linux-kernel - Re: [PATCH v2] landlock: Use kmem for landlock

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Message-ID: <ZgmmnLIal3gz55Q+@ayush-HP-Pavilion-Gaming-Laptop-15-ec0xxx>
Date: Sun, 31 Mar 2024 23:38:28 +0530
From: Ayush Tiwari <ayushtiw0110@...il.com>
To: Greg KH <gregkh@...uxfoundation.org>
Cc: alison.schofield@...el.com, paul@...l-moore.com, mic@...ikod.net,
	fabio.maria.de.francesco@...ux.intel.com,
	linux-kernel@...r.kernel.org, outreachy@...ts.linux.dev,
	linux-security-module@...r.kernel.org
Subject: Re: [PATCH v2] landlock: Use kmem for landlock_object

On Sun, Mar 31, 2024 at 06:54:39PM +0200, Greg KH wrote:
> On Sun, Mar 31, 2024 at 09:12:06PM +0530, Ayush Tiwari wrote:
> > Hello Greg. Thanks for the feedback.
> > On Sat, Mar 30, 2024 at 05:12:18PM +0100, Greg KH wrote:
> > > On Sat, Mar 30, 2024 at 07:24:19PM +0530, Ayush Tiwari wrote:
> > > > Use kmem_cache replace kzalloc() calls with kmem_cache_zalloc() for
> > > > struct landlock_object and update the related dependencies to improve
> > > > memory allocation and deallocation performance.
> > > 
> > > So it's faster?  Great, what are the measurements?
> > > 
> > Thank you for the feedback. Regarding the performance improvements, I
> > realized I should have provided concrete measurements to support the
> > claim. The intention behind switching to kmem_cache_zalloc() was to
> > optimize memory management efficiency based on general principles.
> > Could you suggest some way to get some measurements if possible?
> 
> If you can not measure the difference, why make the change at all?

Kindly refer to this issue: https://github.com/landlock-lsm/linux/issues/19
I have been assigned this issue hence I am focussing on making the
changes that have been listed.
> 
> Again, you need to prove the need for this change, so far I fail to see
> a reason why.
> 
> > > > +static struct kmem_cache *landlock_object_cache;
> > > > +
> > > > +void __init landlock_object_cache_init(void)
> > > > +{
> > > > +	landlock_object_cache = kmem_cache_create(
> > > > +		"landlock_object_cache", sizeof(struct landlock_object), 0,
> > > > +		SLAB_PANIC, NULL);
> > > 
> > > You really want SLAB_PANIC?  Why?
> > >
> > The SLAB_PANIC flag used in kmem_cache_create indicates that if the
> > kernel is unable to create the cache, it should panic. The use of
> > SLAB_PANIC in the creation of the landlock_object_cache is due to the
> > critical nature of this cache for the Landlock LSM's operation. I
> > found it to be a good choice to be used. Should I use some other
> > altrnative?
> 
> Is panicing really a good idea?  Why can't you properly recover from
> allocation failures?

I am relying on SLAB_PANIC because of the reason I mentioned earlier,
and also because it was used in lsm_file_cache that I was asked to look
into as reference. I could try to recover from allocation failures but
currently my focus is on working on the changes that are listed. I will
definitely try to look into it once I am done with all changes.

> > > > +
> > > >  struct landlock_object *
> > > >  landlock_create_object(const struct landlock_object_underops *const underops,
> > > >  		       void *const underobj)
> > > > @@ -25,7 +34,8 @@ landlock_create_object(const struct landlock_object_underops *const underops,
> > > >  
> > > >  	if (WARN_ON_ONCE(!underops || !underobj))
> > > >  		return ERR_PTR(-ENOENT);
> > > > -	new_object = kzalloc(sizeof(*new_object), GFP_KERNEL_ACCOUNT);
> > > > +	new_object =
> > > > +		kmem_cache_zalloc(landlock_object_cache, GFP_KERNEL_ACCOUNT);
> > > 
> > > Odd indentation, why?
> > > 
> > This indentation is due to formatting introduced by running
> > clang-format.
> 
> Why not keep it all on one line?
> 
I kept it all in one line in v1, but Paul and Mickael asked me to use
clang-format, hence it is this way.
> thanks,
> 
> greg k-h