Re: swapfile on btrfs, temporary solution for wiki

[Hugo Mills <hugo@carfax.org.uk>]

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On Sat, Oct 26, 2013 at 10:30:18AM +0100, karim.allah.ahmed@gmail.com wrote:
> Is the swap over NFS code upstream ?

   Yes:

Symbol: NFS_SWAP [=y]
Type  : boolean
Prompt: Provide swap over NFS support
  Location:
    -> File systems
      -> Network File Systems (NETWORK_FILESYSTEMS [=y])
(2)     -> NFS client support (NFS_FS [=m])
  Defined at fs/nfs/Kconfig:89
  Depends on: NETWORK_FILESYSTEMS [=y] && NFS_FS [=m]
  Selects: SUNRPC_SWAP [=y]

   Hugo.

> On 10/25/13, Hugo Mills <hugo@carfax.org.uk> wrote:
> > On Fri, Oct 25, 2013 at 04:35:46PM +0600, Roman Mamedov wrote:
> >> On Thu, 24 Oct 2013 23:52:01 +0300
> >> Timofey Titovets <nefelim4ag@gmail.com> wrote:
> >>
> >> > Hello, i suggest temporary solution to use swap file under btrfs.
> >> > I test it, and it work good.
> >> >
> >> > I invent simple the way, how create and using swap file, just see
> >> > following sh code:
> >> >
> >> > swapfile=$(losetup -f) #free loop device
> >> > truncate -s 8G /swap   #create 8G sparse swap file
> >> > losetup $swapfile /swap #mount file to loop
> >> > mkswap  $swapfile
> >> > swapon  $swapfile
> >> >
> >> > i just adding this to rc.local and this work good.
> >> > May be, add it to btrfs Wiki as temporary solution to using swap file?
> >>
> >> I always thought Btrfs does not allow swap files on purpose, because it is
> >> not
> >> deadlock-proof when used in the swapping context.
> >
> >    It's more that the current swap interface is based on device+block
> > list, and if you balance a filesystem, the blocks for a file move --
> > but there's no way of telling the swap code to cope with that.
> >
> >> Imagine you try swapping out pages to free up some memory, and in the
> >> process
> >> Btrfs needs to allocate some memory to actually perform the write, the
> >> kernel
> >> says "Sure, but for that we need to swap out some more pages..." You see
> >> where
> >> that goes.
> >>
> >> Same issue is possible with swap on other complex filesystems an example
> >> being networked ones like NFS and SMB/CIFS.
> >
> >    The network filesystems have a similar problem as btrfs -- they
> > don't export devices, and you don't get direct access to the low-level
> > blocks under the FS, so the swap code can't deal with it.
> >
> >    That said, there's been a lot of work recently on getting
> > swap-over-NFS to work properly -- effectively giving a new interface
> > for the swap code that doesn't rely on direct mapping to device
> > blocks. That new interface gives us the minimal external
> > infrastructure necessary to consider doing swapfiles on btrfs.
> >
> >> It might work for some time (or even work 99% of time), but you still may
> >> be
> >> endangering your system to a possibility of a lock-up, and certainly
> >> adding
> >> that to any Wiki/FAQ/website as "the solution" might not be the best
> >> choice.
> >
> >    The deadlock situation is dealt with by adding a flag to the memory
> > allocator in the swap-critical paths, which says you're not allowed to
> > swap anything when you make the allocation. That at least allows the
> > memory allocation to fail (hopefully gracefully) without deadlocking.
> > I suspect that this is also part of the swap-on-NFS work -- adding
> > that flag everywhere necessary.
> >
> >    Hugo.
> >

-- 
=== Hugo Mills: hugo@... carfax.org.uk | darksatanic.net | lug.org.uk ===
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