Re: [PATCH v3 3/3] shmem: stable directory offsets

[Chuck Lever III <chuck.lever@oracle.com>]

> On Jun 26, 2023, at 9:18 AM, Bernd Schubert <bernd.schubert@fastmail.fm> wrote:
> 
> 
> 
> On 6/6/23 15:11, Chuck Lever wrote:
>> From: Chuck Lever <chuck.lever@oracle.com>
>> The current cursor-based directory offset mechanism doesn't work
>> when a tmpfs filesystem is exported via NFS. This is because NFS
>> clients do not open directories. Each server-side READDIR operation
>> has to open the directory, read it, then close it. The cursor state
>> for that directory, being associated strictly with the opened
>> struct file, is thus discarded after each NFS READDIR operation.
>> Directory offsets are cached not only by NFS clients, but also by
>> user space libraries on those clients. Essentially there is no way
>> to invalidate those caches when directory offsets have changed on
>> an NFS server after the offset-to-dentry mapping changes. Thus the
>> whole application stack depends on unchanging directory offsets.
>> The solution we've come up with is to make the directory offset for
>> each file in a tmpfs filesystem stable for the life of the directory
>> entry it represents.
>> shmem_readdir() and shmem_dir_llseek() now use an xarray to map each
>> directory offset (an loff_t integer) to the memory address of a
>> struct dentry.
>> Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
>> ---
>>  mm/shmem.c |   39 +++++++++++++++++++++++++++++++++++----
>>  1 file changed, 35 insertions(+), 4 deletions(-)
>> diff --git a/mm/shmem.c b/mm/shmem.c
>> index 721f9fd064aa..fd9571056181 100644
>> --- a/mm/shmem.c
>> +++ b/mm/shmem.c
>> @@ -2410,7 +2410,8 @@ static struct inode *shmem_get_inode(struct mnt_idmap *idmap, struct super_block
>>   /* Some things misbehave if size == 0 on a directory */
>>   inode->i_size = 2 * BOGO_DIRENT_SIZE;
>>   inode->i_op = &shmem_dir_inode_operations;
>> - inode->i_fop = &simple_dir_operations;
>> + inode->i_fop = &stable_dir_operations;
>> + stable_offset_init(inode);
>>   break;
>>   case S_IFLNK:
>>   /*
>> @@ -2950,6 +2951,10 @@ shmem_mknod(struct mnt_idmap *idmap, struct inode *dir,
>>   if (error && error != -EOPNOTSUPP)
>>   goto out_iput;
>>  + error = stable_offset_add(dir, dentry);
>> + if (error)
>> + goto out_iput;
>> +
>>   error = 0;
> 
> This line can be removed?
> 
>>   dir->i_size += BOGO_DIRENT_SIZE;
>>   dir->i_ctime = dir->i_mtime = current_time(dir);
>> @@ -3027,6 +3032,10 @@ static int shmem_link(struct dentry *old_dentry, struct inode *dir, struct dentr
>>   goto out;
>>   }
>>  + ret = stable_offset_add(dir, dentry);
>> + if (ret)
>> + goto out;
>> +
> 
> I think this should call shmem_free_inode() before goto out - reverse what shmem_reserve_inode() has done.
> 
>>   dir->i_size += BOGO_DIRENT_SIZE;
>>   inode->i_ctime = dir->i_ctime = dir->i_mtime = current_time(inode);
>>   inode_inc_iversion(dir);
>> @@ -3045,6 +3054,8 @@ static int shmem_unlink(struct inode *dir, struct dentry *dentry)
>>   if (inode->i_nlink > 1 && !S_ISDIR(inode->i_mode))
>>   shmem_free_inode(inode->i_sb);
>>  + stable_offset_remove(dir, dentry);
>> +
>>   dir->i_size -= BOGO_DIRENT_SIZE;
>>   inode->i_ctime = dir->i_ctime = dir->i_mtime = current_time(inode);
>>   inode_inc_iversion(dir);
>> @@ -3103,24 +3114,37 @@ static int shmem_rename2(struct mnt_idmap *idmap,
>>  {
>>   struct inode *inode = d_inode(old_dentry);
>>   int they_are_dirs = S_ISDIR(inode->i_mode);
>> + int error;
>>     if (flags & ~(RENAME_NOREPLACE | RENAME_EXCHANGE | RENAME_WHITEOUT))
>>   return -EINVAL;
>>  - if (flags & RENAME_EXCHANGE)
>> + if (flags & RENAME_EXCHANGE) {
>> + stable_offset_remove(old_dir, old_dentry);
>> + stable_offset_remove(new_dir, new_dentry);
>> + error = stable_offset_add(new_dir, old_dentry);
>> + if (error)
>> + return error;
>> + error = stable_offset_add(old_dir, new_dentry);
>> + if (error)
>> + return error;
>>   return simple_rename_exchange(old_dir, old_dentry, new_dir, new_dentry);
>> + }
> 
> Hmm, error handling issues? Everything needs to be reversed when any of the operations fails?
> 
>>     if (!simple_empty(new_dentry))
>>   return -ENOTEMPTY;
>>     if (flags & RENAME_WHITEOUT) {
>> - int error;
>> -
>>   error = shmem_whiteout(idmap, old_dir, old_dentry);
>>   if (error)
>>   return error;
>>   }
>>  + stable_offset_remove(old_dir, old_dentry);
>> + error = stable_offset_add(new_dir, old_dentry);
>> + if (error)
>> + return error;
>> +
>>   if (d_really_is_positive(new_dentry)) {
>>   (void) shmem_unlink(new_dir, new_dentry);
>>   if (they_are_dirs) {
>> @@ -3185,6 +3209,11 @@ static int shmem_symlink(struct mnt_idmap *idmap, struct inode *dir,
>>   folio_unlock(folio);
>>   folio_put(folio);
>>   }
>> +
>> + error = stable_offset_add(dir, dentry);
>> + if (error)
>> + goto out_iput;
>> +
> 
> Error handling, there is a kmemdup() above which needs to be freed? I'm not sure about folio, automatically released with the inode?
> 
>>   dir->i_size += BOGO_DIRENT_SIZE;
>>   dir->i_ctime = dir->i_mtime = current_time(dir);
>>   inode_inc_iversion(dir);
>> @@ -3920,6 +3949,8 @@ static void shmem_destroy_inode(struct inode *inode)
>>  {
>>   if (S_ISREG(inode->i_mode))
>>   mpol_free_shared_policy(&SHMEM_I(inode)->policy);
>> + if (S_ISDIR(inode->i_mode))
>> + stable_offset_destroy(inode);
>>  }
>>    static void shmem_init_inode(void *foo)
> 
> Thanks,
> Bernd

Thanks for the review. I think I've addressed the issues you've pointed out.
Watch for v4 of this series.


--
Chuck Lever