Your CVS fix 1.109 to union_vnops.c
Takanori Watanabe
takawata at init-main.com
Sun Oct 3 11:06:58 PDT 2004
In message <41601BE0.4050401 at geminix.org>, Uwe Doering さんいわく:
>takawata at jp.freebsd.org wrote:
>> In message <415FC1A1.3020502 at geminix.org>, Uwe Doering wrote:
>>
>>>Hi there,
>>>
>>>with regard to your above mentioned fix you may be interested in reading
>>>this short discussion, especially my answer to the original article:
>>>
>>>http://docs.freebsd.org/cgi/getmsg.cgi?fetch=116615+0+archive/2004/freebsd-s
>table/20040613.freebsd-stable
>>
>> Thank you for your comment.
>>
>> The code is what nullfs do.
>>
>> static int
>> null_getattr(ap)
>> struct vop_getattr_args /* {
>> struct vnode *a_vp;
>> struct vattr *a_vap;
>> struct ucred *a_cred;
>> struct thread *a_td;
>> } */ *ap;
>> {
>> int error;
>>
>> if ((error = null_bypass((struct vop_generic_args *)ap)) != 0)
>> return (error);
>>
>> ap->a_vap->va_fsid = ap->a_vp->v_mount->mnt_stat.f_fsid.val[0];
>> return (0);
>> }
>>
>> I'm pleased if you explain why it is done
>> for nullfs and not for unionfs, if any.
>> IMHO, there are not so much advantage in assuming
>> exactly same file exists in different filesystem,
>> if the entity is same.
>
>'nullfs' has only one underlying file system, so replacing the file
>system id doesn't break the uniqueness of the va_fsid/va_fileid pair.
>The latter is the inode number in case of UFS.
>
>With 'unionfs' you can have underlying files from two different layers
>(upper and lower) on two different file systems which may, by
>coincidence, have the same inode number. Now, if you override the real
>va_fsid with that of the 'unionfs' mount you'll end up with two
>'unionfs' vnodes that appear to represent the same file (a hard link,
>for instance), but in reality the files are different entities.
>Obviously, both the kernel and applications might draw wrong conclusions
>in this case.
I think the three filesystem entry
1. upper layer file
2. lower layer file
3. unionfs file
can be treated as different.
>> But I want to hear from FS gurus.
>> I found that I reverted the change at CVS rev 1.62.
>> http://www.freebsd.org/cgi/cvsweb.cgi/src/sys/fs/unionfs/union_vnops.c.diff?
>r1=1.61&r2=1.62
>
>Right. Better safe than sorry.
Same change are applyed in nullfs and reverted by bp at .
http://www.freebsd.org/cgi/cvsweb.cgi/src/sys/fs/nullfs/null_vnops.c.diff?r1=1.33&r2=1.34
http://www.freebsd.org/cgi/cvsweb.cgi/src/sys/fs/nullfs/null_vnops.c.diff?r1=1.40&r2=1.41
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