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+[[!meta copyright="Copyright © 2012 Free Software Foundation, Inc."]]
+
+[[!meta license="""[[!toggle id="license" text="GFDL 1.2+"]][[!toggleable
+id="license" text="Permission is granted to copy, distribute and/or modify this
+document under the terms of the GNU Free Documentation License, Version 1.2 or
+any later version published by the Free Software Foundation; with no Invariant
+Sections, no Front-Cover Texts, and no Back-Cover Texts.  A copy of the license
+is included in the section entitled [[GNU Free Documentation
+License|/fdl]]."]]"""]]
+
+[[!tag open_issue_gnumach]]
+
+
+# IRC, freenode, #hurd, 2012-04-23
+
+    <braunr> btw, i'm running a gnumach version using red-black trees for vm
+      map entries
+    <antrik> braunr: sounds fashionable ;-)
+    <youpi> braunr: with some perf improvement?
+    <braunr> looks promising for our ext2fs instances showing several thousands
+      of map entries
+    <braunr> youpi: i'm not using it for lookups yet
+    <braunr> but the tree is there, maintained, used for both regular and copy
+      maps, and it doesn't crash
+    <youpi> good :)
+    <braunr> antrik: isn't it ? :)
+    <braunr> youpi: and the diff stat is like 50/15
+    <antrik> braunr: what's the goal of using the fashionable trees?
+    <braunr> antrik: speeding up lookups in address spaces
+    <antrik> braunr: so the idea is that if we have a heavily fragmented
+      address space, the performance penalty is smaller?
+    <braunr> yes
+    <antrik> OK
+    <antrik> I take it you gave up on attempts to actually decrease
+      fragmentation?...
+    <braunr> it's not as good as reducing fragmentation, which requires
+      implementing a powerful merge, but it's still better
+    <braunr> yes
+    <braunr> it's too messy for my brain :/
+    <antrik> I see
+    <antrik> oh
+    <braunr> it will add some overhead though
+    <youpi> I guess log(n) ?
+    <braunr> but if there is a significant performance gain, it'll be worth it
+    <braunr> yes
+    <braunr> i was more thinking about the memory overhead
+    <antrik> right now it's a linear list?
+    <youpi> I don't think we care nowadays :)
+    <braunr> antrik: yes
+    <antrik> ouch
+    <braunr> antrik: yes ... :>
+    <braunr> the original authors expected vm maps to have like 30 entries
+    <braunr> so they used a list for the maps, and a hash table for the
+      object/offset to physical page lookups
+    <braunr> there is a small lookup cache though, which is a nice optimization
+    <braunr> my code now uses it first, and falls back to the RB tree if the
+      hint didn't help
+    <antrik> braunr: well, don't forget to check whether it actually *is* still
+      an optimisation, when using fashionable trees ;-)
+    <braunr> antrik: i checked that already :)
+    <braunr> i did the same in x15
+    <antrik> I see
+    <braunr> both bsd and linux uses a similar technique
+    <braunr> use*
+    <braunr> (well, bsd actually use what is done in mach :)
+    <antrik> (or perhaps the other way around... ;-) )
+    <braunr> i don't think so, as the bsd vm is really the mach vm
+    <braunr> but we don't care much
+    <antrik> oh, right... that part actually went full circle
+    <braunr> youpi: i have a patch ready for test on machines with significant
+      amounts of map entries (e.g. the buildds ..)
+    <braunr> youpi: i won't have time for tests tonight, are you interested ?
+    <braunr> (i've been running it for 15 minutes without any issue for now)
+    <youpi> I'd say post to the list
+    <braunr> ok
+    <youpi> braunr: your patch uses the rb tree for lookups, right?
+    <youpi> braunr: the buildd using rbtree seems swift
+    <youpi> but maybe it's just a psychologic effect :)
+    <youpi> the chroot ext2fs already has 1392 lines in vminfo
+    <youpi> an rbtree can't hurt  there :)
+    <youpi> braunr: it really seems faster
+    <youpi> the reboot might have helped too
+    <youpi> benchmarks shall say
+    <youpi> for now, I'll just let ironforge use it
+    <antrik> youpi: it's always fast after a reboot ;-)
+    <youpi> sure
+    <youpi> but still
+    <youpi> I mean
+    <youpi> *obviously* the reboot has helped
+    <youpi> but it might not be all
+    <youpi> at least it feels so
+    <youpi> and obviously only benchmarks can say
+    <antrik> the major benefit AIUI is rather that the slowdown happening over
+      time will be less noticable
+
+[[performance/degradation]].
