Re: [PATCH v2 15/54] fs: maintain a list of pinned inodes

[Josef Bacik <josef@toxicpanda.com>]

On Wed, Aug 27, 2025 at 05:20:17PM +0200, Christian Brauner wrote:
> On Tue, Aug 26, 2025 at 11:39:15AM -0400, Josef Bacik wrote:
> > Currently we have relied on dirty inodes and inodes with cache on them
> > to simply be left hanging around on the system outside of an LRU. The
> > only way to make sure these inodes are eventually reclaimed is because
> > dirty writeback will grab a reference on the inode and then iput it when
> > it's done, potentially getting it on the LRU. For the cached case the
> > page cache deletion path will call inode_add_lru when the inode no
> > longer has cached pages in order to make sure the inode object can be
> > freed eventually.  In the unmount case we walk all inodes and free them
> > so this all works out fine.
> > 
> > But we want to eliminate 0 i_count objects as a concept, so we need a
> > mechanism to hold a reference on these pinned inodes. To that end, add a
> > list to the super block that contains any inodes that are cached for one
> > reason or another.
> > 
> > When we call inode_add_lru(), if the inode falls into one of these
> > categories, we will add it to the cached inode list and hold an
> > i_obj_count reference.  If the inode does not fall into one of these
> > categories it will be moved to the normal LRU, which is already holds an
> > i_obj_count reference.
> > 
> > The dirty case we will delete it from the LRU if it is on one, and then
> > the iput after the writeout will make sure it's placed onto the correct
> > list at that point.
> > 
> > The page cache case will migrate it when it calls inode_add_lru() when
> > deleting pages from the page cache.
> > 
> > Signed-off-by: Josef Bacik <josef@toxicpanda.com>
> > ---
> 
> Ok, I'm trying to wrap my head around the justification for this new
> list. Currently we have inodes with a zero reference counts that aren't
> on any LRU. They just appear on sb->i_sb_list and are e.g., dealt with
> during umount (sync_filesystem() followed by evict_inodes()).
> 
> So they're either dealt with by writeback or by the page cache and are
> eventually put on the regular LRU or the filesystem shuts down before
> that happens.
> 
> They're easy to handle and recognize because their inode->i_count is
> zero.
> 
> Now you make the LRUs hold a full reference so it can be grabbed from
> the LRU again avoiding the zombie resurrection from zero. So to
> recognize inodes that are pinned internally due to being dirty or having
> pagecache pages attached to it you need to track them in a new list
> otherwise you can't really differentiate them and when to move them onto
> the LRU after writeback and pagecache is done with them.
> 

Exactly. We need to put them somewhere so we can account for their reference.

We could technically just use a flag and not have a list for this, and just use
the flag to indicate that the inode is pinned and the flag has a full reference
associated with it.

I did it this way because if I had a nickel for every time I needed to figure
out where a zombie inode was and had to do the most grotesque drgn magic to find
it, I'd have like 15 cents, which isn't a lot but weird that it's happened 3
times. Having a list makes it easier from a debugging perspective.

But again, we have ->s_inodes, and I can just scan that list and look for
I_LRU_CACHED. We'd still need to hold a full reference for that, but it would
eliminate the need for another list if that's more preferable?  Thanks,

Josef