Re: [PATCH] VM: kswapd should not do blocking memory allocations

Trond Myklebust
2010-08-18T19:31:17Z

On Wed, 2010-08-18 at 12:24 -0700, Ram Pai wrote:
>
>
> On Wed, Aug 18, 2010 at 12:04 PM, Trond Myklebust
> <Trond.Myklebust@netapp.com> wrote:
> From: Trond Myklebust <Trond.Myklebust@netapp.com>
>
> Allowing kswapd to do GFP_KERNEL memory allocations (or any
> blocking memory
> allocations) is wrong and can cause deadlocks in
> try_to_release_page(), as
> the filesystem believes it is safe to allocate new memory and
> block,
> whereas kswapd is there specifically to clear a low-memory
> situation...
>
> Set the gfp_mask to GFP_IOFS instead.
>
> Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
> ---
>
> mm/vmscan.c | 2 +-
> 1 files changed, 1 insertions(+), 1 deletions(-)
>
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index ec5ddcc..716dd16 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2095,7 +2095,7 @@ static unsigned long
> balance_pgdat(pg_data_t *pgdat, int order)
> unsigned long total_scanned;
> struct reclaim_state *reclaim_state =
> current->reclaim_state;
> struct scan_control sc = {
> - .gfp_mask = GFP_KERNEL,
> + .gfp_mask = GFP_IOFS,
> .may_unmap = 1,
> .may_swap = 1,
> /*
>
> Trond,
>
> Has anyone hit this issue? Or is this based on code
> inspection?
>
> The reason I ask is we are seeing a problem, similar to
> the symptom described, on RH based kernel but have not been able to
> reproduce on 2.6.35.

Hi Ram,

I was seeing it on NFS until I put in the following kswapd-specific hack
into nfs_release_page():

/* Only do I/O if gfp is a superset of GFP_KERNEL */
if (mapping && (gfp & GFP_KERNEL) == GFP_KERNEL) {
int how = FLUSH_SYNC;

/* Don't let kswapd deadlock waiting for OOM RPC calls */
if (current_is_kswapd())
how = 0;
nfs_commit_inode(mapping->host, how);
}

Remove the 'if (current_is_kswapd())' line, and run an mmap() write
intensive workload, and it should hang pretty much every time.

Cheers
Trond