+
+    <youpi> "over time" is actually quite fast
+    <youpi> ext2 fills up very quickly when you build a package
+    <youpi> it went up to 1700 lines very quickly
+    <youpi> and stabilized around there
+    <antrik> yeah, it can be very fast under heavy load
+    <youpi> that's why I say reboot seems not all
+    <youpi> it's already not so fresh
+    <youpi> with 1700 lines in vminfo
+    <antrik> well, I don't know how much of the slowdown I'm experiencing
+      (after doing stuff under memory pressure) is actually related to VM map
+      fragmentation...
+    <antrik> might be all, might be none, might be something in between
+    <youpi> then try his patch
+    <antrik> guess I should play a bit with vminfo...
+    <antrik> well, currently I actually experience pretty little slowdown, as
+      for certain reasons (only indirectly related to the Hurd) I'm not running
+      mutt on this machine, so I don't really have memory pressure...
+
+
+## IRC, freenode, #hurd, 2012-04-24
+
+    <braunr> youpi: yes, it uses bst lookups
+    <braunr> youpi: FYI, the last time i checked, one ext2fs instance had 4k+
+      map entries, and another around 7.5k
+    <braunr> (on ironforge)
+
+
+## IRC, freenode, #hurd, 2012-04-24
+
+    <youpi> braunr: $ sudo vminfo  624 | wc -l
+    <youpi> 22957
+    <youpi> there's no way it can not help :)
+    <braunr> youpi: 23k, that's really huge
+
+
+## IRC, freenode, #hurd, 2012-04-26
+
+    <braunr> youpi: any new numbers wrt the rbtree patch ?
+    <youpi> well, buildd times are not really accurate :)
+    <youpi> but what I can tell is that it managed to build qtwebkit fine
+    <braunr> ok
+    <youpi> so the patch is probably safe :)
+    <braunr> i'll commit it soon then
+    <youpi> I'd say feel free to, yes
+    <braunr> thanks
+
+
+## IRC, freenode, #hurd, 2012-04-27
+
+    <braunr> elmig: don't expect anything grand though, this patch is mostly
+      useful when address spaces get really fragmented, which mainly happens on
+      buildds
+    <braunr> (and it only speeds lookups, which isn't as good as reducing
+      fragmentation; things like fork still have to copy thousands of map
+      entries)
+
+[[glibc/fork]].
+
+
+## IRC, freenode, #hurdfr, 2012-06-02
+
+    <youpi> braunr: oh, un bug de rbtree
+    <youpi> Assertion `diff != 0' failed in file "vm/vm_map.c", line 1002
+    <youpi> c'est dans rbtree_insert()
+    <youpi> vm_map_enter (vm/vm_map.c:1002).
+    <youpi> vm_map (vm/vm_user.c:373).
+    <youpi> syscall_vm_map (kern/ipc_mig.c:657).
+    <youpi> erf j'ai tué mon débuggueur, je ne peux pas inspecter plus
+    <youpi> le peu qui me reste c'est qu'apparemment target_map == 1, size ==
+      0, mask == 0
+    <youpi> anywhere = 1
+    <braunr> youpi: ça signifie sûrement que des adresses overlappent
+    <braunr> je rejetterai un coup d'oeil sur le code demain
+    <braunr> (si ça se trouve c'est un bug rare de la vm, le genre qui fait
+      crasher le noyau)
+    <braunr> (enfin jveux dire, qui faisait crasher le noyau de façon très
+      obscure avant le patch rbtree)
+
+
+### IRC, freenode, #hurd, 2012-07-15
+
+    <bddebian> I get errors in vm_map.c whenever I try to "mount" a CD
+    <bddebian> Hmm, this time it rebooted the machine
+    <bddebian> braunr: The translator set this time and the machine reboots
+      before I can get the full message about vm_map, but here is some of the
+      crap I get:  http://paste.debian.net/179191/
+    <braunr> oh
+    <braunr> nice
+    <braunr> that may be the bug youpi saw with my redblack tree patch
+    <braunr> bddebian: assert(diff != 0); ?
