Re: [PATCH v4 0/5] Fix easy bits of the negative dentry problem

[Ian Kent <raven@themaw.net>]

On Mon, 2026-10-19 at 11:00 +1100, NeilBrown wrote:
> For this v4 I decided to focus on just fsnotify, as that is where
> I've
> received the most useful review, and that is where problem reports
> seem
> to focus.
> 
> Other places that walk the ->d_children list (and don't already use
> cond_resched()) are specific to just one file-system-type.  fsnotify
> can
> affect (almost) any file-system.  So it is reasonable that it gets
> priority.
> 
> So here we just fix the refcount overflow, and the ->d_children walk
> for
> fsnotify.
> 
> If/when this lands, I can suggest patches to various file-system
> maintainers.
> 
> Thanks,
> NeilBrown
> 
> 
> Original intro - revised:
> 
>  I was reading about the negative dentry problem in
>  https://lwn.net/Articles/1079407/ and thought I would have a go at
> the
>  easy bits.  Given how involved some of that was I'm glad I didn't
> try
>  the tricky bits!
> 
>  So I didn't try to reduce the number of negative dentries or handle
>  them differently to positive dentries.  I just tried to address
>  specific symptoms.
> 
>  This series addresses two reported symptoms.
>   1/ refcounts on parents can potentially overflow if there are
> billions
>      of (negative) children.  This was the easiest easy bit.  The
> first
>      patch is all that is needed.
> 
>   2/ Walking the ->d_children list can take arbitrarily long and can
>      trigger soft lockups.  I haven't addresses the "arbitrarily
> long"
>      part, but the "soft lockups" is relatively easy.
>      With this in place I would feel more confident addressing
>      the "arbitrarily long" part if that seemed worth while.
> 
>  The core idea is to use a common helper for all walking of
>  ->d_children, and teach it to drop the lock and schedule when
> needed,
>  inserting a cursor to keep its place.  This simple idea requires
> that
>  nothing should be holding any other lock while walking ->d_children

As is so often the case you've come up with an interesting and useful
proposal, ;)

Nothing stood to me out as problematic given the scope of the series.

> 
> 
>  [PATCH v4 1/5] VFS: don't count references through ->d_parent
>  [PATCH v4 2/5] fsnotify: update children flags only when adding mask
>  [PATCH v4 3/5] fsnotify: don't hold i_lock while walking d_children
>  [PATCH v4 4/5] Add d_for_each_positive_child family of iterators
>  [PATCH v4 5/5] Use d_for_each_positive_child() in fsnotify.

Did you have any thoughts on how to contain the number of negative
dentries, (perhaps) including directory entry counts, in an intelligent
way (meaning "as you go" rather than "brute force" when needed)?

A significant problem, particularly on systems that count their memory
in TBs rather than GBs.

Ian