---- 在 星期二, 2020-11-03 01:30:52 Jan Kara <jack@xxxxxxx> 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@xxxxxxxxxxxx>
> > ---
> > 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
