Re: [fuse-devel] Difference between invalidating and deleting dentry

[Nikolaus Rath <Nikolaus@rath.org>]

Hi Amir,

On Oct 10 2016, Amir Goldstein <amir73il@gmail.com> wrote:
> Hi Nikolaus,
>
> On Sun, Oct 9, 2016 at 7:37 AM, Nikolaus Rath <Nikolaus@rath.org> wrote:
>> Hello,
>>
>> I just added an example to FUSE that illustrates use of the
>> fuse_lowlevel_notify_inval_entry() function. However, when writing it I
>> realized that I don't actually fully understand how this function
>> differs from fuse_lowlevel_notify_delete(). Could someone shed some
>> light on this?
>>
>> Currently, the FUSE documentation says:
>>
>> fuse_lowlevel_notify_inval_entry:
>>    Notify to invalidate parent attributes and the dentry matching
>>    parent/name
>>
>> fuse_lowlevel_notify_delete:
>>    Notify to invalidate parent attributes and delete the dentry matching
>>    parent/name if the dentry's inode number matches child (otherwise it
>>    will invalidate the matching dentry).
>>
>>
>> But what exactly is the difference between deleting and invalidating a
>> dentry?
>
> That is the difference:
>
> /*
>  * d_drop() unhashes the entry from the parent dentry hashes, so that it won't
>  * be found through a VFS lookup any more. Note that this is different from
>  * deleting the dentry - d_delete will try to mark the dentry negative if
>  * possible, giving a successful _negative_ lookup, while d_drop will
>  * just make the cache lookup fail.
>  */

Alright, so at this point I thought I understood the difference and got
ready to update the documentation, but then you got me very confused:

> But since fuse_lowlevel_notify_delete does among other things:
> d_invalidate->...d_drop()
> d_delete()
>
> You may still ask yourself what is the purpose of d_delete() after d_drop(),
> because there is no cache entry to make negative...

So, in other words, FUSE's notify_delete will *not* store a negative
dentry, but will just drop the dentry?

>> In each case, isn't the resulting behavior the same, in that the
>> next time someone tries to access this (parent_inode,entry_name)
>> combination a lookup() request will be send to the FUSE process?
>
> You are right about the next lookup behavior being the same, but there
> are other things that d_delete() does which d_invalidate does not, which
> are important, like calling fsnotify_nameremove() and update the cached
> inode and dentry that are referenced by open files.

Hmm. So when should one use notify_delete() and when
notify_inval_entry()? I understand there is a difference, but I'm
uncertain about the practical consequences...


Thanks!
-Nikolaus

-- 
GPG encrypted emails preferred. Key id: 0xD113FCAC3C4E599F
Fingerprint: ED31 791B 2C5C 1613 AF38 8B8A D113 FCAC 3C4E 599F

             »Time flies like an arrow, fruit flies like a Banana.«