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    SubjectRe: linux-next: manual merge of the akpm tree with the nfs tree
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    On Tue, 2012-07-31 at 16:19 +0100, Mel Gorman wrote:
    > 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.

    Right, but in this case, we're talking about a GFP_KERNEL allocation
    that always happens in an rpciod workqueue process context, so we still
    won't be able to access the PFMEMALLOC reserves if I understand you
    correctly?

    I understand the value of preventing the page reclaim recursing into
    itself, but in this case, we're talking about a separate process that is
    operating on behalf of the allocator (much like kswapd does).

    Cheers
    Trond
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