Re: [PATCH] ext4: cache es->s_journal_inum in ext4_sb_info

[Ojaswin Mujoo]

On Sat, Mar 15, 2025 at 09:41:28PM -0400, Theodore Ts'o wrote:
> On Fri, Mar 14, 2025 at 05:11:43PM +0530, Ojaswin Mujoo wrote:
> > Currently, we access journal ino through sbi->s_es->s_journal_inum,
> > which directly reads from the ext4 sb buffer head. If someone modifies
> > this underneath us then the s_journal_inum field might get corrupted.
> > 
> > Although direct block device modifications can be expected to cause
> > issues in the FS, let's cache s_journal_inum in sbi->s_journal_ino so
> > our checks can be more resillient.
> 
> The reason why the block validity checks need to check against
> s_journal_ino is to exempt the lookups done by ext4_journal_bmap()
> from running afoul of the system zone checks, since the journal's data
> blocks are considered part of the system zone.
> 
> So this is something we need to do if the journal is actived, and if
> it's active, then sbi->s_journal will be non-NULL, and so we can just
> check to see if inode == sbi->s_journal instead.  This will simplify

I believe you mean inode == sbi->s_journal->j_inode here right?

since that is enough to confirm if we are called via ext4_journal_bmap
so we can avoid validity checks.

Regards,
ojaswin

> the code, without needing to expand the ext4_sb_info structure.
> 
> Cheers,
> 
> 						- Ted