Date: Fri, 29 Jun 2018 15:17:25 -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: <63B2C22B-9B7C-4F8C-A153-8B4A6618558A@FreeBSD.org> In-Reply-To: <CANCZdfoTY3kFYj7kthBwhZfudPvOuynd6O_V12XDftC7qBQ3aA@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> <7C4AEADC-1973-482B-A7A5-75F23397D44E@FreeBSD.org> <CANCZdfoTY3kFYj7kthBwhZfudPvOuynd6O_V12XDftC7qBQ3aA@mail.gmail.com>
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> On Jun 28, 2018, at 12:36 AM, Warner Losh <imp@bsdimp.com> wrote: >=20 >=20 >=20 > On Thu, Jun 28, 2018 at 12:54 AM, Devin Teske <dteske@freebsd.org = <mailto:dteske@freebsd.org>> wrote: >=20 >> On Jun 27, 2018, at 7:35 PM, Warner Losh <imp@bsdimp.com = <mailto: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 >=20 > Given: >=20 > struct vnode *vp, const char *fi_name >=20 > Here's a rough (not optimized; unchecked) crack at a C equivalent: >=20 > Thanks, but this code suffers from the same problem that = vn_fullpath_global() suffers from: we clear out parts of the cache when = we open an existing file with O_CREAT. This one NULL leads to a cascade = of failures, as show below. >=20 >=20 >=20 > #include <sys/mount.h> > #include <sys/vnode.h> >=20 > #include <limits.h> >=20 > 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 *, ""); >=20 > if (vp !=3D NULL) { > ncp =3D vp->v_cache_dst.tqh_first; >=20 > For my case, this is NULL since we clear out the cache when we = re-create the file. I confirmed this by adding a printf where I put in = my workaround and I get: > vp->v_cache_dst.tqh_first is 0xfffff80003bd7a10 (first time, no = cat.core exists) > vp->v_cache_dst.tqh_first is 0 (second time, cat.core does exist) >=20 > 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; > } >=20 > since ncp is NULL here, we never set nc[0] >=20 > =20 > if (nc[0] =3D=3D NULL || *nc[0] =3D=3D '\0' || !strcmp(nc[0], = "/")) > dvp =3D NULL; >=20 > so dvp is NULL > =20 > /* > * BEGIN Pathname-depth loop > */ > for (i =3D 1; i < max_depth; i++) { > if (dvp =3D=3D NULL) break; >=20 > and we break out of this loop without setting any other nc[] entries. > =20 > 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); >=20 > which joins a lot of empty strings together > =20 > 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); >=20 > since we don't have the fi_name from the VOP_CREATE where we want to = do this, we have nothing for here. > =20 > } >=20 > So I appreciate the effort here, but I'm not sure that it's useful in = this context. vn_fullpath* already can do this sort of thing, but can't = in the case where vp->v_cache_dst.tqh_first is NULL. >=20 Thank you so much for the thoughtful response. It means a lot really. --=20 Cheers, Devin=
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