Re: [PATCH rc 1/4] iommufd/selftest: Fix iommufd_dirty_tracking with large hugepage sizes

[Jason Gunthorpe <jgg@nvidia.com>]

On Tue, Jun 17, 2025 at 04:46:57PM -0700, Nicolin Chen wrote:
> 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?

My patch calls free on it which removes the metadata and might munmap
the range (or re-use it, but that doesn't matter)

Jason