linux-kernel - Re: [PATCH] mm/pgtable-generic.c: optimize the VM_BUG_ON condition in pmdp_huge_clear

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Date:   Thu, 4 Feb 2021 12:36:05 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Miaohe Lin <linmiaohe@...wei.com>
Cc:     akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/pgtable-generic.c: optimize the VM_BUG_ON condition
 in pmdp_huge_clear_flush()

On Wed, Feb 03, 2021 at 03:41:37AM -0500, Miaohe Lin wrote:
> The developer will have trouble figuring out why the BUG actually triggered
> when there is a complex expression in the VM_BUG_ON. Because we can only
> identify the condition triggered BUG via line number provided by VM_BUG_ON.
> Optimize this by spliting such a complex expression into two simple
> conditions.

>  	pmd_t pmd;
>  	VM_BUG_ON(address & ~HPAGE_PMD_MASK);
> -	VM_BUG_ON(!pmd_present(*pmdp) || (!pmd_trans_huge(*pmdp) &&
> -					  !pmd_devmap(*pmdp)));
> +	VM_BUG_ON(!pmd_present(*pmdp));
> +	/* Below assumes pmd_present() is true */
> +	VM_BUG_ON(!pmd_trans_huge(*pmdp) && !pmd_devmap(*pmdp));

This is not a complex condition.  We're in the huge PMD handling case
and we're looking at a PMD which either isn't present or isn't huge.
It might be useful to print out the PMD in such a case, but splitting
this into the two cases of pmd-not-present and pmd-isn't-huge isn't
particularly useful.

I think you know that, or you wouldn't feel the need to put in a
comment explaining it!