Re: linux-next: manual merge of the akpm tree with the nfs tree

[Mel Gorman <mgorman@suse.de>]

On Tue, Jul 31, 2012 at 02:37:24PM +0000, Myklebust, Trond wrote:
> On Tue, 2012-07-31 at 11:33 +0100, Mel Gorman wrote:
> > On Tue, Jul 31, 2012 at 02:24:41PM +1000, Stephen Rothwell wrote:
> > > Hi Andrew,
> > > 
> > > Today's linux-next merge of the akpm tree got a conflict in
> > > net/sunrpc/xprtsock.c between commit 5cf02d09b50b ("nfs: skip commit in
> > > releasepage if we're freeing memory for fs-related reasons") from the nfs
> > > tree and commit "nfs: enable swap on NFS" from the akpm tree.
> > > 
> > > Just context changes?  I fixed it up (I think - see below) and can carry
> > > the fix as necessary.
> > 
> > Functionally it looks fine. As you say, it all looks like context
> > changes. Arguably code like this
> > 
> > current->flags &= ~PF_FSTRANS
> > 
> > could use tsk_restore_flags instead() even though it should never be
> > necessary as PF_FSTRANS would not be set on function entry. However,
> > it would set up a depedency between the patch sets that is undesirable.
> > If both sets get merged then it might make sense as a cleanup to use
> > tsk_restore_flags() but not until then.
> > 
> > Thanks Stephen.
> > 
> 
> Do we really need to set both PF_FSTRANS and PF_MEMALLOC here? The
> reason why I merged the PF_FSTRANS patch is that we have the deadlock
> problem when allocating a new socket even before we add swap-over-nfs.
> Adding PF_FSTRANS to disallow entry into the NFS layer by the memory
> allocator fixes that issue.

PF_FSTRANS is to prevent recursion into NFS and is set whether swap-over-NFS
is used or not and for all requests.

> What value does PF_MEMALLOC add? Is that in order to prevent recursion
> into other areas of the swap code (say, if you mix swap-over-nfs with
> ordinary swap-to-disk)?
> 

PF_MEMALLOC is normally to prevent the page reclaim recursing into
itself. Page reclaim can call the page allocator and that cannot re-enter
page reclaim.

In the case of swap-over-NFS, PF_MEMALLOC is set only if the socket is
being used for swapping. In softirq context, the allocation request is
allowed to use PFMEMALLOC reserves to avoid deadlock.

I do not see an obvious way to collapse the two flags together.
PF_FSTRANS should not mean the PFMEMALLOC reserves can be used and
PFMEMALLOC is not set for all requests.

-- 
Mel Gorman
SUSE Labs