Why does readdir() open files ?

Ben RUBSON ben.rubson@gmail.com
Wed Mar 28 17:51:00 GMT 2018


On 28 Mar 2018 19:03, Corinna Vinschen wrote:

> On Mar 28 18:35, Ben RUBSON wrote:
>> On 28 Mar 2018, Ben RUBSON wrote:
>>
>>> Hi,
>>>
>>> I'm porting a FUSE FS to Cygwin, and I focus especially on readdir()
>>> performance.
>>>
>>> What I see is that when readdir() occurs, each file of the directory is
>>> as expected (at least I think) stated (getattr), but also opened (open).
>>> The first block of the file was even red, until I set the notexec mount
>>> option.
>>>
>>> My question is, why is every file opened ?
>>> This is quite a performance killer, particularly for a FUSE FS.
>>>
>>> I thought this was to calculate the inode number, I then set the ihash
>>> mount option, but it did not help.
>>
>> I may be totally wrong, but perhaps the culprit is the following code  
>> path ?
>> https://github.com/mirror/newlib-cygwin/blob/master/winsup/cygwin/fhandler_disk_file.cc#L2262
>> Perhaps we simply don't need to open the file if (!hasgood_inode ()) ?
>>
>> That would be a nice performance improvement !
>
> You should make sure that the hasgood_inode() check works for your FUSE
> FS.  Check at the end of fs_info::update():
>
>   has_acls (flags () & FS_PERSISTENT_ACLS);
>   /* Netapp inode numbers are fly-by-night. */
>   hasgood_inode ((has_acls () && !is_netapp ()) || is_nfs ());
>
> So your filesystem should return the FS_PERSISTENT_ACLS flags in a call
> to NtQueryVolumeInformationFile(..., FileFsAttributeInformation)

Thank you for your answer Corinna.

You assume FS_PERSISTENT_ACLS must be set, otherwise I would not pass the  
following dir->__flags condition ?
if (de->d_ino == 0 && (dir->__flags & dirent_set_d_ino))
https://github.com/mirror/newlib-cygwin/blob/master/winsup/cygwin/fhandler_disk_file.cc#L2225

So, as goal is to avoid opening files, I then should remove this flag from  
my FS ?

I found that hasgood_inode() is also defined in winsup/cygwin/path.h :
bool hasgood_inode () const {return !(path_flags & PATH_IHASH); }

And when I set the ihash mount option, it is correctly honoured : generated  
inode numbers differ from the "true" ones.
Can't we also rely on the ihash mount option to avoid opening files ?

Thank you again for your clarification & support !

Ben



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