+    <bddebian> Aye
+    <braunr> good
+    <braunr> it means we're trying to insert a vm_map_entry at a region in a
+      map which is already occupied
+    <bddebian> Oh
+    <braunr> and unlike the previous code, the tree actually checks that
+    <braunr> it has to
+    <braunr> so you just simply use the iso9660fs translator and it crashes ?
+    <bddebian> Well it used to on just trying to set the translator.  This time
+      I was able to set the translator but as soon as I cd to the mount point I
+      get all that crap
+    <braunr> that's very good
+    <braunr> more test cases to fix the vm
+
+
+### IRC, freenode, #hurd, 2012-11-01
+
+    <youpi> braunr: Assertion `diff != 0' failed in file "vm/vm_map.c", line
+      1002
+    <youpi> that's in rbtree_insert
+    <braunr> youpi: the problem isn't the tree, it's the map entries
+    <braunr> some must overlap
+    <braunr> if you can inspect that, it would be helpful
+    <youpi> I have a kdb there
+    <youpi> it's within a port_name_to_task system call
+    <braunr> this assertion basically means there already is an item in the
+      tree where the new item is supposed to be inserted
+    <youpi> this port_name_to_task presence in the stack is odd
+    <braunr> it's in vm_map_enter
+    <youpi> there's a vm_map just after that (and the assembly trap code
+      before)
+    <youpi> I know
+    <youpi> I'm wondering about the caller
+    <braunr> do you have a way to inspect the inserted map entry ?
+    <youpi> I'm actually wondering whether I have the right kernel in gdb
+    <braunr> oh
+    <youpi> better
+    <youpi> with the right kernel :)
+    <youpi> 0x80039acf (syscall_vm_map)
+      (target_map=d48b6640,address=d3b63f90,size=0,mask=0,anywhere=1)
+    <youpi> size == 0 seems odd to me
+    <youpi> (same parameters for vm_map)
+    <braunr> right
+    <braunr> my code does assume an entry has a non null size
+    <braunr> (in the entry comparison function)
+    <braunr>        EINVAL (since Linux 2.6.12) length was 0.
+    <braunr> that's a quick glance at mmap(2)
+    <braunr> might help track bugs from userspace (e.g. in exec .. :))
+    <braunr> posix says the saem
+    <braunr> same*
+    <braunr> the gnumach manual isn't that precise
+    <youpi> I don't seem to manage to read the entry
+    <youpi> but I guess size==0 is the problem anyway
+    <mcsim> youpi, braunr: Is there another kernel fault? Was that in my
+      kernel?
+    <braunr> no that's another problem
+    <braunr> which became apparent following the addition of red black trees in
+      the vm_map code
+    <braunr> (but which was probably present long before)
+    <mcsim> braunr: BTW, do you know if there where some specific circumstances
+      that led to memory exhaustion in my code? Or it just aggregated over
+      time?
+    <braunr> mcsim: i don't know
+    <mcsim> s/where/were
+    <mcsim> braunr: ok
+
+
+### IRC, freenode, #hurd, 2012-11-05
+
+    <tschwinge> braunr: I have now also hit the diff != 0 assertion error;
+      sitting in KDB, waiting for your commands.
+    <braunr> tschwinge: can you check the backtrace, have a look at the system
+      call and its parameters like youpi did ?
+    <tschwinge> If I manage to figure out how to do that...  :-)
+    * tschwinge goes read scrollback.
+    <braunr> "trace" i suppose
+    <braunr> if running inside qemu, you can use the integrated gdb server
+    <tschwinge> braunr: No, hardware.  And work intervened.  And mobile phone
+      <-> laptop via bluetooth didn't work.  But now:
+    <tschwinge> Pretty similar to Samuel's:
+    <tschwinge>     Assert([...])
+    <tschwinge>     vm_map_enter(0xc11de6c8, 0xc1785f94, 0, 0, 1)
+    <tschwinge>     vm_map(0xc11de6c8, 0xc1785f94, 0, 0, 1)
+    <tschwinge>     syscall_vm_map(1, 0x1024a88, 0, 0, 1)
+    <tschwinge>     mach_call_call(1, 0x1024a88, 0, 0, 1)
+    <braunr> thanks
+    <braunr> same as youpi observed, the requested size for the mapping is 0
+    <braunr> tschwinge: thanks
+    <tschwinge> braunr: Anything else you'd like to see before I reboot?
