linux-kernel - Re: [RESEND PATCH v5] arm64: Enable vmalloc-huge with ptdump

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Message-ID: <aItVswTcGVz_A6K8@arm.com>
Date: Thu, 31 Jul 2025 12:38:27 +0100
From: Catalin Marinas <catalin.marinas@....com>
To: Dev Jain <dev.jain@....com>
Cc: Ryan Roberts <ryan.roberts@....com>, will@...nel.org,
	anshuman.khandual@....com, quic_zhenhuah@...cinc.com,
	kevin.brodsky@....com, yangyicong@...ilicon.com, joey.gouly@....com,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	david@...hat.com, mark.rutland@....com, urezki@...il.com
Subject: Re: [RESEND PATCH v5] arm64: Enable vmalloc-huge with ptdump

On Thu, Jul 31, 2025 at 10:00:15AM +0530, Dev Jain wrote:
> On 30/07/25 11:59 pm, Ryan Roberts wrote:
> > On 30/07/2025 18:00, Catalin Marinas wrote:
> > > On Wed, Jul 23, 2025 at 09:48:27PM +0530, Dev Jain wrote:
> > > > +	 * mmap_write_lock/unlock in T1 be called CS (the critical section).
> > > > +	 *
> > > > +	 * Claim: The CS of T1 will never operate on a freed PMD table.
> > > > +	 *
> > > > +	 * Proof:
> > > > +	 *
> > > > +	 * Case 1: The static branch is visible to T2.
> > > > +	 *
> > > > +	 * Case 1 (a): T1 acquires the lock before T2 can.
> > > > +	 * T2 will block until T1 drops the lock, so pmd_free() will only be
> > > > +	 * executed after T1 exits CS.
> > > 
> > > This assumes that there is some ordering between unlock and pmd_free()
> > > (e.g. some poisoning of the old page). The unlock only gives us release
> > > semantics, not acquire. It just happens that we have an atomic
> > > dec-and-test down the __free_pages() path but I'm not convinced we
> > > should rely on it unless free_pages() has clear semantics on ordering
> > > related to prior memory writes.
> > 
> > I can understand how pmd_free() could be re-ordered before the unlock, but
> > surely it can't be reorded before the lock? I need to go unlearn everything I
> > thought I understood about locking if that's the case...

Indeed, it can't be reordered before the lock as it has acquire
semantics.

> You are correct, what Catalin is saying is that my reasoning has a hole.
> There is no obvious ordering between unlock and free(), but
> 
> mmap_write_unlock() will happen before mmap_read_lock() ... (i)
> mmap_read_lock() will happen before pmd_free() ... (ii)
> 
> which lets us conclude that mmap_write_unlock() will happen before pmd_free().

Yes, in this sub-case, mmap_write_unlock() on T1 will happen before
pmd_free() on T2 if T2 waits on the lock.

-- 
Catalin