Re: [PATCH v2 1/3] jbd2: store jinode dirty range in PAGE_SIZE units

[Andreas Dilger]

On Feb 19, 2026, at 04:46, Li Chen <me@linux.beauty> wrote:
> 
> jbd2_inode fields are updated under journal->j_list_lock, but some paths
> read them without holding the lock (e.g. fast commit helpers and ordered
> truncate helpers).
> 
> READ_ONCE() alone is not sufficient for i_dirty_start/end as they are
> loff_t and 32-bit platforms can observe torn loads. Store the dirty range
> in PAGE_SIZE units as pgoff_t so lockless readers can take non-torn
> snapshots.

When making semantic changes like this, it is best to change the variable
names as well, so that breaks compilation if bisection happens to land
between these patches.  Otherwise, that could cause some random behavior
if jbd2 is treating these as pages, but ext4/ocfs2 are treating them as
bytes or vice versa.

Something like i_dirty_{start,end} -> i_dirty_{start,end}_page would make
this very clear what the units are.

To avoid breakage between the patches (which is desirable to avoid problems
with automated bisection) you should make an initial patch with wrappers to
access these values and convert ext4/ocfs2 to use them:

static inline loff_t jbd2_jinode_dirty_start(struct jbd2_inode *jinode)
{
	return jinode->i_dirty_start;
}

static inline loff_t jbd2_jinode_dirty_end(struct jbd2_inode *jinode)
{
	return jinode->i_dirty_end;
}

then change this in the jbd2 patch at the end, which would then be self-contained:

static inline loff_t jbd2_jinode_dirty_start(struct jbd2_inode *jinode)
{
	return (loff_t)jinode->i_dirty_start_page << PAGE_SHIFT;
}

static inline loff_t jbd2_jinode_dirty_end(struct jbd2_inode *jinode)
{
	return ((loff_t)jinode->i_dirty_end_page << PAGE_SHIFT) + ~PAGE_MASK;
}


Cheers, Andreas

> Use READ_ONCE() on the read side and WRITE_ONCE() on the write side for
> the dirty range and i_flags to match the existing lockless access pattern.
> 
> Suggested-by: Jan Kara <jack@xxxxxxx>
> Reviewed-by: Jan Kara <jack@xxxxxxx>
> Signed-off-by: Li Chen <me@linux.beauty>
> ---
>