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

[Nicolin Chen <nicolinc@nvidia.com>]

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)?

[1] posix_memalign()
[2] mmap()
[3] munmap()

> > // Step 1: Use posix_memalign to get an aligned pointer
> > if (posix_memalign(&ptr, alignment, size) != 0) {
> >     perror("posix_memalign");
> >     return -1;
> > }
> 
> Also no, the main point of this is to inject MAP_SHARED which
> posix_memalign cannot not do.

I see.

> > Instead, it suggests a cleaner version without posix_memalign():
> > ---code---
> > void *addr = mmap(NULL, variant->buffer_size, PROT_READ | PROT_WRITE,
> >                   MAP_SHARED | MAP_ANONYMOUS | MAP_HUGETLB | MAP_POPULATE,
> > 		  -1, 0);
> > if (addr == MAP_FAILED) { perror("mmap"); return -1; }
> > ---end---
> 
> Yes, we could do this only for MAP_HUGETLB, but it doesn't help the
> normal case with MAP_SHARED.
> 
> So I would leave it alone, use the version I showed.

OK. Will respin a v2 with that.

Thanks
Nicolin