linux-kernel - Re: mm/DAMON: Profiling enhancements for DAMON

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Date: Fri, 15 Dec 2023 10:10:08 -0800
From: Dave Hansen <dave.hansen@...el.com>
To: Aravinda Prasad <aravinda.prasad@...el.com>, damon@...ts.linux.dev,
 linux-mm@...ck.org, sj@...nel.org, linux-kernel@...r.kernel.org
Cc: s2322819@...ac.uk, sandeep4.kumar@...el.com, ying.huang@...el.com,
 dan.j.williams@...el.com, sreenivas.subramoney@...el.com,
 antti.kervinen@...el.com, alexander.kanevskiy@...el.com,
 Alan Nair <alan.nair@...el.com>
Subject: Re: mm/DAMON: Profiling enhancements for DAMON

On 12/14/23 23:46, Aravinda Prasad wrote:
> +static int damon_young_pmd(pmd_t *pmd, unsigned long addr,
> +		unsigned long next, struct mm_walk *walk)
> +{
> +	spinlock_t *ptl;
> +	struct damon_young_walk_private *priv = walk->private;
> +
> +	if (!pmd_present(*pmd) || pmd_none(*pmd))
> +		goto out;
> +
> +	ptl = pmd_lock(walk->mm, pmd);
> +	if (pmd_young(*pmd) || mmu_notifier_test_young(walk->mm, addr))
> +		priv->young = true;
> +
> +	*priv->folio_sz = (1UL << PMD_SHIFT);
> +	spin_unlock(ptl);
> +out:
> +	return 0;
> +}

There are a number of paired p*_present() and p_*none() checks in this
patch.  What is their function (especially pmd_none())?  For instance,
damon_young_pmd() gets called via the pagewalk infrastructure:

> static int walk_pmd_range(pud_t *pud, unsigned long addr, unsigned long end,
>                           struct mm_walk *walk)
> {
...
>                 next = pmd_addr_end(addr, end);
>                 if (pmd_none(*pmd)) {
...
>                         continue;
>                 }
...
>                 if (ops->pmd_entry)
>                         err = ops->pmd_entry(pmd, addr, next, walk);

I'd suggest taking a closer look at the code you are replacing and other
mm_walk users to make sure the handlers are doing appropriate checks.