linux-kernel - Re: [PATCH 00/18 V2] Repair SWAP-over-NFS

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Date:   Fri, 17 Dec 2021 16:29:04 -0500
From:   Anna Schumaker <anna.schumaker@...app.com>
To:     NeilBrown <neilb@...e.de>
Cc:     Trond Myklebust <trond.myklebust@...merspace.com>,
        Chuck Lever <chuck.lever@...cle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mel Gorman <mgorman@...e.de>,
        Christoph Hellwig <hch@...radead.org>,
        David Howells <dhowells@...hat.com>,
        Linux NFS Mailing List <linux-nfs@...r.kernel.org>,
        linux-mm@...ck.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 00/18 V2] Repair SWAP-over-NFS

Hi Neil,

On Thu, Dec 16, 2021 at 7:07 PM NeilBrown <neilb@...e.de> wrote:
>
> swap-over-NFS currently has a variety of problems.
>
> swap writes call generic_write_checks(), which always fails on a swap
> file, so it completely fails.
> Even without this, various deadlocks are possible - largely due to
> improvements in NFS memory allocation (using NOFS instead of ATOMIC)
> which weren't tested against swap-out.
>
> NFS is the only filesystem that has supported fs-based swap IO, and it
> hasn't worked for several releases, so now is a convenient time to clean
> up the swap-via-filesystem interfaces - we cannot break anything !
>
> So the first few patches here clean up and improve various parts of the
> swap-via-filesystem code.  ->activate_swap() is given a cleaner
> interface, a new ->swap_rw is introduced instead of burdening
> ->direct_IO, etc.
>
> Current swap-to-filesystem code only ever submits single-page reads and
> writes.  These patches change that to allow multi-page IO when adjacent
> requests are submitted.  Writes are also changed to be async rather than
> sync.  This substantially speeds up write throughput for swap-over-NFS.
>
> Some of the NFS patches can land independently of the MM patches.  A few
> require the MM patches to land first.

Thanks for fixing swap-over-NFS! Looks like it passes all the
swap-related xfstests except for generic/357 on NFS v4.2. This test
checks that we get -EINVAL on a reflinked swapfile, but I'm not sure
if there is a way to check for that on the client side but if you have
any ideas it would be nice to get that test passing while you're at
it!

Anna

>
> Thanks,
> NeilBrown
>
>
> ---
>
> NeilBrown (18):
>       Structural cleanup for filesystem-based swap
>       MM: create new mm/swap.h header file.
>       MM: use ->swap_rw for reads from SWP_FS_OPS swap-space
>       MM: perform async writes to SWP_FS_OPS swap-space
>       MM: reclaim mustn't enter FS for SWP_FS_OPS swap-space
>       MM: submit multipage reads for SWP_FS_OPS swap-space
>       MM: submit multipage write for SWP_FS_OPS swap-space
>       MM: Add AS_CAN_DIO mapping flag
>       NFS: rename nfs_direct_IO and use as ->swap_rw
>       NFS: swap IO handling is slightly different for O_DIRECT IO
>       SUNRPC/call_alloc: async tasks mustn't block waiting for memory
>       SUNRPC/auth: async tasks mustn't block waiting for memory
>       SUNRPC/xprt: async tasks mustn't block waiting for memory
>       SUNRPC: remove scheduling boost for "SWAPPER" tasks.
>       NFS: discard NFS_RPC_SWAPFLAGS and RPC_TASK_ROOTCREDS
>       SUNRPC: improve 'swap' handling: scheduling and PF_MEMALLOC
>       NFSv4: keep state manager thread active if swap is enabled
>       NFS: swap-out must always use STABLE writes.
>
>
>  drivers/block/loop.c            |   4 +-
>  fs/fcntl.c                      |   5 +-
>  fs/inode.c                      |   3 +
>  fs/nfs/direct.c                 |  56 ++++++----
>  fs/nfs/file.c                   |  25 +++--
>  fs/nfs/inode.c                  |   1 +
>  fs/nfs/nfs4_fs.h                |   1 +
>  fs/nfs/nfs4proc.c               |  20 ++++
>  fs/nfs/nfs4state.c              |  39 ++++++-
>  fs/nfs/read.c                   |   4 -
>  fs/nfs/write.c                  |   2 +
>  fs/open.c                       |   2 +-
>  fs/overlayfs/file.c             |  10 +-
>  include/linux/fs.h              |   2 +-
>  include/linux/nfs_fs.h          |  11 +-
>  include/linux/nfs_xdr.h         |   2 +
>  include/linux/pagemap.h         |   3 +-
>  include/linux/sunrpc/auth.h     |   1 +
>  include/linux/sunrpc/sched.h    |   1 -
>  include/linux/swap.h            | 121 --------------------
>  include/linux/writeback.h       |   7 ++
>  include/trace/events/sunrpc.h   |   1 -
>  mm/madvise.c                    |   9 +-
>  mm/memory.c                     |   3 +-
>  mm/mincore.c                    |   1 +
>  mm/page_alloc.c                 |   1 +
>  mm/page_io.c                    | 189 ++++++++++++++++++++++++++------
>  mm/shmem.c                      |   1 +
>  mm/swap.h                       | 140 +++++++++++++++++++++++
>  mm/swap_state.c                 |  32 ++++--
>  mm/swapfile.c                   |   6 +
>  mm/util.c                       |   1 +
>  mm/vmscan.c                     |  31 +++++-
>  net/sunrpc/auth.c               |   8 +-
>  net/sunrpc/auth_gss/auth_gss.c  |   6 +-
>  net/sunrpc/auth_unix.c          |  10 +-
>  net/sunrpc/clnt.c               |   7 +-
>  net/sunrpc/sched.c              |  29 +++--
>  net/sunrpc/xprt.c               |  19 ++--
>  net/sunrpc/xprtrdma/transport.c |  10 +-
>  net/sunrpc/xprtsock.c           |   8 ++
>  41 files changed, 558 insertions(+), 274 deletions(-)
>  create mode 100644 mm/swap.h
>
> --
> Signature
>