Re: [PATCH] mm/shmem: use invalidate_lock to fix hole-punch race

[Pedro Falcato <pfalcato@suse.de>]

On Thu, Mar 26, 2026 at 11:26:11AM -0500, Gregory Price wrote:
> Inflating a VM's balloon while vhost-user-net fork+exec's a helper
> triggers "still mapped when deleted" on the memfd backing guest RAM:
> 
>   BUG: Bad page cache in process __balloon  pfn:6520704
>   page dumped because: still mapped when deleted
>   ...
>   shmem_undo_range+0x3fa/0x570
>   shmem_fallocate+0x366/0x4d0
>   vfs_fallocate+0x13c/0x310
> 
> This BUG also resulted in guests seeing stale mappings backed by a
> zeroed page, causing guest kernel panics.  I was unable to trace that
> specific interaction, but it appears to be related to THP splitting.
> 
> Two races allow PTEs to be re-installed for a folio that fallocate
> is about to remove from page cache:

Hmm, I don't see how your patch fixes anything.

> 
> Race 1 — fault-around (filemap_map_pages):
> 
>   fallocate              fault-around           fork
>   --------               ------------           ----
>   set i_private
>   unmap_mapping_range()
>   # zaps PTEs
>                        filemap_map_pages()
>                         # re-maps folio!
>                                               dup_mmap()
>                                               # child VMA
>                                               # in tree
>   shmem_undo_range()
>     lock folio
>     unmap_mapping_folio()
	spin_lock(ptl);
>     # child VMA:
>     #   no PTE, skip
	spin_unlock(ptl);
>                                             copy_page_range()
                                               spin_lock(dst_ptl);
					       spin_lock(src_ptl);
						/* does not copy PTE. either
						 * we find a zapped PTE, or unmap_mapping_folio()
						 * finds two mappings instead of one. */
>						# copies PTE
>     # parent VMA:
>     #   zaps PTE
>   filemap_remove_folio()
>     # mapcount=1, BUG!
> 
> filemap_map_pages() is called directly as .map_pages, bypassing
> shmem_fault()'s i_private synchronization.
> 
> Race 2 — shmem_fault TOCTOU:
> 
>   fallocate                   shmem_fault
>   --------                    -----------
>                             check i_private → NULL
>   set i_private
>   unmap_mapping_range()
>   # zaps PTEs
>                             shmem_get_folio_gfp()
>                               # finds folio in cache
>                             finish_fault()
>                               # installs PTE
>   shmem_undo_range()
>     truncate_inode_folio()
	truncate_inode_folio() zaps the PTEs, thus mapcount = 0.
	shmem folio is locked by both truncate and shmem_fault().
>       # mapcount=1, BUG!
> 
> Fix both races with invalidate_lock.
> 

I don't see what you're seeing? Note that both map_pages and fault()
take the folio lock (map_pages does a trylock) to exclude against truncate
as well.

-- 
Pedro