Re: [PATCH v5 1/5] mm/zone_device: Reinitialize large zone device private folios

[Matthew Brost <matthew.brost@intel.com>]

On Wed, Jan 14, 2026 at 03:34:21PM -0800, Matthew Brost wrote:
> On Wed, Jan 14, 2026 at 01:48:25PM -0800, Andrew Morton wrote:
> > On Wed, 14 Jan 2026 20:19:52 +0100 Francois Dugast <francois.dugast@intel.com> wrote:
> > 
> > > Reinitialize metadata for large zone device private folios in
> > > zone_device_page_init prior to creating a higher-order zone device
> > > private folio. This step is necessary when the folio’s order changes
> > > dynamically between zone_device_page_init calls to avoid building a
> > > corrupt folio. As part of the metadata reinitialization, the dev_pagemap
> > > must be passed in from the caller because the pgmap stored in the folio
> > > page may have been overwritten with a compound head.
> > 
> > Thanks.  What are the worst-case userspace-visible effects of the bug?
> 
> If you reallocate a subset of pages from what was originally a large
> device folio, the pgmap mapping becomes invalid because it was
> overwritten by the compound head, and this can crash the kernel.
> 
> Alternatively, consider the case where the original folio had an order
> of 9 and _nr_pages was set. If you then reallocate the folio plus one as

s/_nr_pages/the order was encoded the page flags.

Not clearing _nr_pages is probably bad too, not sure what the side
affect of that is, but it can't be good. 

> an individual page, the flags would still have PG_locked set, causing a
> hang the next time you try to lock the page.
> 
> This is pretty bad if drivers implement a buddy allocator for device
> pages (Xe does; Nouveau doesn’t, which is why they haven’t hit this
> issue). Only Nouveau enables large device pages in 6.19 but probably
> best to have kernel flying around with known issues.

s/best to have kernel/best to not have kernels

Matt

> 
> Matt