Re: [PATCH v4 0/3] FUSE: Implement atomic lookup + open/create

[Vivek Goyal]

On Wed, May 11, 2022 at 11:40:59AM +0200, Miklos Szeredi wrote:
> On Thu, 5 May 2022 at 21:59, Vivek Goyal <vgoyal@xxxxxxxxxx> wrote:
> 
> > Oh, I have no issues with the intent. I will like to see cut in network
> > traffic too (if we can do this without introducing problems). My primary
> > interest is that this kind of change should benefit virtiofs as well.
> 
> One issue with that appears to be checking permissions.   AFAIU this
> patchset only enables the optimization if default_permissions is
> turned off (i.e. all permission checking is done by the server).  But
> virtiofs uses the default_permissions model.

IIUC, only 3rd patch mentions that default_permission should be turned
off. IOW, first patch where lookup + create + open is a single operation
and second patch which does "lookup + open" in a single operation does
not seem to require that default_permissions are not in effect.

So if first two patches work fine, I think virtiofs should benefit too.
(IMHO, 3rd patch is too hacky anyway)

W.r.t permission checks, looks like may_open() will finally be called
after ->atomic_open(). So even if we open the file, we should still be
able to check whether we have permissions to open the file or not
after the fact.

fs/namei.c

path_openat()
{
	open_last_lookups()  <--- This calls ->atomic_open()
	do_open()  <--- This calls may_open()
}

Thanks
Vivek

> 
> I'm not quite sure about this limitation, guessing that it's related
> to the fact that the permissions may be stale at the time of checking?
> 
> Thanks,
> Miklos
>