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# Implementation

  * [[filetype]] option

  * [[Hurd-specific_extensions]]

  * [[Page_cache]]

  * [[metadata_caching]]

  * [[internal_allocator]]


## Large Stores

The `ext2fs` translator from the upstream Hurd code base can only handle file
systems with sizes of less than roughly 2 GiB.

[[!tag open_issue_hurd]]


### Ognyan's Work

  * Ognyan Kulev, [[*Supporting Large ext2 File Systems in the
    Hurd*|ogi-fosdem2005.mgp]], 2005, at FOSDEM

  * Ognyan Kulev, [[large_stores]]

  * <http://kerneltrap.org/node/4429>

Ognyan's patch lifts this limitation (and is being used in the
[[Debian_GNU/Hurd_distribution|running/debian]]), but it introduces another
incompatibility: `ext2fs` then only supports block sizes of 4096 bytes.
Smaller block sizes are commonly automatically selected by `mke2fs` when using
small backend stores, like floppy devices.


#### IRC, freenode, #hurd, 2012-06-30

    <braunr> at least having the same api in the debian package and the git
      source would be great (in reference to the large store patch ofc)
    <youpi> braunr: the api part could be merged perhaps
    <youpi> it's very small apparently
    <antrik> braunr: the large store patch is a sad story. when it was first
      submitted, one of the maintainers raised some concerns. the other didn't
      share these (don't remember who is who), but the concerned one never
      followed up with details. so it has been in limbo ever since. tschwinge
      once promised to take it up, but didn't get around to it so far. plus,
      the original author himself mentioned once that he didn't consider it
      finished...
    <youpi> antrik: it's clearly not finished
    <youpi> there are XXXs here and there
    <braunr> it's called an RC1 and RC2 is mentioned in the release notes
    <antrik> youpi: well, that doesn't stop most other projects from commiting
      stuff... including most emphatically the original Hurd code :-)
    <youpi> what do you refer to my "that" ? :)
    <braunr> "XXX"
    <youpi> right
    <youpi> at the time it made sense to delay applying
    <youpi> but I guess by nowadays standard we should just as well commit it
    <youpi> it works enough for Debian, already
    <youpi> there is just one bug I nkow about
    <youpi> the apt database file keeps haveing the wrong size, fixed by e2fsck
    <pinotree> youpi: remember that patch should be fixed in the offset
      declaration in diskfs.h
    <youpi> I don't remember about that
    <youpi> did we fix it in the debian package?
    <pinotree> nope
    <youpi> you had issues when fixing it, didn't you?
    <youpi> (I don't remember where I can find the details about this)
    <pinotree> i changed it, recompiled hurd and installed it, started a perl
      rebuild and when running one of the two lfs tests it hard locked the vm
      after ext2fs was taking 100% cpu for a bit
    <pinotree> i don't exclude i could have done something stupid on my side
      though
    <youpi> or there could just be actual issues, uncovered here
    <youpi> which can be quite probable


##### IRC, freenode, #hurd, 2013-03-19

    <braunr> youpi: i'm back on polishing the large store patch
    <braunr> did you remember seeing something else than the bzero/memset
      out-of-scope changes ?
    <braunr> (i mean, readily noticeable)
    <youpi> I don't remember
    <braunr> ok
    <braunr> the original code already assumes mmap succeeds
    <braunr> is it ok to consider the patch can do the same ?
    <youpi> I'd say so
    <braunr> ok
    <braunr> youpi: actually, it looks like a good part of the patch isn't
      related to large stores
    <braunr> for example, in ext2fs/inode.c, there are calls to
      dino_ref/dino_deref
    <youpi> hum
    <braunr> i'm not sure at all these have anything to do with handling large
      stores
    <youpi> but dino_ref is introduced by this patch, isn't it?
    <braunr> it replaces dino
    <youpi> yes, it replaces it because the dino() approach can't work beyond
      2GiB
    <braunr> i see
    <braunr> youpi: i'd like to replace the recursive call to
      disk_cache_block_ref with a goto, is that fine with you ?
    <youpi> looks ok to me
    <youpi> better than relying on tail recursion
    <braunr> that's the idea :)


