Re: [PATCH v6 5/5] libfs: Use d_children list to iterate simple_offset directories

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

On 12/23/24 11:40 PM, yangerkun wrote:
> 
> 
> 在 2024/12/23 22:44, Chuck Lever 写道:
>> On 12/23/24 9:21 AM, yangerkun wrote:
>>>
>>>
>>> 在 2024/12/20 23:33, cel@kernel.org 写道:
>>>> From: Chuck Lever <chuck.lever@oracle.com>
>>>>
>>>> The mtree mechanism has been effective at creating directory offsets
>>>> that are stable over multiple opendir instances. However, it has not
>>>> been able to handle the subtleties of renames that are concurrent
>>>> with readdir.
>>>>
>>>> Instead of using the mtree to emit entries in the order of their
>>>> offset values, use it only to map incoming ctx->pos to a starting
>>>> entry. Then use the directory's d_children list, which is already
>>>> maintained properly by the dcache, to find the next child to emit.
>>>>
>>>> One of the sneaky things about this is that when the mtree-allocated
>>>> offset value wraps (which is very rare), looking up ctx->pos++ is
>>>> not going to find the next entry; it will return NULL. Instead, by
>>>> following the d_children list, the offset values can appear in any
>>>> order but all of the entries in the directory will be visited
>>>> eventually.
>>>>
>>>> Note also that the readdir() is guaranteed to reach the tail of this
>>>> list. Entries are added only at the head of d_children, and readdir
>>>> walks from its current position in that list towards its tail.
>>>>
>>>> Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
>>>> ---
>>>>   fs/libfs.c | 84 ++++++++++++++++++++++++++++++++++++ 
>>>> +-----------------
>>>>   1 file changed, 58 insertions(+), 26 deletions(-)
>>>>
>>>> diff --git a/fs/libfs.c b/fs/libfs.c
>>>> index 5c56783c03a5..f7ead02062ad 100644
>>>> --- a/fs/libfs.c
>>>> +++ b/fs/libfs.c
>>>> @@ -247,12 +247,13 @@ EXPORT_SYMBOL(simple_dir_inode_operations);
>>>>   /* simple_offset_add() allocation range */
>>>>   enum {
>>>> -    DIR_OFFSET_MIN        = 2,
>>>> +    DIR_OFFSET_MIN        = 3,
>>>>       DIR_OFFSET_MAX        = LONG_MAX - 1,
>>>>   };
>>>>   /* simple_offset_add() never assigns these to a dentry */
>>>>   enum {
>>>> +    DIR_OFFSET_FIRST    = 2,        /* Find first real entry */
>>>>       DIR_OFFSET_EOD        = LONG_MAX,    /* Marks EOD */
>>>>   };
>>>> @@ -458,51 +459,82 @@ static loff_t offset_dir_llseek(struct file 
>>>> *file, loff_t offset, int whence)
>>>>       return vfs_setpos(file, offset, LONG_MAX);
>>>>   }
>>>> -static struct dentry *offset_find_next(struct offset_ctx *octx, 
>>>> loff_t offset)
>>>> +static struct dentry *find_positive_dentry(struct dentry *parent,
>>>> +                       struct dentry *dentry,
>>>> +                       bool next)
>>>>   {
>>>> -    MA_STATE(mas, &octx->mt, offset, offset);
>>>> +    struct dentry *found = NULL;
>>>> +
>>>> +    spin_lock(&parent->d_lock);
>>>> +    if (next)
>>>> +        dentry = d_next_sibling(dentry);
>>>> +    else if (!dentry)
>>>> +        dentry = d_first_child(parent);
>>>> +    hlist_for_each_entry_from(dentry, d_sib) {
>>>> +        if (!simple_positive(dentry))
>>>> +            continue;
>>>> +        spin_lock_nested(&dentry->d_lock, DENTRY_D_LOCK_NESTED);
>>>> +        if (simple_positive(dentry))
>>>> +            found = dget_dlock(dentry);
>>>> +        spin_unlock(&dentry->d_lock);
>>>> +        if (likely(found))
>>>> +            break;
>>>> +    }
>>>> +    spin_unlock(&parent->d_lock);
>>>> +    return found;
>>>> +}
>>>> +
>>>> +static noinline_for_stack struct dentry *
>>>> +offset_dir_lookup(struct dentry *parent, loff_t offset)
>>>> +{
>>>> +    struct inode *inode = d_inode(parent);
>>>> +    struct offset_ctx *octx = inode->i_op->get_offset_ctx(inode);
>>>>       struct dentry *child, *found = NULL;
>>>> -    rcu_read_lock();
>>>> -    child = mas_find(&mas, DIR_OFFSET_MAX);
>>>> -    if (!child)
>>>> -        goto out;
>>>> -    spin_lock(&child->d_lock);
>>>> -    if (simple_positive(child))
>>>> -        found = dget_dlock(child);
>>>> -    spin_unlock(&child->d_lock);
>>>> -out:
>>>> -    rcu_read_unlock();
>>>> +    MA_STATE(mas, &octx->mt, offset, offset);
>>>> +
>>>> +    if (offset == DIR_OFFSET_FIRST)
>>>> +        found = find_positive_dentry(parent, NULL, false);
>>>> +    else {
>>>> +        rcu_read_lock();
>>>> +        child = mas_find(&mas, DIR_OFFSET_MAX);
>>>
>>> Can this child be NULL?
>>
>> Yes, this mas_find() call can return NULL. find_positive_dentry() should
>> then return NULL. Kind of subtle.
>>
>>
>>> Like we delete some file after first readdir, maybe we should break 
>>> here, or we may rescan all dentry and return them to userspace again?
>>
>> You mean to deal with the case where the "next" entry has an offset
>> that is lower than @offset? mas_find() will return the entry in the
>> tree that is "at or after" mas->index.
>>
>> I'm not sure either "break" or returning repeats is safe. But, now that
>> you point it out, this function probably does need additional logic to
>> deal with the offset wrap case.
>>
>> But since this logic already exists here, IMO it is reasonable to leave
>> that to be addressed by a subsequent patch. So far there aren't any
>> regression test failures that warn of a user-visible problem the way it
>> is now.
> 
> Sorry for the confusing, the case I am talking is something like below:
> 
> mkdir /tmp/dir && cd /tmp/dir
> touch file1 # offset is 3
> touch file2 # offset is 4
> touch file3 # offset is 5
> touch file4 # offset is 6
> touch file5 # offset is 7
> first readdir and get file5 file4 file3 file2 #ctx->pos is 3, which
> means we will get file1 for second readdir
> 
> unlink file1 # can not get entry for ctx->pos == 3
> 
> second readdir # offset_dir_lookup will use mas_find but return NULL,
> and we will get file5 file4 file3 file2 again?

