Re: [PATCH v4 3/3] shmem: stable directory offsets

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

> On Jun 27, 2023, at 10:06 AM, Bernd Schubert <bernd.schubert@fastmail.fm> wrote:
> 
> 
> 
> On 6/26/23 20:21, 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 |   54 +++++++++++++++++++++++++++++++++++++++++++++++-------
>>  1 file changed, 47 insertions(+), 7 deletions(-)
>> diff --git a/mm/shmem.c b/mm/shmem.c
>> index 721f9fd064aa..89012f3583b1 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:
>>   /*
>> @@ -2950,7 +2951,10 @@ shmem_mknod(struct mnt_idmap *idmap, struct inode *dir,
>>   if (error && error != -EOPNOTSUPP)
>>   goto out_iput;
>>  - error = 0;
>> + error = stable_offset_add(dir, dentry);
>> + if (error)
>> + goto out_iput;
>> +
>>   dir->i_size += BOGO_DIRENT_SIZE;
>>   dir->i_ctime = dir->i_mtime = current_time(dir);
>>   inode_inc_iversion(dir);
>> @@ -3027,6 +3031,13 @@ static int shmem_link(struct dentry *old_dentry, struct inode *dir, struct dentr
>>   goto out;
>>   }
>>  + ret = stable_offset_add(dir, dentry);
>> + if (ret) {
>> + if (inode->i_nlink)
>> + shmem_free_inode(inode->i_sb);
>> + goto out;
>> + }
>> +
>>   dir->i_size += BOGO_DIRENT_SIZE;
>>   inode->i_ctime = dir->i_ctime = dir->i_mtime = current_time(inode);
>>   inode_inc_iversion(dir);
>> @@ -3045,6 +3056,8 @@ 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 +3116,41 @@ 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) {
>> + error = stable_offset_add(new_dir, old_dentry);
>> + if (error)
>> + return error;
> 
> Won't this fail in stable_offset_add() with -EBUSY, because dentry->d_offset is set?
> 
>> + error = stable_offset_add(old_dir, new_dentry);
>> + if (error) {
>> + stable_offset_remove(new_dir, old_dentry);
>> + return error;
>> + }
>> + stable_offset_remove(old_dir, old_dentry);
>> + stable_offset_remove(new_dir, new_dentry);
> 
> Assuming stable_offset_add() would have succeeded (somehow), old_dentry and new_dentry would have gotten reset their dentry->d_offset?

Probably. I can have another look.


>> +
>> + /* Always returns zero */
>>   return simple_rename_exchange(old_dir, old_dentry, new_dir, new_dentry);
>> + }
> 
> Hmm, let's assume simple_rename_exchange() fails, stable entries are now the other way around?

Today it never fails. We can add an assertion here.

Otherwise cleaning up after a simple_rename_exchange() failure is going to be even more hairy.


> I actually start to wonder if we need to introduce error injection for RENAME_EXCHANGE, I think it the most complex part in the series.

And it's the least frequently used mode of rename.


>>     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) {
>> @@ -3164,19 +3194,23 @@ static int shmem_symlink(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;
>> +
>>   inode->i_size = len-1;
>>   if (len <= SHORT_SYMLINK_LEN) {
>>   inode->i_link = kmemdup(symname, len, GFP_KERNEL);
>>   if (!inode->i_link) {
>>   error = -ENOMEM;
>> - goto out_iput;
>> + goto out_remove_offset;
>>   }
>>   inode->i_op = &shmem_short_symlink_operations;
>>   } else {
>>   inode_nohighmem(inode);
>>   error = shmem_get_folio(inode, 0, &folio, SGP_WRITE);
>>   if (error)
>> - goto out_iput;
>> + goto out_remove_offset;
>>   inode->i_mapping->a_ops = &shmem_aops;
>>   inode->i_op = &shmem_symlink_inode_operations;
>>   memcpy(folio_address(folio), symname, len);
>> @@ -3185,12 +3219,16 @@ static int shmem_symlink(struct mnt_idmap *idmap, struct inode *dir,
>>   folio_unlock(folio);
>>   folio_put(folio);
>>   }
>> +
>>   dir->i_size += BOGO_DIRENT_SIZE;
>>   dir->i_ctime = dir->i_mtime = current_time(dir);
>>   inode_inc_iversion(dir);
>>   d_instantiate(dentry, inode);
>>   dget(dentry);
>>   return 0;
>> +
>> +out_remove_offset:
>> + stable_offset_remove(dir, dentry);
>>  out_iput:
>>   iput(inode);
>>   return error;
>> @@ -3920,6 +3958,8 @@ 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)


--
Chuck Lever