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

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Message-ID: <mafs0a50bzczf.fsf@kernel.org>
Date: Mon, 24 Nov 2025 16:06:44 +0100
From: Pratyush Yadav <pratyush@...nel.org>
To: Mike Rapoport <rppt@...nel.org>
Cc: Pasha Tatashin <pasha.tatashin@...een.com>,  pratyush@...nel.org,
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Subject: 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:
>> From: Pratyush Yadav <ptyadav@...zon.de>
>> 
>> 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 <ptyadav@...zon.de>
>> Signed-off-by: Pasha Tatashin <pasha.tatashin@...een.com>
>> ---
[...]
>> 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.

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

-- 
Regards,
Pratyush Yadav