Re: [PATCH v7 11/22] mm: shmem: allow freezing inode mapping

[Mike Rapoport]

On Sat, Nov 22, 2025 at 05:23:38PM -0500, Pasha Tatashin wrote:
> From: Pratyush Yadav <ptyadav@xxxxxxxxx>
> 
> To prepare a shmem inode for live update, 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 <ptyadav@xxxxxxxxx>
> Signed-off-by: Pasha Tatashin <pasha.tatashin@xxxxxxxxxx>
> ---
>  include/linux/shmem_fs.h | 17 +++++++++++++++++
>  mm/shmem.c               | 19 ++++++++++++++++---
>  2 files changed, 33 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/shmem_fs.h b/include/linux/shmem_fs.h
> index 650874b400b5..d34a64eafe60 100644
> --- a/include/linux/shmem_fs.h
> +++ b/include/linux/shmem_fs.h
> @@ -24,6 +24,14 @@ struct swap_iocb;
>  #define SHMEM_F_NORESERVE	BIT(0)
>  /* Disallow swapping. */
>  #define SHMEM_F_LOCKED		BIT(1)
> +/*
> + * Disallow growing, shrinking, or hole punching in the inode. Combined with
> + * folio pinning, makes sure the inode's mapping stays fixed.
> + *
> + * In some ways similar to F_SEAL_GROW | F_SEAL_SHRINK, but can be removed and
> + * isn't directly visible to userspace.
> + */
> +#define SHMEM_F_MAPPING_FROZEN	BIT(2)
>  
>  struct shmem_inode_info {
>  	spinlock_t		lock;
> @@ -186,6 +194,15 @@ static inline bool shmem_file(struct file *file)
>  	return shmem_mapping(file->f_mapping);
>  }
>  
> +/* Must be called with inode lock taken exclusive. */
> +static inline void shmem_freeze(struct inode *inode, bool freeze)
> +{
> +	if (freeze)
> +		SHMEM_I(inode)->flags |= SHMEM_F_MAPPING_FROZEN;
> +	else
> +		SHMEM_I(inode)->flags &= ~SHMEM_F_MAPPING_FROZEN;
> +}
> +
>  /*
>   * If fallocate(FALLOC_FL_KEEP_SIZE) has been used, there may be pages
>   * beyond i_size's notion of EOF, which fallocate has committed to reserving:
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 1d5036dec08a..cb74a5d202ac 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -1292,9 +1292,13 @@ 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)) ||
> -		    (newsize > oldsize && (info->seals & F_SEAL_GROW)))
> -			return -EPERM;
> +		if (newsize != oldsize) {
> +			if (info->flags & SHMEM_F_MAPPING_FROZEN)
> +				return -EPERM;
> +			if ((newsize < oldsize && (info->seals & F_SEAL_SHRINK)) ||
> +			    (newsize > oldsize && (info->seals & F_SEAL_GROW)))
> +				return -EPERM;
> +		}
>  
>  		if (newsize != oldsize) {

I'd stick 

			if (info->flags & SHMEM_F_MAPPING_FROZEN)
				return -EPERM;

here and leave the seals check alone.

Other than than

Reviewed-by: Mike Rapoport (Microsoft) <rppt@xxxxxxxxxx>

-- 
Sincerely yours,
Mike.