Date: Thu, 05 Feb 2015 08:25:41 -0500 From: John Baldwin <jhb@freebsd.org> To: Konstantin Belousov <kostikbel@gmail.com> Cc: Eric Badger <eric@badgerio.us>, freebsd-current@freebsd.org Subject: Re: Filepaths in VM map for tmpfs files Message-ID: <14469498.JC5CzcnZkB@ralph.baldwin.cx> In-Reply-To: <20150205083755.GG42409@kib.kiev.ua> References: <54CCEFAB.9040406@badgerio.us> <1601131.aIB9RoRbLs@ralph.baldwin.cx> <20150205083755.GG42409@kib.kiev.ua>
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On Thursday, February 05, 2015 10:37:55 AM Konstantin Belousov wrote: > On Wed, Feb 04, 2015 at 10:15:04AM -0500, John Baldwin wrote: > > On Tuesday, February 03, 2015 10:33:36 PM Konstantin Belousov wrote: > > > On Mon, Feb 02, 2015 at 09:50:22PM -0600, Eric Badger wrote: > > > > On 02/02/2015 03:30 AM, Konstantin Belousov wrote: > > > > > On Sun, Feb 01, 2015 at 08:38:29PM -0600, Eric Badger wrote: > > > > >> On 01/31/2015 09:36 AM, Konstantin Belousov wrote: > > > > >>> First, shouldn't the kve_type changed to KVME_TYPE_VNODE as well ? > > > > >> > > > > >> My thinking is no, because KVME_TYPE_SWAP is in fact the correct > > > > >> type; > > > > >> I'd opine that it is better to be transparent than make it look > > > > >> like > > > > >> there is an OBJT_VNODE object there. It may be that some programs > > > > >> would > > > > >> be confused by VNODE info returned on a SWAP type mapping, though I > > > > >> know > > > > >> that dtrace handles it OK. > > > > > > > > > > kve_vn_* and kve_path fields are defined only for KVME_TYPE_VNODE > > > > > kve_type. > > > > > So this is in fact a bug in whatever used the API to access kve_path > > > > > for KVE_TYPE_SWAP. > > > > > > > > Hmm, is that documented anywhere? I think it's fair to assume that > > > > kve_vn* applies only to the VNODE type, > > > > but I know there are several in-tree users that reference kve_path > > > > regardless of type (ostensibly relying > > > > on the default of an empty string). Maybe one could determine the > > > > validity of the kve_vn* fields by > > > > inspecting the kve_vn_type (not sure of all the consequences of that)? > > > > Or change it to KVME_TYPE_VNODE > > > > and deal with the below problem... > > > > > > There is no useful documentation for the kern.proc. sysctls. > > > My word (and statements from other involved developers) could be > > > considered as close to the truth as it can be. > > > Somebody taking the efforts to document the stuff would make very > > > valuable contribution. > > > > I think that kve_path should be valid for all types (e.g. shm_open() > > is not a vnode but has a pathname, and that should be fixed to display > > if possible). In the equivalent for files (kinfo_file), the pathname > > is type-independent and always valid. > > Well, this means that it should be valid for vnodes and shm. My point > is that kvme_vn_path should be used only after the check for type. > We can and do set it to nul string, but using the path unconditionally > is a bug in the user code. The problem is that shm's can have different types (DEFAULT vs SWAP vs PHYS). :) For kinfo_file, tools like fstat always print kf_path regardless of type. I do think it would be more consistent if the path in a kvme worked the same way. Then you don't have to update all the tools each time a type starts populating the path. > > That said, I think tmpfs nodes should be exposed as files. It is an > > implementation detail of tmpfs that they are swap-backed, but from a > > user's perspective these are files, and if you want to expose other > > vnode-specific fields than just the path, KVME_TYPE_VNODE would be > > more correct. > > I agree, but doing it is not easy, since there might be no vnode > to get the required information from. We do know that this swap > object is for tmpfs node, but currently we only store pointer to > object in the node, not pointer to node from the object. When the > vnode exists, pointer to vnode is stored in the object. > > To fix the issue, we should store pointer to node. Code was not done > this way, because VM code which handles special-case for OBJT_TMPFS, > would need to know tmpfs internals. Right now, code knows about vnodes > anyway, so object->vnode does not bring tmpfs internals into vm. I'm more arguing in support of your original proposal. Doing a best effort if the vnode exists would certainly be an improvement over what we have now. -- John Baldwin
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