Date: Sun, 08 Feb 2015 09:51:11 -0600 From: Eric Badger <eric@badgerio.us> To: John Baldwin <jhb@freebsd.org>, freebsd-current@freebsd.org, kostikbel@gmail.com Subject: Re: Filepaths in VM map for tmpfs files Message-ID: <54D785EF.7020802@badgerio.us> In-Reply-To: <14469498.JC5CzcnZkB@ralph.baldwin.cx> References: <54CCEFAB.9040406@badgerio.us> <1601131.aIB9RoRbLs@ralph.baldwin.cx> <20150205083755.GG42409@kib.kiev.ua> <14469498.JC5CzcnZkB@ralph.baldwin.cx>
next in thread | previous in thread | raw e-mail | index | archive | help
On 02/05/2015 07:25 AM, John Baldwin wrote: > 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. Re: the kve_vn* fields, isn't setting kve_status = KF_ATTR_VALID the way to mark them as valid (irrespective of kve_type)? As for path name, I'd agree that there's no inherent need to restrict it by type. The field is somewhat self-validating (if something other than an empty string was returned in the path name field, this field is obviously valid). > >>> 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. > I'll make one more brief case for returning tmpfs vm objects as KVME_TYPE_SWAP. Isn't the purpose of this sysctl for debugging, or to help a user understand what is going on internally? I can imagine scenarios where knowing that a mapped file is swap backed is relevant information, and returning it as KVME_TYPE_VNODE would hide this information. I'd put forth a vote for return vnode info on a best-effort basis, at least for now. Eric
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?54D785EF.7020802>