+    <braunr> tschwinge: no, that's enough for now, and the other kind of info
+      i'd like are much more difficult to obtain
+    <braunr> if we still have the problem once a small patch to prevent null
+      size is applied, then it'll be worth looking more into it
+    <pinotree> isn't it possible to find out who called with that size?
+    <braunr> not easy, no
+    <braunr> it's also likely that the call that fails isn't the first one
+    <pinotree> ah sure
+    <pinotree> braunr: making mmap reject 0 size length could help? posix says
+      such size should be rejected straight away
+    <braunr> 17:09 < braunr> if we still have the problem once a small patch to
+      prevent null size is applied, then it'll be worth looking more into it
+    <braunr> that's the idea
+    <braunr> making faulty processes choke on it should work fine :)
+    <pinotree> «If len is zero, mmap() shall fail and no mapping shall be
+      established.»
+    <pinotree> braunr: should i cook up such patch for mmap?
+    <braunr> no, the change must be applied in gnumach
+    <pinotree> sure, but that could simply such condition in mmap (ie avoiding
+      to call io_map on a file)
+    <braunr> such calls are erroneous and rare, i don't see the need
+    <pinotree> ok
+    <braunr> i bet it comes from the exec server anyway :p
+    <tschwinge> braunr: Is the mmap with size 0 already a reproducible testcase
+      you can use for the diff != 0 assertion?
+    <tschwinge> Otherwise I'd have a reproducer now.
+    <braunr> tschwinge: i'm not sure but probably yes
+    <tschwinge> braunr: Otherwise, take GDB sources, then: gcc -fsplit-stack
+      gdb/testsuite/gdb.base/morestack.c && ./a.out
+    <tschwinge> I have not looked what exactly this does; I think -fsplit-stack
+      is not really implemented for us (needs something in libgcc we might not
+      have), is on my GCC TODO list already.
+    <braunr> tschwinge: interesting too :)
+
+
+### IRC, freenode, #hurd, 2012-11-19
+
+    <tschwinge> braunr: Hmm, I have now hit the diff != 0 GNU Mach assertion
+      failure during some GCC invocation (GCC testsuite) that does not relate
+      to -fsplit-stack (as the others before always have).
+    <tschwinge> Reproduced:
+      /media/erich/home/thomas/tmp/gcc/hurd/master.build/gcc/xgcc
+      -B/media/erich/home/thomas/tmp/gcc/hurd/master.build/gcc/
+      /home/thomas/tmp/gcc/hurd/master/gcc/testsuite/gcc.dg/torture/pr42878-1.c
+      -fno-diagnostics-show-caret -O2 -flto -fuse-linker-plugin
+      -fno-fat-lto-objects -fcompare-debug -S -o pr42878-1.s
+    <tschwinge> Will check whether it's the same backtrace in GNU Mach.
+    <tschwinge> Yes, same.
+    <braunr> tschwinge: as youpi seems quite busy these days, i'll cook a patch
+      and commit it directly
+    <tschwinge> braunr: Thanks!  I have, by the way, confirmed that the
+      following is enough to trigger the issue: vm_map(mach_task_self(), 0, 0,
+      0, 1, 0, 0, 0, 0, 0, 0);
+    <tschwinge> ... and before the allocator patch, GNU Mach did accept that
+      and return 0 -- though I did not check what effect it actually has.  (And
+      I don't think it has any useful one.)  I'm also reading that as of lately
+      (Linux 2.6.12), mmap (length = 0) is to return EINVAL, which I think is
+      the foremost user of vm_map.
+    <pinotree> tschwinge: posix too says to return EINVAL for length = 0
+    <braunr> yes, we checked that earlier with youpi
+
+[[!message-id "87sj8522zx.fsf@kepler.schwinge.homeip.net"]].
+
+    <braunr> tschwinge: well, actually your patch is what i had in mind
+      (although i'd like one in vm_map_enter to catch wrong kernel requests
+      too)
+    <braunr> tschwinge: i'll work on it tonight, and do some testing to make
+      sure we don't regress critical stuff (exec is another major direct user
+      of vm_map iirc)
+    <tschwinge> braunr: Oh, OK.  :-)