Hi Amir,
On Oct 10 2016, Amir Goldstein <amir73il@xxxxxxxxx> wrote:
> Hi Nikolaus,
>
> On Sun, Oct 9, 2016 at 7:37 AM, Nikolaus Rath <Nikolaus@xxxxxxxx> 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
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