Date: Wed, 27 Jun 2018 19:14:47 -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: <623F6077-0A89-4D35-9E83-76BF40CC290F@FreeBSD.org> In-Reply-To: <CANCZdfpJzFn0tj-PGY8Zp%2BMT7AZrYxTR5DEjHsFeDFfiDCnxGw@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>
next in thread | previous in thread | raw e-mail | index | archive | help
> On Jun 27, 2018, at 6:58 PM, Warner Losh <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 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: 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 Devin=
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?623F6077-0A89-4D35-9E83-76BF40CC290F>