Re: [PATCH v11] ovl: Improving syncfs efficiency

[Miklos Szeredi <miklos@szeredi.hu>]

On Thu, Apr 16, 2020 at 8:09 AM Chengguang Xu <cgxu519@mykernel.net> wrote:
>
>  ---- 在 星期四, 2020-04-16 03:19:50 Miklos Szeredi <miklos@szeredi.hu> 撰写 ----
>  > On Mon, Feb 10, 2020 at 4:10 AM Chengguang Xu <cgxu519@mykernel.net> wrote:

>  > > +               if (current->flags & PF_MEMALLOC) {
>  > > +                       spin_lock(&inode->i_lock);
>  > > +                       ovl_set_flag(OVL_WRITE_INODE_PENDING, inode);
>  > > +                       wqh = bit_waitqueue(&oi->flags,
>  > > +                                       OVL_WRITE_INODE_PENDING);
>  > > +                       prepare_to_wait(wqh, &wait.wq_entry,
>  > > +                                       TASK_UNINTERRUPTIBLE);
>  > > +                       spin_unlock(&inode->i_lock);
>  > > +
>  > > +                       ovl_wiw.inode = inode;
>  > > +                       INIT_WORK(&ovl_wiw.work, ovl_write_inode_work_fn);
>  > > +                       schedule_work(&ovl_wiw.work);
>  > > +
>  > > +                       schedule();
>  > > +                       finish_wait(wqh, &wait.wq_entry);
>  >
>  > What is the reason to do this in another thread if this is a PF_MEMALLOC task?
>
> Some underlying filesystems(for example ext4) check the flag in ->write_inode()
> and treate it as an abnormal case.(warn and return)
>
> ext4_write_inode():
>         if (WARN_ON_ONCE(current->flags & PF_MEMALLOC) ||
>                 sb_rdonly(inode->i_sb))
>                         return 0;
>
> overlayfs inodes are always keeping clean even after wring/modifying  upperfile ,
> so they are right target of kswapd  but in the point of lower layer, ext4 just thinks
> kswapd is choosing a wrong dirty inode to reclam memory.

I don't get it.  Why can't overlayfs just skip the writeback of upper
inode in the reclaim case?  It will be written back through the normal
relcaim channels.

Thanks,
Miklos