#### [[libpager]] API change

##### IRC, freenode, #hurd, 2013-03-04

    <braunr> youpi: i don't remember exactly your answer when i asked about
      considering the ext2 large store patch for merging
    <youpi> there's just an API change that it introduces
    <youpi> but otherwise I'd say we should just do it
    <braunr> ok
    <youpi> I've just checked the API change again
    <youpi> it's simply adding a notify_on_evict parameter
    <youpi> and a pager_notify_evict callback
    <braunr> yes
    <youpi> I'd say we mostly need to polish this
    <youpi> ah, there is the same parameter on diskfs_start_disk_pager


##### IRC, freenode, #hurd, 2013-04-23

    <braunr> and i'm working again on the ext2fs large store patch
    <braunr> i finished separating the libpager interface change from the rest,
      as Thomas suggested, so a new version should be ready soon


#### `EXT2FS_DEBUG`

##### IRC, freenode, #hurd, 2013-03-04

    <braunr> youpi: do we want EXT2FS_DEBUG defined upstream ?
    <youpi> I don't really have an opinion on this
    <youpi> stuffing it in the large store patch is not good of course
    <youpi> I wonder whether we want it by default.
    <braunr> it is currently defined by the patch
    <braunr> (in the debian package i mean)
    <youpi> I've just seen that yes
    <braunr> i won't include it upstream, and if we decide to keep this
      behaviour, we can add a patch just for that
    <braunr> or a define
    <braunr> err
    <braunr> a configure option
    <youpi> ok


#### IRC, freenode, #hurd, 2013-10-08

    <braunr> ogi: your ext2fs patches were finally merged upstream :)


## Sync Interval

[[!tag open_issue_hurd]]


### IRC, freenode, #hurd, 2012-10-08

    <braunr> btw, how about we increase our ext2 sync interval to 30 seconds,
      like others do ?
    <braunr> not really because others do it that way, but because it severely
      breaks performance on the hurd
    <braunr> and 30 seems like a reasonable amount (better than 5 at least)

That would be a nice improvement, but only after writeback throttling is implemented.


## Stripped vs. Unstripped `ext2fs.static`

[[!tag open_issue_hurd]]


### IRC, freenode, #hurd, 2013-09-17

    <teythoon> I always had some trouble with dropping a rebuild ext2fs.static
      into my test system and I never figured out why
    <teythoon> I just followed a hunch and stripped the binary, and all of the
      sudden it works
    <teythoon> any ideas why?
    <tschwinge> teythoon: I quick search found me:
      <https://savannah.gnu.org/bugs/?8497> and
      <http://news.gmane.org/find-root.php?message_id=%3c4090243E.2040605%40comcast.net%3e>.
    <teythoon> tschwinge: ugh, thanks for the pointers ;)
    <tschwinge> teythoon: They won't help too much I fear.  Anyway, good
      intuition (or whatever) ;-) that you found this out.
    <tschwinge> teythoon: Not exactly related to stripped/unstripped per se
      (that is, debug information), but in the past we've had an issue about
      relro (see binutils/glibc, <http://www.airs.com/blog/archives/189>),
      where a variable (that erroneously happend to be in such a read-only
      section, if I remember correct) was tried to be modified -- which worked
      "sometimes": depending on where exactly it was located in the binary
      (which shifted around a page
    <tschwinge> boundary by stripped/unstripped), it'd segfault or not.  Burnt
      several days on that before Samuel (IIRC) eventually figured it out.
    <teythoon> tschwinge: well, thanks anyway ;)


## IRC, freenode, #hurd, 2014-02-11

    <gg0> task with pid 5 deallocating an invalid port 4622, most probably a
      bug.
    <gg0> ext2fs
    <teythoon> yes, i've seen this
    <teythoon> e.g. when a passive translator starts
    <teythoon> i guess it is in libfshelp/translator-list.c


