Date: Wed, 27 Jun 2018 20:35:46 -0600 From: Warner Losh <imp@bsdimp.com> To: Devin Teske <dteske@freebsd.org> Cc: 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: <CANCZdfonitWdaAXXoQnPZtMd_ytumGdWr=WYfX5CHYMytsXuOA@mail.gmail.com> In-Reply-To: <623F6077-0A89-4D35-9E83-76BF40CC290F@FreeBSD.org> 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>
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On Wed, Jun 27, 2018 at 8:14 PM, Devin Teske <dteske@freebsd.org> wrote: > > On Jun 27, 2018, at 6:58 PM, Warner Losh <imp@bsdimp.com> wrote: > > > > On Wed, Jun 27, 2018 at 7:49 PM, Devin Teske <dteske@freebsd.org> wrote: > >> >> On Jun 27, 2018, at 5:59 PM, Warner Losh <imp@bsdimp.com> wrote: >> >> >> >> On Wed, Jun 27, 2018 at 5:52 PM, Gleb Smirnoff <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 >>> W> >>> W> Log: >>> W> Fix devctl generation for core files. >>> W> >>> 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. >>> >>> Is this going to work when a core is dumped not at current working >>> directory, >>> but at absolute path? e.g. kern.corefile=/var/log/cores/%N.core >>> >> >> Yes. That works. >> >> >>> Looks like the vn_fullpath_global needs to be fixed rather than problem >>> workarounded. >>> >> >> 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. >> >> >> They said it couldn't be done, but I personally have done it ... >> >> I map vnodes to full paths in dwatch. It's not impossible, just >> implausibly hard. >> >> 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). >> >> You're welcome to see how it's done in /usr/libexec/dwatch/vop_create >> >> 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). >> >> 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). >> >> 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. >> >> Maybe some of this can be useful? If not, just ignore me. >> > > 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. > > I look forward to seeing your conversion of the D to C that works, even > when there's namespace cache pressure. > > > 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. > 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... Warnerhome | help
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