linux-ext4 - Re: [PATCH v2 1/8] migrate: fix skipping metadata buffer heads on migration

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Message-ID: <20250415-freihalten-tausend-a9791b9c3a03@brauner>
Date: Tue, 15 Apr 2025 11:05:38 +0200
From: Christian Brauner <brauner@...nel.org>
To: Luis Chamberlain <mcgrof@...nel.org>
Cc: Jan Kara <jack@...e.cz>, tytso@....edu, adilger.kernel@...ger.ca, 
	linux-ext4@...r.kernel.org, riel@...riel.com, dave@...olabs.net, willy@...radead.org, 
	hannes@...xchg.org, oliver.sang@...el.com, david@...hat.com, axboe@...nel.dk, 
	hare@...e.de, david@...morbit.com, djwong@...nel.org, ritesh.list@...il.com, 
	linux-fsdevel@...r.kernel.org, linux-block@...r.kernel.org, linux-mm@...ck.org, 
	gost.dev@...sung.com, p.raghav@...sung.com, da.gomez@...sung.com, 
	syzbot+f3c6fda1297c748a7076@...kaller.appspotmail.com
Subject: Re: [PATCH v2 1/8] migrate: fix skipping metadata buffer heads on
 migration

On Mon, Apr 14, 2025 at 03:19:33PM -0700, Luis Chamberlain wrote:
> On Mon, Apr 14, 2025 at 02:09:46PM -0700, Luis Chamberlain wrote:
> > On Thu, Apr 10, 2025 at 02:05:38PM +0200, Jan Kara wrote:
> > > > @@ -859,12 +862,12 @@ static int __buffer_migrate_folio(struct address_space *mapping,
> > > >  			}
> > > >  			bh = bh->b_this_page;
> > > >  		} while (bh != head);
> > > > +		spin_unlock(&mapping->i_private_lock);
> > > 
> > > No, you've just broken all simple filesystems (like ext2) with this patch.
> > > You can reduce the spinlock critical section only after providing
> > > alternative way to protect them from migration. So this should probably
> > > happen at the end of the series.
> > 
> > So you're OK with this spin lock move with the other series in place?
> > 
> > And so we punt the hard-to-reproduce corruption issue as future work
> > to do? Becuase the other alternative for now is to just disable
> > migration for jbd2:
> > 
> > diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> > index 1dc09ed5d403..ef1c3ef68877 100644
> > --- a/fs/ext4/inode.c
> > +++ b/fs/ext4/inode.c
> > @@ -3631,7 +3631,6 @@ static const struct address_space_operations ext4_journalled_aops = {
> >  	.bmap			= ext4_bmap,
> >  	.invalidate_folio	= ext4_journalled_invalidate_folio,
> >  	.release_folio		= ext4_release_folio,
> > -	.migrate_folio		= buffer_migrate_folio_norefs,
> >  	.is_partially_uptodate  = block_is_partially_uptodate,
> >  	.error_remove_folio	= generic_error_remove_folio,
> >  	.swap_activate		= ext4_iomap_swap_activate,
> 
> BTW I ask because.. are your expectations that the next v3 series also
> be a target for Linus tree as part of a fix for this spinlock
> replacement?

Since this is fixing potential filesystem corruption I will upstream
whatever we need to do to fix this. Ideally we have a minimal fix to
upstream now and a comprehensive fix and cleanup for v6.16.