linux-kernel - Re: [PATCH 5/6] eventfs: get rid of dentry pointers without refcounts

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Date: Tue, 30 Jan 2024 15:25:07 -0800
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: Masami Hiramatsu <mhiramat@...nel.org>, linux-kernel@...r.kernel.org, 
	linux-trace-kernel@...r.kernel.org
Subject: Re: [PATCH 5/6] eventfs: get rid of dentry pointers without refcounts

On Tue, 30 Jan 2024 at 15:10, Steven Rostedt <rostedt@...dmis.org> wrote:
> >
> > At which point a name pointer would *just* fit in 96 bytes.
>
> Does that mean I should keep the kstrdup_const()?

You should check that my math matches reality and relevant
configurations, but yes, at 96 bytes that should fit exactly in one
slab entry on both x86-64 and arm64 (now that arm finally does sane
kmalloc sizes - for the longest time they were all 128-byte aligned
due to historical horrible DMA coherency issues).

But you might also add a comment to the eventfs_inode definition,
because if that ever changes, the math changes again. For example,
adding just one more attribute data would make it all fall apart.

If you *really* want to optimize that data structure, I think you can
make the child list be a 'hlist'. That makes the list head ('children'
becomes a 'hlist_head') smaller, even if the list entry ('list'
becomes a 'hlist_node') stays the same size.

             Linus