## Inode Sizes, Fragment and Block Sizes

### IRC, freenode, #hurd, 2014-02-12

    <gg0> this might be interesting and could make people not to fsck hurd
      filesystem on linux:
    <gg0> start ext2fs: ext2fs: device:hd0s1
    <braunr> ?
    <gg0> : panic: get_hypermetadata: inode size 256 isn't supported
    <gg0> (wait, also a bad typist)
    <braunr> well, if the file system was created from the hurd, or with -o
      hurd, as it ought to be, you wouldn't have this problem
    <gg0> oh, good to know, especially before restoring :p
    <braunr> i suspect your mkfs command to have created an ext4 fs
    <gg0> nope mkfs.ext2
    <braunr> hm ok, so it seems to create 256 size inodes by default there
    <gg0> i guess -o hurd would set some os-specific properties
    <braunr> it merely enforces a few restrictions
    <gg0> some predefined defaults
    <braunr> fragments and blocks are 4k
    <braunr> and apparently inodes are 128 bytes
    <gg0> because it can't support bigger values? is it worth working on remove
      such restrictions?
    <braunr> probably not so far
    <braunr> certainly not the fragment/block size restriction
    <braunr> it matches the page size
    <braunr> larger inode sizes could be supported if they're dependencies for
      other worthwhile features such as those someone would add in an ext4
      translator


## Linux' `CONFIG_EXT4_USE_FOR_EXT23`

### IRC, freenode, #hurd, 2014-02-12

    <gg0> why the hell i have thousands of Inode 839, i_blocks is 248, should
      be 256.  Fix<y>? yes
    <gg0> in all cases i_blocks should be +8
    <gg0> and /dev/sda1: (There are 245635 inodes containing multiply-claimed
      blocks.)
    <gnu_srs1> 10:50:08< gg0> start ext2fs: Hurd server bootstrap:
      ext2fs[device:hd0s1] exec
    <gnu_srs1> That's exactly where my image boot hangs!
    <gg0> start ext2fs: Hurd server bootstrap: ext2fs[gunzip:device:rd0] exec
    <AliciaC> gnu_srs1: you might want to check that linux isn't using the ext4
      module to handle ext2 and ext3 filesystems
    <AliciaC> gnu_srs1: as I understand it the idea is that the ext4 module
      treats them as ext2/ext3 filesystems, just avoiding code duplication from
      having three separate modules for related filesystems, so it shouldn't
      change it from ext2 at all, but it does do something strange with it
    <AliciaC> but I'm not sure if that's the case or if it's converting it to
      ext4. last I heard Hurd doesn't support anything beyond ext2
    <gnu_srs1> AliciaC: I did use ext2 when mounting from Linux: mount -t ext2
      /dev/loop0 /mnt
    <gnu_srs1> and when not mounted: e2fsck /dev/loop0
    <AliciaC> gnu_srs1: I'd check the kernel config to be sure,
      CONFIG_EXT4_USE_FOR_EXT23 must be disabled
    <braunr> you can use the ext4 driver for ext2
    <braunr> that's not a problem here
    <braunr> the problem happens long before, when the file system gets
      corrupted
    <braunr> you must understand why
    <AliciaC> I have done some testing on this, mounting a Hurd ext2 filesystem
      with the ext4 module broke it for me, an easily repeated issue
    <AliciaC> mounting Debian's ext2 image and unmounting it with ext4 broke
      it, resulting precisely in the kind of hang ups mentioned by gnu_srs1 and
      gg0
    <braunr> interesting
    <AliciaC> that's with a clean image with nothing corrupting it before hand,
      tested to be working as well
    <braunr> ok so the ext4 driver must ignore hurd specific stuff
    <braunr> that's strange because i recall using it to perform small repairs
      on darnassus and never had any issue
    <braunr> even on the root file syste
    <braunr> but my repairs were very quick and targetted
    <AliciaC> different linux versions maybe
    <AliciaC> when I was testing it I didn't even need to do anything in the
      filesystem to trigger the issue, just mount and unmount
    <gnu_srs1> I repaired filesystems before like this, has something happened
      with later versions of Linux?
    <gnu_srs1> One of my boxes is ext3 (probably worked before) another ext4
      (the one breaking things, but worked before)
    <gnu_srs1> ext3 and ext4 box: CONFIG_EXT4_USE_FOR_EXT23=y same kernel
      3.12-1.amd64
    <gnu_srs1> what about mounting with bs=4096 (used by hurd)
    <braunr> -t ext2 should work fine
    <braunr> just don't use the ext4 driver if in doubt
    <gg0> no difference between specifying -t or not, in both cases EXT4-fs
      (sda1): mounting ext2 file system using the ext4 subsystem
    <braunr> hmm
    <braunr> you're screwed then ;
    <braunr> ;p
    <braunr> or maybe -t ext3 .. :)
    <braunr> although i suspect ext4 would be used then too
    <gg0> linux-image-3.2.0-4-amd64:
      /lib/modules/3.2.0-4-amd64/kernel/fs/ext2/ext2.ko
    <gg0> wheezy still has it. then something between 3.2.0 and 3.13(?) removed
      it
    <braunr> check the config file
    <gg0> i mean ext2 module
    <braunr> check if the config file enables it
    <gnu_srs1> It's not: # CONFIG_EXT2_FS is not set
    <gg0> 14:42 < gg0> wheezy still has it. then something between 3.2.0 and
      3.13(?) removed it
    <braunr> how about retrying what you did without ever mounting from linux ?
    <braunr> gg0: it wasn't clear enough that you meant removed from
      configuration
    <braunr> (for example, it could have been blacklisted)
    <gg0> or present not as a module
    <braunr> maybe yes, although it's unusual to see generic kernels embedding
      file systems these days
    <AliciaC> the CONFIG_EXT4_USE_FOR_EXT23 option isn't available if either
      ext2 or ext3 are enabled though, even just as loadable modules
    <gnu_srs1> The ext2 and ext3 modules were there in  3.10-3, not in 3.12-1
    <gnu_srs1> (14:48:59) <srs>: It's not: # CONFIG_EXT2_FS is not set --
      3.12-1
    <gg0> https://bugs.debian.org/731072
    * gg0 rsync'ing back to new fs with 3.10 kernel
    <gnu_srs1> seems like this bug was archived without being closed??
    <gg0> someone should produce a testcase and file another one btw
    <gnu_srs1> but that bug was for files systems up to 4MB, not 4GB?
    <gg0> i pasted it just because submitter talks about config option in
      question and when was enabled
    <gg0> don't we want to thank AliciaC who pointed it out and who could
      precisely file a bug? :)
    <gg0> filed http://bugs.debian.org/738758
    <braunr> gg0: thanks
    <braunr> AliciaC: and thanks too


