linux-mm.kvack.org archive mirror
 help / color / mirror / Atom feed
From: Andrew Morton <akpm@linux-foundation.org>
To: Miaohe Lin <linmiaohe@huawei.com>
Cc: <linux-mm@kvack.org>, <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] mm: simplify the VM_BUG_ON condition in pmdp_huge_clear_flush()
Date: Mon, 1 Feb 2021 15:33:54 -0800	[thread overview]
Message-ID: <20210201153354.e640247cb5ab306e909322d0@linux-foundation.org> (raw)
In-Reply-To: <20210201114319.34720-1-linmiaohe@huawei.com>

On Mon, 1 Feb 2021 06:43:19 -0500 Miaohe Lin <linmiaohe@huawei.com> wrote:

> The condition (A && !C && !D) || !A is equivalent to !A || (A && !C && !D)
> and can be further simplified to !A || (!C && !D).
> 
> ..
>
> --- a/mm/pgtable-generic.c
> +++ b/mm/pgtable-generic.c
> @@ -135,8 +135,8 @@ pmd_t pmdp_huge_clear_flush(struct vm_area_struct *vma, unsigned long address,
>  {
>  	pmd_t pmd;
>  	VM_BUG_ON(address & ~HPAGE_PMD_MASK);
> -	VM_BUG_ON((pmd_present(*pmdp) && !pmd_trans_huge(*pmdp) &&
> -			   !pmd_devmap(*pmdp)) || !pmd_present(*pmdp));
> +	VM_BUG_ON(!pmd_present(*pmdp) || (!pmd_trans_huge(*pmdp) &&
> +					  !pmd_devmap(*pmdp)));
>  	pmd = pmdp_huge_get_and_clear(vma->vm_mm, address, pmdp);
>  	flush_pmd_tlb_range(vma, address, address + HPAGE_PMD_SIZE);
>  	return pmd;

True, and the resulting code is still readable enough.

But a problem with such a complex expression is that the developer will
have trouble figuring out why the BUG actually triggered.

If we had a VM_BUG_ON_PMD() then we could print the pmd's value and
permit diagnosis from that.  But we don't have such a thing.

So I suggest that it would be better to have

	VM_BUG_ON((pmd_present(*pmdp) && !pmd_trans_huge(*pmdp) &&
			   !pmd_devmap(*pmdp)));
	VM_BUG_ON(!pmd_present(*pmdp));

This way, the BUG()'s file-n-line output will tell us more about why the
kernel went splat.


I suppose maybe this could be optimized the same way, as

	VM_BUG_ON(!pmd_present(*pmdp));
	/* Below assumes pmd_present() is true */
	VM_BUG_ON(!pmd_trans_huge(*pmdp) && !pmd_devmap(*pmdp));

Which works because VM_BUG_ON is, depending up Kconfig, either a no-op
or a noreturn-if-it-triggered.  I'm not sure if I like this trick much though.



  reply	other threads:[~2021-02-01 23:33 UTC|newest]

Thread overview: 3+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-02-01 11:43 Miaohe Lin
2021-02-01 23:33 ` Andrew Morton [this message]
2021-02-02  2:09   ` Miaohe Lin

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20210201153354.e640247cb5ab306e909322d0@linux-foundation.org \
    --to=akpm@linux-foundation.org \
    --cc=linmiaohe@huawei.com \
    --cc=linux-kernel@vger.kernel.org \
    --cc=linux-mm@kvack.org \
    --subject='Re: [PATCH] mm: simplify the VM_BUG_ON condition in pmdp_huge_clear_flush()' \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).