Re: git: 460ed6106cf0 - main - Add support for managing UFS/FFS snapshots to fsck_ffs(8).

On Thu, Nov 10, 2022 at 02:31:22AM +0000, Glen Barber wrote:
> On Wed, Nov 09, 2022 at 06:46:51PM +0000, Kirk McKusick wrote:
> > The branch main has been updated by mckusick:
> > 
> > URL: https://cgit.FreeBSD.org/src/commit/?id=460ed6106cf0854caff62e4eeba8ffcd00ab0690
> > 
> > commit 460ed6106cf0854caff62e4eeba8ffcd00ab0690
> > Author:     Kirk McKusick <mckusick@FreeBSD.org>
> > AuthorDate: 2022-11-09 18:44:03 +0000
> > Commit:     Kirk McKusick <mckusick@FreeBSD.org>
> > CommitDate: 2022-11-09 18:46:31 +0000
> > 
> >     Add support for managing UFS/FFS snapshots to fsck_ffs(8).
> >     
> >     The kernel handles the managment of UFS/FFS snapshots. Since UFS/FFS
> >     updates filesystem data (rather than always writing changes to new
> >     locations like ZFS), the kernel must check every filesystem write
> >     to see if the block being written is part of a snapshot. If it is
> >     part of a snapshot, then the kernel must make a copy of the old
> >     block value into a newly allocated block for the snapshot before
> >     allowing the write to be done. Similarly, if a block is being freed,
> >     the kernel must check to see if it is part of a snapshot and let
> >     the snapshot claim the block rather than freeing it for future use.
> >     When a snapshot is freed, its blocks need to be offered to older
> >     snapshots and freed only if no older snapshots wish to claim them.
> >     
> >     When snapshots were added to UFS/FFS they were integrated into soft
> >     updates and just a small part of the management of snapshots needed
> >     to be added to fsck_ffs(8) as soft updates minimized the set of
> >     snapshot changes that might need correction. When journaling was
> >     added to soft updates a much more complete knowledge of snapshots
> >     needed to be added to fsck_ffs(8) for it to be able to properly
> >     handle the filesystem changes that a journal rollback needs to do
> >     (specifically the freeing and allocation of blocks). Since this
> >     functionality was unavailable, the use of snapshots was disabled
> >     when running with journaled soft updates.
> >     
> >     This set of changes imports the kernel code for the management of
> >     snapshots to fsck_ffs(8). With this code in place it will become
> >     possible to enable snapshots when running with journalled soft
> >     updates. The most immediate benefit will be the ability to use
> >     snapshots to take consistent filesystem dumps on live filesystems.
> >     Future work will be done to update fsck_ffs(8) to be able to use
> >     snapshots to run in background on live filesystems running with
> >     journaled soft updates.
> >     
> 
> I think this broke the main build.
> 
> Notably:
> 
> % @@ -483,11 +507,38 @@ flush(int fd, struct bufarea *bp)
> %                 }
> %                 /* FALLTHROUGH */
> %         default:
> % +               copyonwrite(&sblock, bp, std_checkblkavail);
> %                 blwrite(fd, bp->b_un.b_buf, bp->b_bno, bp->b_size);
> %                 break;
> 
> https://ci.freebsd.org/job/FreeBSD-main-i386-build/24590/console
> 

Oddly, the failure email I received was from stable/13, not main, but it
was the exact same failure case according to the logs.

That said, I see you had committed a fix already, so please disregard
this email for now.

Glen