Date: Wed, 27 Jun 2018 23:54:41 -0700 From: Devin Teske <dteske@FreeBSD.org> To: Warner Losh <imp@bsdimp.com> Cc: Devin Teske <dteske@FreeBSD.org>, Gleb Smirnoff <glebius@freebsd.org>, Warner Losh <imp@freebsd.org>, src-committers <src-committers@freebsd.org>, svn-src-all@freebsd.org, svn-src-head@freebsd.org Subject: Re: svn commit: r335690 - head/sys/kern Message-ID: <7C4AEADC-1973-482B-A7A5-75F23397D44E@FreeBSD.org> In-Reply-To: <CANCZdfonitWdaAXXoQnPZtMd_ytumGdWr=WYfX5CHYMytsXuOA@mail.gmail.com> References: <201806270411.w5R4B9ZB078994@repo.freebsd.org> <20180627235216.GO1165@FreeBSD.org> <CANCZdfpdBOin=w0qEGXM%2BciTAbETezYTOPq1nuEV1pT5QATEHA@mail.gmail.com> <16B9F36D-E92A-4FEC-B096-5E24A4E180FC@FreeBSD.org> <CANCZdfpJzFn0tj-PGY8Zp%2BMT7AZrYxTR5DEjHsFeDFfiDCnxGw@mail.gmail.com> <623F6077-0A89-4D35-9E83-76BF40CC290F@FreeBSD.org> <CANCZdfonitWdaAXXoQnPZtMd_ytumGdWr=WYfX5CHYMytsXuOA@mail.gmail.com>
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> On Jun 27, 2018, at 7:35 PM, Warner Losh <imp@bsdimp.com> wrote: >=20 >=20 >=20 > On Wed, Jun 27, 2018 at 8:14 PM, Devin Teske <dteske@freebsd.org = <mailto:dteske@freebsd.org>> wrote: >=20 >> On Jun 27, 2018, at 6:58 PM, Warner Losh <imp@bsdimp.com = <mailto:imp@bsdimp.com>> wrote: >>=20 >>=20 >>=20 >> On Wed, Jun 27, 2018 at 7:49 PM, Devin Teske <dteske@freebsd.org = <mailto:dteske@freebsd.org>> wrote: >>=20 >>> On Jun 27, 2018, at 5:59 PM, Warner Losh <imp@bsdimp.com = <mailto:imp@bsdimp.com>> wrote: >>>=20 >>>=20 >>>=20 >>> On Wed, Jun 27, 2018 at 5:52 PM, Gleb Smirnoff <glebius@freebsd.org = <mailto:glebius@freebsd.org>> wrote: >>> On Wed, Jun 27, 2018 at 04:11:09AM +0000, Warner Losh wrote: >>> W> Author: imp >>> W> Date: Wed Jun 27 04:11:09 2018 >>> W> New Revision: 335690 >>> W> URL: https://svnweb.freebsd.org/changeset/base/335690 = <https://svnweb.freebsd.org/changeset/base/335690>; >>> W>=20 >>> W> Log: >>> W> Fix devctl generation for core files. >>> W> =20 >>> W> We have a problem with vn_fullpath_global when the file exists. = Work >>> W> around it by printing the full path if the core file name = starts with /, >>> W> or current working directory followed by the filename if not. >>>=20 >>> Is this going to work when a core is dumped not at current working = directory, >>> but at absolute path? e.g. kern.corefile=3D/var/log/cores/%N.core >>>=20 >>> Yes. That works. >>> =20 >>> Looks like the vn_fullpath_global needs to be fixed rather than = problem >>> workarounded. >>>=20 >>> It can't be fixed reliably. FreeBSD does not and cannot map a vnode = to a name. The only reason we're able to at all is due to the name = cache. And when we recreate a file, we invalidate the name cache. And = even if we fixed that, there's no guarantee the name cache won't get = flushed before we translate the name.... Linux can do this because it = keeps the path name associated with the inode. FreeBSD simply doesn't. >>>=20 >>=20 >> They said it couldn't be done, but I personally have done it ... >>=20 >> I map vnodes to full paths in dwatch. It's not impossible, just = implausibly hard. >>=20 >> I derived my formula by reading the C code which was very = twisty-turny and rather hard to read at times (and I have been using C = since 1998 on everything from AIX and OSF/1 to HP/UX, Cygwin, MinGW, = FreeBSD, countless Linux-like things, IRIX, and a God-awful remainder of = many others; the vnode code was an adventure to say the least). >>=20 >> You're welcome to see how it's done in /usr/libexec/dwatch/vop_create >>=20 >> I load up a clause-local variable named "path" with a path = constructed from a pointer to a vnode structure argument to = VOP_CREATE(9). >>=20 >> The D code could easily be rewritten back into C to walk the vnode to = completion (compared to the D code which is bounded by depth-limit since = DTrace doesn't provide loops; so you have to, as I have done, use a = higher-level language wrapper