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SubjectRe: [PATCH 00/25] vfs: atomic open RFC
Date
Christoph Hellwig <hch@infradead.org> writes:

> Do we really need the opendata structure?
>
> It seems like we could just pass a struct path instead of the dentry
> passed directly and the vfsmount in it. There should be no need to
> preallocate the file before calling into ->atomic_open, as it's only
> used to pass around f_flags - but we already pass that one to
> ->atomic_open directly and might as well pass it on to finish_open and
> allocate the file there.

We really don't want to get into the situation where the open fails
after a successful create(*). Which means the file needs to be allocated
prior to calling ->atomic_open and needs to be passed to finish_open()
toghether with the vfsmount and dentry.

In the first version of the patch I set filp->f_path.mnt to nd->path.mnt
and passed the half initialized filp to ->atomic_open. But then decided
that it's confusing for the filesystem code to deal with a half baked
filp (does it need to be fput on error? etc...)

Doing it with an opaque opendata makes this cleaner I think.

Thanks,
Miklos


(*)
commit a1a5b3d93ca45613ec1d920fdb131b69b6553882
Author: Peter Staubach <staubach@redhat.com>
Date: Tue Sep 13 01:25:12 2005 -0700

[PATCH] open returns ENFILE but creates file anyway

When open(O_CREAT) is called and the error, ENFILE, is returned, the file
may be created anyway. This is counter intuitive, against the SUS V3
specification, and may cause applications to misbehave if they are not
coded correctly to handle this semantic. The SUS V3 specification
explicitly states "No files shall be created or modified if the function
returns -1.".

The error, ENFILE, is used to indicate the system wide open file table is
full and no more file structs can be allocated.

This is due to an ordering problem. The entry in the directory is created
before the file struct is allocated. If the allocation for the file struct
fails, then the system call must return an error, but the directory entry
was already created and can not be safely removed.

The solution to this situation is relatively easy. The file struct should
be allocated before the directory entry is created. If the allocation
fails, then the error can be returned directly. If the creation of the
directory entry fails, then the file struct can be easily freed.


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