Re: [PATCH] NFS: don't raise i_size for DEALLOCATE in _nfs42_proc_fallocate()

[Joseph Qi]

Hi,

On 11/4/26 4:11 AM, Anna Schumaker wrote:
> Hi Joseph,
> 
> On Tue, Nov 3, 2026, at 1:16 AM, Joseph Qi wrote:
>> _nfs42_proc_fallocate() raises i_size to offset + len on success, so
>> that the cached size is not left stale when a delegation blocks the
>> post-op attribute refresh.  The helper is shared by all three
>> fallocate modes, but DEALLOCATE always carries FALLOC_FL_KEEP_SIZE
>> and never extends the file on the server, so punching a hole that
>> reaches past EOF now wrongly enlarges the cached i_size.
>>
>> The post-op update would normally restore the size from the returned
>> attributes, but nfs_update_inode() applies a differing size only when
>> no delegation is held, and then only when no write is in flight or the
>> new size is larger:
>>
>> 	if (new_isize != cur_isize && !have_delegation) {
>> 		if (!nfs_have_writebacks(inode) || new_isize > cur_isize)
>> 			i_size_write(inode, new_isize);
>> 	}
>>
>> With a concurrent mmap writer (nrequests != 0) or any outstanding
>> delegation, the shrinking server size is therefore ignored and the
>> client keeps reporting a file larger than the server has, in violation
>> of KEEP_SIZE, until some later revalidation without writebacks happens
>> to fix it up.
>>
>> Skip the i_size bump for the DEALLOCATE procedure.  ALLOCATE (mode 0)
>> and ZERO_RANGE (which nfs42_fallocate() accepts only without
>> FALLOC_FL_KEEP_SIZE) do extend the file when the range reaches past
>> EOF, so they keep the update.
>>
>> Fixes: 35168eb947f2 ("NFS: fix eof updates after NFSv4.2 fallocate/zero-range")
>> Cc: stable@xxxxxxxxxxxxxxx
>> Signed-off-by: Joseph Qi <joseph.qi@xxxxxxxxxxxxxxxxx>
>> ---
>>  fs/nfs/nfs42proc.c | 11 ++++++++++-
>>  1 file changed, 10 insertions(+), 1 deletion(-)
>>
>> diff --git a/fs/nfs/nfs42proc.c b/fs/nfs/nfs42proc.c
>> index ab86246fc364f..914499d30d47e 100644
>> --- a/fs/nfs/nfs42proc.c
>> +++ b/fs/nfs/nfs42proc.c
>> @@ -84,7 +84,16 @@ static int _nfs42_proc_fallocate(struct rpc_message 
>> *msg, struct file *filep,
>>  		loff_t newsize = offset + len;
>>
>>  		spin_lock(&inode->i_lock);
>> -		if (newsize > i_size_read(inode))
>> +		/*
>> +		 * DEALLOCATE always carries FALLOC_FL_KEEP_SIZE and never
>> +		 * extends the file on the server, so only ALLOCATE and
>> +		 * ZERO_RANGE may raise i_size here.  A wrongly raised size
>> +		 * can persist: nfs_update_inode() ignores a shrinking
>> +		 * post-op size while writebacks are outstanding or a
>> +		 * delegation is held.
>> +		 */
> 
> Is the comment really necessary? It's just restating things from the commit
> message, and could quickly go stale if the behavior of nfs_update_inode()
> ever needs to change in the future.
> 

Thanks for reviewing this.
So any review comments on the code change? I'll update the comments and
send v2 if you're fine with fix.

Thanks,
Joseph