Re: Inode 2885482 (000000008e814f64): i_reserved_data_blocks (2) not cleared!

[Gao Xiang]

On Thu, Oct 14, 2021 at 05:57:32PM -0400, Theodore Ts'o wrote:
> On Fri, Oct 15, 2021 at 02:06:52AM +0800, Gao Xiang wrote:
> > On Thu, Oct 14, 2021 at 12:54:14PM +0000, Rantala, Tommi T. (Nokia - FI/Espoo) wrote:
> > > Hi,
> > > 
> > > I'm seeing these i_reserved_data_blocks not cleared! messages when using ext4
> > > with nodelalloc, message added in:
> > > 
> > >   commit 6fed83957f21eff11c8496e9f24253b03d2bc1dc
> > >   Author: Jeffle Xu <jefflexu@xxxxxxxxxxxxxxxxx>
> > >   Date:   Mon Aug 23 14:13:58 2021 +0800
> > > 
> > >       ext4: fix reserved space counter leakage
> > > 
> > > I can quickly reproduce in 5.15.0-rc5-00041-g348949d9a444 by doing some
> > > filesystem I/O while toggling delalloc:
> > > 
> > > 
> > > while true; do mount -o remount,nodelalloc /; sleep 1; mount -o remount,delalloc /; sleep 1; done &
> > > git clone linux xxx; rm -rf xxx
> > 
> > If I understand correctly, switching such option implies
> > sync inodes to write back exist delayed allocation blocks.
> 
> Well, no.  What it implies is that all writes after the remount into
> an unallocated portion of the file will be allocated at the time when
> the page is dirtied, instead of when the page is written back.  It's
> possible for some pages to be written using delayed allocation, and
> some other pages in the legacy "allocate on page dirty" mechanism.
> This can happen when the file system is remounted; it can also happen
> when the file system starts getting close to 100% full.  See the
> comment in ext4_nonda_switch:
> 
> 	/*
> 	 * switch to non delalloc mode if we are running low
> 	 * on free block. The free block accounting via percpu
> 	 * counters can get slightly wrong with percpu_counter_batch getting
> 	 * accumulated on each CPU without updating global counters
> 	 * Delalloc need an accurate free block accounting. So switch
> 	 * to non delalloc when we are near to error range.
> 	 */

Hi Ted,

Ok, thanks for the detailed behavior explanation yet I guess several
checks of "test_opt(inode->i_sb, DELALLOC)" could be somewhat racy
then? For example a check in __es_remove_extent() of extents_status.c?

Thanks,
Gao Xiang

> 
> Cheers,
> 
> 					- Ted