### IRC, freenode, #hurd, 2014-02-13

    <gnu_srs> gg0: Did you create and test with an ext2 Linux image too on
      3.10/3.12?
    <gnu_srs> here is a diff: http://paste.debian.net/81837/
    <gnu_srs> visible differences: Filesystem features:filetype (linux only)
      and Free inodes:1268(hurd)  / 1269(linux)
    <AliciaC> between one created with -o Hurd and one created with -o Linux
      (or no -o)?
    <gnu_srs> AliciaC: -o Hurd and -b 4096 (no -o)
    <AliciaC> I wonder if that would show any interesting difference between an
      untouched -o Hurd ext2 image and a copy of it that has been mounted with
      the ext4 module
    <gnu_srs> AliciaC: here: http://paste.debian.net/81857/
    <gnu_srs> there is a difference of one in the number of free inodes!
    <gnu_srs> cf the number of free inodes for linux
    <AliciaC> gnu_srs: thanks :) though I don't know what to make of that, I
      guess just adding an inode shouldn't break anything
    <AliciaC> wait, no, removing an inode?
    <AliciaC> bleh, too tired, read it wrong
    <gnu_srs> this line should read:(11:37:48) srs: visible differences:
      Filesystem features:filetype (linux only) and Free inodes:1268(linux)  /
      1269(hurd)
    <gnu_srs> There are differences in ext2.h and ext4.h in the Linux source
      code wrt hurd1, hurd2 structs.
    <gnu_srs> one change might be interesting: http://paste.debian.net/81864/
    <braunr> gnu_srs: probably not
    <gnu_srs> If not, where to look?
    <braunr> well, the first thing would be to create a (small) ext2 file
      system, usable on the hurd, with a few files and directories
    <braunr> save it
    <braunr> mount it with the ext4 driver
    <braunr> and make a binary comparison
    <braunr> you could use a modified ext2fs translator to tell you exactly
      what's wrong when loading the file system
    <braunr> and then look at the corresponding code in the ext4 driver
    <gnu_srs1> braunr: here is a binary diff of the unmounted and mounted e2fs
      files: http://paste.debian.net/81896/
    <braunr> gnu_srs1: i'm not going to analyze it
    <braunr> you are
    <braunr> :p
    <gnu_srs1> many of them can be removed: e.g. /mnt and bug000
    <braunr> ?
    <gnu_srs1> many diff entries*
    <braunr> but why ?
    <braunr> you shouldn't have changed the content at all
    <gnu_srs1> If I don't add a file, the fs is not corrupted
    <gnu_srs1> this is with two vers small files created as in gg0s bug report
    <gnu_srs1> very*
    <braunr> ok
    <braunr> i guess checking the source code first and the binary diffs next
      is easier
    <gnu_srs1> OK, I have to find out how the ext2fs files are organized.
    <gnu_srs1> I.e. reading mke2fs source code
    <braunr> no
    <braunr> read the ext4 driver
    <braunr> how a directory entry is created
    <braunr> how a directory is saved back on the block device
    <braunr> how any potential conversion could be triggered
    <gnu_srs1> k, will do
    <braunr> read about the ext2fs format if doing that first doesn't help
    <braunr> learning a file system is complicated and long
    <gnu_srs1> What is the inode size for Hurd/Linux?
    <braunr> probably 128
    <gnu_srs1> same for both?
    <braunr> what is "Hurd/Linux" ?
    <gnu_srs1> on Hurd / on Linux
    <braunr> 128 on hurd, variable on linux
    <braunr> 128-512 i'd say
    <gnu_srs1> ext2 on linux
    <gnu_srs1> found it from dumpe2fs: 128 for both
    <braunr> no, it can vary on linux
    <braunr> although once a file system is built, the inode size cannot be
      changed
    <gnu_srs1> k, the file created with mke2fs has 128


