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SubjectRe: Inode 2885482 (000000008e814f64): i_reserved_data_blocks (2) not cleared!
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@linux.alibaba.com>
> > > 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

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