Re: [PATCH 00/11] Swap-over-NFS without deadlocking V3

[Eric B Munson <emunson@mgebm.net>]

[-- Attachment #1: Type: text/plain, Size: 3547 bytes --]

On Mon, 16 Apr 2012, Mel Gorman wrote:

> Changelog since V2
> o Nothing significant, just rebases. A radix tree lookup is replaced with
>   a linear search would be the biggest rebase artifact
> 
> This patch series is based on top of "Swap-over-NBD without deadlocking v9"
> as it depends on the same reservation of PF_MEMALLOC reserves logic.
> 
> When a user or administrator requires swap for their application, they
> create a swap partition and file, format it with mkswap and activate it with
> swapon. In diskless systems this is not an option so if swap if required
> then swapping over the network is considered.  The two likely scenarios
> are when blade servers are used as part of a cluster where the form factor
> or maintenance costs do not allow the use of disks and thin clients.
> 
> The Linux Terminal Server Project recommends the use of the Network
> Block Device (NBD) for swap but this is not always an option.  There is
> no guarantee that the network attached storage (NAS) device is running
> Linux or supports NBD. However, it is likely that it supports NFS so there
> are users that want support for swapping over NFS despite any performance
> concern. Some distributions currently carry patches that support swapping
> over NFS but it would be preferable to support it in the mainline kernel.
> 
> Patch 1 avoids a stream-specific deadlock that potentially affects TCP.
> 
> Patch 2 is a small modification to SELinux to avoid using PFMEMALLOC
> 	reserves.
> 
> Patch 3 adds three helpers for filesystems to handle swap cache pages.
> 	For example, page_file_mapping() returns page->mapping for
> 	file-backed pages and the address_space of the underlying
> 	swap file for swap cache pages.
> 
> Patch 4 adds two address_space_operations to allow a filesystem
> 	to pin all metadata relevant to a swapfile in memory. Upon
> 	successful activation, the swapfile is marked SWP_FILE and
> 	the address space operation ->direct_IO is used for writing
> 	and ->readpage for reading in swap pages.
> 
> Patch 5 notes that patch 3 is bolting
> 	filesystem-specific-swapfile-support onto the side and that
> 	the default handlers have different information to what
> 	is available to the filesystem. This patch refactors the
> 	code so that there are generic handlers for each of the new
> 	address_space operations.
> 
> Patch 6 adds an API to allow a vector of kernel addresses to be
> 	translated to struct pages and pinned for IO.
> 
> Patch 7 updates NFS to use the helpers from patch 3 where necessary.
> 
> Patch 8 avoids setting PF_private on PG_swapcache pages within NFS.
> 
> Patch 9 implements the new swapfile-related address_space operations
> 	for NFS and teaches the direct IO handler how to manage
> 	kernel addresses.
> 
> Patch 10 prevents page allocator recursions in NFS by using GFP_NOIO
> 	where appropriate.
> 
> Patch 11 fixes a NULL pointer dereference that occurs when using
> 	swap-over-NFS.
> 
> With the patches applied, it is possible to mount a swapfile that is on an
> NFS filesystem. Swap performance is not great with a swap stress test taking
> roughly twice as long to complete than if the swap device was backed by NBD.
> 


FWIW, I'd like to see these go in, I use them for giving ARM boards with NFS
root file systems swap space.

I have tested these with an artificial swap benchmark and with a large project
compile on a beagle board.  They work great for me.

Tested-by: Eric B Munson <emunson@mgebm.net>

[-- Attachment #2: Digital signature --]
[-- Type: application/pgp-signature, Size: 836 bytes --]