linux-kernel - Re: [PATCH rc 1/4] iommufd/selftest: Fix iommufd_dirty

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Message-ID: <aFH+cUSi1GITvF3F@nvidia.com>
Date: Tue, 17 Jun 2025 16:46:57 -0700
From: Nicolin Chen <nicolinc@...dia.com>
To: Jason Gunthorpe <jgg@...dia.com>
CC: <kevin.tian@...el.com>, <shuah@...nel.org>, <joao.m.martins@...cle.com>,
	<steven.sistare@...cle.com>, <iommu@...ts.linux.dev>,
	<linux-kselftest@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<thomas.weissschuh@...utronix.de>
Subject: Re: [PATCH rc 1/4] iommufd/selftest: Fix iommufd_dirty_tracking with
 large hugepage sizes

On Tue, Jun 17, 2025 at 08:01:36PM -0300, Jason Gunthorpe wrote:
> On Tue, Jun 17, 2025 at 02:23:41PM -0700, Nicolin Chen wrote:
> > On Tue, Jun 17, 2025 at 08:59:48AM -0300, Jason Gunthorpe wrote:
> > > On Mon, Jun 16, 2025 at 07:02:08PM -0700, Nicolin Chen wrote:
> > > > ---breakdown---
> > > > After `posix_memalign()`:
> > > >   [ posix_memalign() memory ]  ← malloc thinks it owns this
> > > > 
> > > > Then `mmap(aligned_ptr, ..., MAP_FIXED)`:
> > > >   [ anonymous mmap region ]    ← malloc still thinks it owns this (!)
> > > >   ↑ mapped
> > > > ---end---
> > > 
> > > Yes, this is correct and what we are doing here. The allocator always
> > > owns it and we are just replacing the memory with a different mmap.
> > 
> > Hmm, if allocator always owns it. Does that mean the munmap() [3]
> > will release what [1] and [2] do (allocating and replacing)?
> 
> No, munmap doesn't destroy the allocator meta data.

Should we do something to that meta data?