### IRC, freenode, #hurd, 2014-03-10

    <mcsim> hi. I'm having problems with copying directory from host machine to
      hurd image. I mount hurd image at host and than use "cp -r" to copy
      directory. Then I unmount image and start qemu. When I try to see
      contents of the directory I get error "Computer bought the farm".
    <braunr> mcsim: are you using the ext4 driver from linux ?
    <mcsim> I use image debian-hurd-20140211.img from 
    <braunr> if so, avoid it
    <mcsim> ok
    <mcsim> i'll try
    <mcsim> braunr: do I have to compile module on my own? because modprobe
      ext2 && lsmod | grep ext2 give no output?
    <braunr> maybe
    <braunr> mcsim: what kernel are you using ?
    <mcsim> 3.12
    <braunr> oh
    <mcsim> braunr: what is wrong with it?
    <braunr> grep 'EXT[234]' /boot/config*
    <braunr> it probably uses ext4 for ext2 and ext3
    <braunr> and i don't think it's possible to simply load the ext2fs module
    <braunr> because the options are mutually exclusive iirc
    <mcsim> # CONFIG_EXT2_FS is not set
    <mcsim> is there another way to copy data?
    <braunr> probably not
    <braunr> (check for CONFIG_EXT4_USE_FOR_EXT23)
    <gg0> mcsim: install 3.10
      http://snapshot.debian.org/package/linux/3.10.11-1/#linux-image-3.10-3-686-pae_3.10.11-1
    <gg0> https://bugs.debian.org/738758
    <gg0> assuming host is debian
    <mcsim> gg0: thank you
    * gg0 redirects thanks to AliciaC who caught it
    <AliciaC> gg0: thanks :)


## `ext2fs: ../../ext2fs/pager.c:401: file_pager_write_page: Assertion 'block' failed.`

### IRC, freenode, #hurd, 2014-02-19

    <pere> "ext2fs: ../../ext2fs/pager.c:401: file_pager_write_page: Assertion
      'block' failed." in the console.

[[user/Maksym_Planeta]] also has hit that one.

