Re: [PATCH 35/46] mm: fix unmap_mapping_range() beyond 4 GiB on 32-bit

[Pedro Falcato]

On Tue, Oct 27, 2026 at 12:17:55AM +0100, David Hildenbrand (Arm) wrote:
> From: Josef Bacik <josef@xxxxxxxxxxxxxx>
> 
> Commit 9eab0421fa94 ("mm: fix unmap_mapping_range high bits shift bug")
> made holebegin and holelen unsigned before they are shifted, by casting
> them to pgoff_t. That is an unsigned long: on a 32-bit kernel the cast
> cuts off the upper half of the two loff_t values, so a hole which begins
> at 4 GiB or beyond is taken for one in the first 4 GiB of the file.
> 
> truncate_pagecache() passes the new size of the file and asks for private
> COWed pages to be unmapped as well. If a file of 6 GiB is truncated to
> 5 GiB, everything from 1 GiB on is unmapped, and what a process has
> written to a private mapping of the range between 1 GiB and 5 GiB is
> lost: it reads the contents of the file again.
> 
> Where the range has an end, the wrong range is unmapped instead of the
> right one. The page cache copes with that, as it unmaps every folio that
> it removes. The callers which only have unmap_mapping_range() to get
> rid of user mappings do not, for example drm_vma_node_unmap(), whose fake
> offsets reach beyond 4 GiB on 32-bit, and revoke_iomem() with physical
> addresses of more than 32 bits.
> 
> Cast to u64 instead. Nothing changes for 64-bit kernels.
> 
> Fix generated by an LLM and sent by Josef Bacik in private.
> 
> Fixes: 9eab0421fa94 ("mm: fix unmap_mapping_range high bits shift bug")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Josef Bacik <josef@xxxxxxxxxxxxxx>
> Acked-by: Lorenzo Stoakes (ARM) <ljs@xxxxxxxxxx>
> Signed-off-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>

Reviewed-by: Pedro Falcato <pfalcato@xxxxxxx>

-- 
Pedro