Re: [PATCH 1/3] x86/resctrl: Remove superfluous kernfs_get() calls to prevent refcount leak

From: Borislav Petkov
Date: Fri Nov 20 2020 - 11:14:02 EST


On Sat, Oct 31, 2020 at 03:03:58AM +0800, Xiaochen Shen wrote:
> Willem reported growing of kernfs_node_cache entries in slabtop when
> repeatedly creating and removing resctrl subdirectories as well as when
> repeatedly mounting and unmounting resctrl filesystem.
>
> On resource group (control as well as monitoring) creation via a mkdir
> an extra kernfs_node reference is obtained to ensure that the rdtgroup
> structure remains accessible for the rdtgroup_kn_unlock() calls where it
> is removed on deletion. The kernfs_node reference count is dropped by
> kernfs_put() in rdtgroup_kn_unlock().
>
> With the above explaining the need for one kernfs_get()/kernfs_put()
> pair in resctrl there are more places where a kernfs_node reference is
> obtained without a corresponding release. The excessive amount of
> reference count on kernfs nodes will never be dropped to 0 and the
> kernfs nodes will never be freed in the call paths of rmdir and umount.
> It leads to reference count leak and kernfs_node_cache memory leak.
>
> Remove the superfluous kernfs_get() calls and expand the existing
> comments surrounding the remaining kernfs_get()/kernfs_put() pair that
> remains in use.
>
> Superfluous kernfs_get() calls are removed from two areas:
>
> (1) In call paths of mount and mkdir, when kernfs nodes for "info",
> "mon_groups" and "mon_data" directories and sub-directories are
> created, the reference count of newly created kernfs node is set to 1.
> But after kernfs_create_dir() returns, superfluous kernfs_get() are
> called to take an additional reference.
>
> (2) kernfs_get() calls in rmdir call paths.
>
> Cc: stable@xxxxxxxxxxxxxxx
> Fixes: 17eafd076291 ("x86/intel_rdt: Split resource group removal in two")
> Fixes: 4af4a88e0c92 ("x86/intel_rdt/cqm: Add mount,umount support")
> Fixes: f3cbeacaa06e ("x86/intel_rdt/cqm: Add rmdir support")
> Fixes: d89b7379015f ("x86/intel_rdt/cqm: Add mon_data")
> Fixes: c7d9aac61311 ("x86/intel_rdt/cqm: Add mkdir support for RDT monitoring")
> Fixes: 5dc1d5c6bac2 ("x86/intel_rdt: Simplify info and base file lists")
> Fixes: 60cf5e101fd4 ("x86/intel_rdt: Add mkdir to resctrl file system")
> Fixes: 4e978d06dedb ("x86/intel_rdt: Add "info" files to resctrl file system")

Are those 8(!) Fixes tags supposed to list *all* commits which add those
wrong kernfs_get() calls?

--
Regards/Gruss,
Boris.

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