    <braunr> wow oO
    <braunr> using debian hurd right ?
    <pere> power cycling.
    <pere> yes.
    <braunr> with hurd 1:0.5.git20140203-1 and glibc 2.17-98~1 ?
    <pere> braunr: not sure how to check.
    <braunr> pere: dpkg -l | grep .. i suppose
    <pere> gah, autofsck do not work.. :(
    <braunr> it does :(
    <braunr> unstable is easy to mess it seems
    <pere> had to run fsck -y / manually...
    <braunr> i suspect you were using a corrupted file system at mount time
    <braunr> ah that
    <braunr> yes it is sometimes needed
    <braunr> but ext2 is reliable enough that only temporary files get their
      way into lost+found
    <braunr> temporary/recently created
    <braunr> the crash you had, on the other hand, looks more serious
    <braunr> it seems like you mounted a corrupted file system
    <pere> could be.  
    <pere> hurd v1:0.5.git20140203-1 and libc0.3 v2.17-98~1, it seem.
    <braunr> good
    <braunr> you shouldn't have such problems then, i suspect a mess up on your
      part
    <braunr> but you're not the only one to have had weird file systems
      problems lately
    <pere> hah.  I blame the hurd. :P
    <braunr> heh :)
    <pere> gah, another crash. :(
    <braunr> Oo
    <braunr> same assertion ?
    <pere> same place, or almost the same place.
    <pere> yes.
    <braunr> hm
    <pere> same crash. :(
    <braunr> what kind of machine do you run the hurd on ?
    <pere> kvm
    <braunr> how much memory ?
    <pere> 1G
    <braunr> did you see if the system was swapping ?
    <pere> no idea.
    <braunr> i suggest always running top/htop on the hurd ;p
    <braunr> and monitor memory usage closely
    <gg0> unless pere lately mounted/fsck'ed fs in question with a recent linux
      kernel, there should not be particular problems
    <braunr> it definitely doesn't look like it was mounted by an ext4 driver,
      no
    <braunr> which means it's something else entirely and this is scary
    <pere> I didn't.  I fetched the prebuild image, upgraded it, switched it to
      sysvinit and started working.
    <braunr> sorry i can't be of more help about that
    <braunr> ext2fs has been quite solid on my machines for a long time :(
    <braunr> there are known assertions that trigger under some special
      pressure, but that's not what you're having here
    <braunr> pere: anything particular in fstab ?
    <pere> nope, have not touched /etc/fstab.
    <braunr> hm stupid question
    <braunr> are you sure it's not full ?
    <pere> nothing look full to me.
    <pere> neither the disk nor the host file system.


### IRC, freenode, #hurd, 2014-02-20

    <pere> braunr: do you remember my ext2fs crash from yesterday?  I could
      avoid it by interrupting the triggering build and running sync once in a
      while.  and it show up again if I do not sync in between. :)
    <braunr> ?
    <braunr> are you sure you're not swapping ?
    <pere> I have no idea. still. :)
    <braunr> again, i recommend you run top/htop and monitor that 
    <braunr> pere: is your patch needed to trigger the assertion ?

[[open_issues/ti-rpc_then_nfs]].

    <pere> braunr: well, without it, the package do not build, so yeah. :)
    <braunr> ok
    <pere> tested again, and is not swapping.  850MB free memory.
    <braunr> ok
    <braunr> so this might be a real file system bug
    <braunr> let me see
    <braunr> pere: libtirpc built fine here ..
    <braunr> pere: do you have a separate /home partition ?
    <braunr> or any separate file system for builds
    <pere> braunr: nope, everything on /
    <braunr> pere: i wouldn't recommend that
    <braunr> there very probably are bugs in the file system code and using
      separate partitions is a way to alleviate them


## `ext2fs: ../../libdiskfs/rdwr-internal.c:42: _diskfs_rdwr_internal: Assertion `!diskfs_readonly' failed.`

### IRC, freenode, #hurd, 2014-02-22

    <gg0> login: init: notifying pfinet of shutdown...init: notifying tmpfs
      none of shutdown...init: notifying tmpfs none of shutdown...init:
      notifyi.
    <gg0> ext2fs: ../../libdiskfs/rdwr-internal.c:42: _diskfs_rdwr_internal:
      Assertion `!diskfs_readonly' failed.
    <gg0> In tight loop: hit ctl-alt-del to reboot


# Documentation

  * <http://e2fsprogs.sourceforge.net/ext2.html>

  * <http://www.nongnu.org/ext2-doc/>