITE))
 > > +            ovl_mark_inode_dirty(file_inode(file));
 > > +
 > 
 > But does this work reliably? I mean once writeback runs, your inode (as
 > well as upper inode) is cleaned. Then a page fault comes so file has dirty
 > pages again and would need flushing but overlayfs inode stays clean? Am I
 > missing something?
 > 

Yeah, this is key point of this approach, in order to  fix the issue I explicitly set 
I_DIRTY_SYNC flag in ovl_mark_inode_dirty(), so what i mean is during writeback
we will call into ->write_inode() by this flag(I_DIRTY_SYNC) and at that place
we get chance to check mapping and re-dirty overlay's inode. The code logic
like below in ovl_write_inode().

    if (mapping_writably_mapped(upper->i_mapping) ||
         mapping_tagged(upper->i_mapping, PAGECACHE_TAG_WRITEBACK))
                 iflag |= I_DIRTY_PAGES; 




Thanks,
Chengguang