to repeat the code to some desired limit; = dwatch defaulting to 64 for directory depth limit). >>=20 >> Compared this, say, to vfssnoop.d from Chapter 5 of the DTrace book = which utilizes the vfs:namei:lookup: probes. My approach in dwatch is = far more accurate, produces full paths, and I've benchmarked it at lower = overhead with better results. >>=20 >> Maybe some of this can be useful? If not, just ignore me. >>=20 >> IMHO, there's no benefit from the crazy hoops than the super simple = heuristic I did: if the path starts with / print it. If the path doesn't = print cwd / and then the path. >>=20 >> I look forward to seeing your conversion of the D to C that works, = even when there's namespace cache pressure. >>=20 >=20 > I was looking at the output of "dwatch -dX vop_create" (the -d flag to = dwatch causes the generated dwatch to be printed instead of executed) = and thinking to myself: >=20 > I could easily convert this, as I can recognize the flattened loop = structure. However, not many people will be able to do it. I wasn't = really volunteering, but now I'm curious what other people see when they = run "dwatch -dX vop_create". If it's half as bad as I think it is, then = I'll gladly do it -- but will have to balance it against work = priorities. >=20 > We don't have the data you do in vop_create anymore, just the vp at = the point in the code where we want to do the reverse translation... But = we also have a name that's been filled in from the template and cwd, = which gives us almost the same thing as we'd hoped to dig out of the = name cache... >=20 Given: struct vnode *vp, const char *fi_name Here's a rough (not optimized; unchecked) crack at a C equivalent: #include <sys/mount.h> #include <sys/vnode.h> #include <limits.h> int i, max_depth =3D 64; char *d_name =3D NULL; char *fi_fs =3D NULL; char *fi_mount =3D NULL; char *cp; struct namecache *ncp =3D NULL; struct mount *mount =3D NULL; struct vnode *dvp =3D NULL; char *nc[max_depth+1]; char path[PATH_MAX] =3D __DECONST(char *, ""); if (vp !=3D NULL) { ncp =3D vp->v_cache_dst.tqh_first; mount =3D vp->v_mount; /* ptr to vfs we are in */ if (vp->v_cache_dd !=3D NULL) d_name =3D vp->v_cache_dd->nc_name; } if (mount !=3D NULL) { fi_fs =3D mount->mnt_stat.f_fstypename; if (fi_fs !=3D NULL && *fi_fs !=3D '\0') { if (!strcmp(fi_fs, "devfs")) ncp =3D NULL; } else { if (fi_name =3D=3D NULL || *fi_name =3D=3D '\0') = ncp =3D NULL; } fi_mount =3D mount->mnt_stat.f_mntonname; } else { ncp =3D NULL; } for (i =3D 0; i <=3D max_depth; i++) { nc[i] =3D NULL; } if (ncp !=3D NULL) { /* Reach into vnode of parent of name */ if (ncp->nc_dvp !=3D NULL) dvp =3D ncp->nc_dvp->v_cache_dst.tqh_first; if (dvp !=3D NULL) nc[0] =3D dvp->nc_name; } if (nc[0] =3D=3D NULL || *nc[0] =3D=3D '\0' || !strcmp(nc[0], = "/")) dvp =3D NULL; /* * BEGIN Pathname-depth loop */ for (i =3D 1; i < max_depth; i++) { if (dvp =3D=3D NULL) break; if (dvp->nc_dvp !=3D NULL) dvp =3D dvp->nc_dvp->v_cache_dst.tqh_first; else dvp =3D NULL; if (dvp !=3D NULL) nc[i] =3D dvp->nc_name; } /* * END Pathname-depth loop */ if (dvp !=3D NULL) { if (dvp->nc_dvp !=3D NULL) dvp =3D dvp->nc_dvp->v_cache_dst.tqh_first; else dvp =3D NULL; if (dvp !=3D NULL && dvp->nc_dvp !=3D NULL) nc[max_depth] =3D __DECONST(char *, "..."); } if (fi_mount !=3D NULL) { /* * Join full path * NB: Up-to but not including the parent directory = (joined below) */ strcat(path, fi_mount); if (strcmp(fi_mount, "/")) strcat(path, "/"); for (i =3D max_depth; i >=3D 0; i--) { char *namei =3D nc[i]; strcat(path, namei =3D=3D NULL ? "" : namei); if (namei !=3D NULL) strcat(path, "/"); } /* Join the parent directory name */ if (d_name !=3D NULL && *d_name !=3D '\0') { strcat(path, d_name); strcat(path, "/"); } /* Join the entry name */ if (fi_name !=3D NULL && *fi_name !=3D '\0') strcat(path, fi_name); } =09 --=20 Devin=
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