Re: [PATCH v6 12/20] mm: shmem: allow freezing inode mapping

On Mon, Nov 17 2025, Mike Rapoport wrote:

On Sat, Nov 15, 2025 at 06:33:58PM -0500, Pasha Tatashin wrote:

quoted

From: Pratyush Yadav <redacted>

To prepare a shmem inode for live update via the Live Update
Orchestrator (LUO), its index -> folio mappings must be serialized. Once
the mappings are serialized, they cannot change since it would cause the
serialized data to become inconsistent. This can be done by pinning the
folios to avoid migration, and by making sure no folios can be added to
or removed from the inode.

While mechanisms to pin folios already exist, the only way to stop
folios being added or removed are the grow and shrink file seals. But
file seals come with their own semantics, one of which is that they
can't be removed. This doesn't work with liveupdate since it can be
cancelled or error out, which would need the seals to be removed and the
file's normal functionality to be restored.

Introduce SHMEM_F_MAPPING_FROZEN to indicate this instead. It is
internal to shmem and is not directly exposed to userspace. It functions
similar to F_SEAL_GROW | F_SEAL_SHRINK, but additionally disallows hole
punching, and can be removed.

Signed-off-by: Pratyush Yadav <redacted>
Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>
---

[...]

quoted

diff --git a/mm/shmem.c b/mm/shmem.c
index 1d5036dec08a..05c3db840257 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c

@@ -1292,7 +1292,8 @@ static int shmem_setattr(struct mnt_idmap *idmap,
 		loff_t newsize = attr->ia_size;
 
 		/* protected by i_rwsem */
-		if ((newsize < oldsize && (info->seals & F_SEAL_SHRINK)) ||
+		if ((info->flags & SHMEM_F_MAPPING_FROZEN) ||

A corner case: if newsize == oldsize this will be a false positive

Good catch. Though I wonder why anyone would do a truncate with the same
size.

quoted

+		    (newsize < oldsize && (info->seals & F_SEAL_SHRINK)) ||
 		    (newsize > oldsize && (info->seals & F_SEAL_GROW)))
 			return -EPERM;
 

[...]

-- 
Regards,
Pratyush Yadav