After this patch, directory entries are reported in descending
cookie order. Therefore, should this patch replace the mas_find() call
with mas_find_rev() ?


> And for the offset wrap case, I prefer it's safe with your patch if we 
> won't unlink file between two readdir. The second readdir will use an
> active ctx->pos which means there is a active dentry attach to this
> ctx->pos. find_positive_dentry will stop once we meet the last child.
> 
> 
> I am not sure if I understand correctly, if not, please point out!
> 
> Thanks!
> 
>>
>>
>>>> +        found = find_positive_dentry(parent, child, false);
>>>> +        rcu_read_unlock();
>>>> +    }
>>>>       return found;
>>>>   }
>>>>   static bool offset_dir_emit(struct dir_context *ctx, struct dentry 
>>>> *dentry)
>>>>   {
>>>>       struct inode *inode = d_inode(dentry);
>>>> -    long offset = dentry2offset(dentry);
>>>> -    return ctx->actor(ctx, dentry->d_name.name, dentry->d_name.len, 
>>>> offset,
>>>> -              inode->i_ino, fs_umode_to_dtype(inode->i_mode));
>>>> +    return dir_emit(ctx, dentry->d_name.name, dentry->d_name.len,
>>>> +            inode->i_ino, fs_umode_to_dtype(inode->i_mode));
>>>>   }
>>>> -static void offset_iterate_dir(struct inode *inode, struct 
>>>> dir_context *ctx)
>>>> +static void offset_iterate_dir(struct file *file, struct 
>>>> dir_context *ctx)
>>>>   {
>>>> -    struct offset_ctx *octx = inode->i_op->get_offset_ctx(inode);
>>>> +    struct dentry *dir = file->f_path.dentry;
>>>>       struct dentry *dentry;
>>>> +    dentry = offset_dir_lookup(dir, ctx->pos);
>>>> +    if (!dentry)
>>>> +        goto out_eod;
>>>>       while (true) {
>>>> -        dentry = offset_find_next(octx, ctx->pos);
>>>> -        if (!dentry)
>>>> -            goto out_eod;
>>>> +        struct dentry *next;
>>>> -        if (!offset_dir_emit(ctx, dentry)) {
>>>> -            dput(dentry);
>>>> +        ctx->pos = dentry2offset(dentry);
>>>> +        if (!offset_dir_emit(ctx, dentry))
>>>>               break;
>>>> -        }
>>>> -        ctx->pos = dentry2offset(dentry) + 1;
>>>> +        next = find_positive_dentry(dir, dentry, true);
>>>>           dput(dentry);
>>>> +
>>>> +        if (!next)
>>>> +            goto out_eod;
>>>> +        dentry = next;
>>>>       }
>>>> +    dput(dentry);
>>>>       return;
>>>>   out_eod:
>>>> @@ -541,7 +573,7 @@ static int offset_readdir(struct file *file, 
>>>> struct dir_context *ctx)
>>>>       if (!dir_emit_dots(file, ctx))
>>>>           return 0;
>>>>       if (ctx->pos != DIR_OFFSET_EOD)
>>>> -        offset_iterate_dir(d_inode(dir), ctx);
>>>> +        offset_iterate_dir(file, ctx);
>>>>       return 0;
>>>>   }
>>>
>>
>>
> 


-- 
Chuck Lever