Re: [DISCUSSION] Revisiting Slab Movable Objects

[Al Viro <viro@zeniv.linux.org.uk>]

On Wed, Apr 23, 2025 at 04:20:20PM +0900, Harry Yoo wrote:

> If we can't migrate or reclaim dentries with a nonzero refcount,
> can we at least prevent slab pages from containing a mix of dentries
> with zero and nonzero refcounts?
> 
> An idea: "Migrate a dentry (and inode?) _before_ it becomes unrelocatable"
> This is somewhat similar to "Migrate a page out of the movable area before
> pinning it" in MM.
> 
> For example, suppose we have two slab caches for dentry:
> dentry_cache_unref and dentry_cache_ref.
> 
> When a dentry with a zero refcount is about to have its refcount
> incremented, the VFS allocates a new object from dentry_cache_ref, copies
> the dentry into it, frees the original dentry back to
> dentry_cache_unref, and returns the newly allocated object.
> 
> Similarly when a dentry with a nonzero refcount drops to zero,
> it is migrated to dentry_cache_unref. This should be handled on the VFS
> side rather than by the slab allocator.
> 
> This approach could, at least, help reduce fragmentation.

No.  This is utterly insane - you'd need to insert RCU delay on each
of those transitions and that is not to mention the frequency with
which those will happen on a lot of loads (any kind of builds included).
Not a chance.