linux-kernel - Re: [PATCH] maple_tree: add mas_node_count() before going to slow_path in mas_wr

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Date: Mon, 3 Jun 2024 08:29:41 -0400
From: "Liam R. Howlett" <Liam.Howlett@...cle.com>
To: JaeJoon Jung <rgbi3307@...il.com>
Cc: maple-tree@...ts.infradead.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] maple_tree: add mas_node_count() before going to
 slow_path in mas_wr_modify()

* JaeJoon Jung <rgbi3307@...il.com> [240602 05:06]:
> Hello, Liam.
> Thank you very much for the detailed answer and explanation.
> 
> I tested this patch in userspace.
> In user space, this phenomenon always occurs when kmem_cache_alloc()
> is executed to allocate a new node.

This is expected in the userspace test program.  We test the error path
most frequently, with only a bypass for those who wish to test an
initial success - such as preallocations.  I was concerned about the
testing since your first patch had a syntax error with a correction
quickly after.  It is good news that you were able to find and use the
maple tree testing framework, though.

> I will try to test it in more detail in kernel space.

If you test in kernel space, you will have to hit a low memory scenario
to see a difference.  stress-ng would probably help.

> I will also refer to the notes from the email list you shared
> and send results once a more clear analysis has been made.

I don't think you need to continue with this work as you will find that
the low memory situation is going to be rare and in a very slow path
already.

Thanks,
Liam