Re: [PATCH v2 13/22] iommufd: Add mmap interface

Jason Gunthorpe
2025-05-05T16:55:56Z

On Tue, Apr 29, 2025 at 02:46:25PM -0700, Nicolin Chen wrote:
> > > > > > > > + immap = kzalloc(sizeof(*immap), GFP_KERNEL);
> > > > > > > > + if (!immap)
> > > > > > > > + return -ENOMEM;
> > > > > > > > + immap->pfn_start = base >> PAGE_SHIFT;
> > > > > > > > + immap->pfn_end = immap->pfn_start + (size >> PAGE_SHIFT) - 1;
> > > > > > > > +
> > > > > > > > + rc = mtree_alloc_range(&ictx->mt_mmap, immap_id, immap, sizeof(immap),
> > > > > > >
> > > > > > > I believe this should be sizeof(*immap) ?
> > > > > >
> > > > > > Ugh, Sorry, shouldn't this be size >> PAGE_SHIFT (num_indices to alloc) ?
> > > > >
> > > > > mtree_load() returns a "struct iommufd_map *" pointer.
> > > >
> > > > I'm not talking about mtree_load. I meant mtree_alloc_range takes in a
> > > > "size" parameter, which is being passed as sizeof(imap) in this patch.
> > > > IIUC, the mtree_alloc_range, via mas_empty_area, gets a range that is
> > > > sufficient for the given "size".
> > > >
> > > > Now in this case, "size" would be the no. of pfns which are mmap-able.
> > > > By passing sizeof(immap), we're simply reserving sizeof(ptr) i.e. 8 pfns
> > > > for a 64-bit machine. Whereas we really, just want to reserve a range
> > > > for size >> PAGE_SHIFT pfns.
> > >
> > > But we are not storing pfns but the immap pointer..

That doesn't seem right, the entire point of using a maple tree is to
manage the pfn number space, ie the pgoff argument to mmap.

So when calling mtree_alloc_range:

int mtree_alloc_range(struct maple_tree *mt, unsigned long *startp,
void *entry, unsigned long size, unsigned long min,
unsigned long max, gfp_t gfp)

size should be the number of PFNs this mmap is going to use, which is
not sizeof() anything

min should be 0 and max should be uh.. U32_MAX >> PAGE_SHIFT
IIRC.. There is a different limit for pgof fon 32 bit mmap()

> > Ohh... so we are storing the raw pointer in the mtree.. I got confused
> > with the `LONG_MAX >> PAGE_SHIFT`.. Sorry about the confusion!
>
> Yes. We want the pointer at mtree_load(). The pfn range is for
> validation after mtree_load(). And we are likely to stuff more
> bits into the immap structure for other verifications.

Validation is fine, but you still have to reserve the whole pfn number
space to get sensible non-overlapping pgoffs out of the allocator.

Jason