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

[Gregory Price]

On Thu, Mar 26, 2026 at 07:21:21PM +0000, Matthew Wilcox wrote:
> On Thu, Mar 26, 2026 at 11:26:11AM -0500, Gregory Price wrote:
> > This also requires removing the rcu_read_lock() from
> > do_fault_around() so that .map_pages may use sleeping locks.
> 
> NACK.
> 
> ->map_pages() is called when VM asks to map easy accessible pages.
> Filesystem should find and map pages associated with offsets from "start_pgoff"
> till "end_pgoff". ->map_pages() is called with the RCU lock held and must
> not block.  If it's not possible to reach a page without blocking,
> filesystem should skip it. Filesystem should use set_pte_range() to setup
> page table entry. Pointer to entry associated with the page is passed in
> "pte" field in vm_fault structure. Pointers to entries for other offsets
> should be calculated relative to "pte".
> 

Hm, I follow. I was originally thinking this was scoping issue given
we take the rcu_read_lock shortly after the call anyway, but I see.

If the invalidate lock ends up being needed then i could leave rcu
and just use trylock/fallback to fault.

But I need to test a few things, nothing else protects filemap_map_pages
with the invalidate lock at the moment but only shmem appears broken.

~Gregory