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

[yangerkun <yangerkun@huaweicloud.com>]

在 2024/12/24 21:52, Chuck Lever 写道:
> 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() ?

Emm... The reason that why readdir report file with descending cookie
order is d_alloc will insert child dentry to the list head of
&parent->d_subdirs, and find_positive_dentry will get child in order. So
it seems this won't work?

> 
> 
>> 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;
>>>>>   }
>>>>
>>>
>>>
>>
> 
>