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    SubjectRe: [PATCH v2 3/3] NFS: Fix a memory leak in nfs_readdir
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    On Wed, 2010-12-01 at 14:38 -0800, Andrew Morton wrote:
    > On Wed, 1 Dec 2010 14:24:42 -0800
    > Linus Torvalds <torvalds@linux-foundation.org> wrote:
    >
    > > On Wed, Dec 1, 2010 at 2:13 PM, Andrew Morton <akpm@linux-foundation.org> wrote:
    > > > On Wed, 01 Dec 2010 16:51:12 -0500
    > > > Trond Myklebust <Trond.Myklebust@netapp.com> wrote:
    > > >
    > > >> On Wed, 2010-12-01 at 13:38 -0800, Andrew Morton wrote:
    > > >> > Probably on most call paths we'll be OK - if a process is in the middle
    > > >> > of a file truncate, holdin a file* ref which holds an inode ref then
    > > >> > nobody will be unmounting that fs and hence nobody will be unloading
    > > >> > that module.
    > > >> >
    > > >> > However on the random_code->alloc_page->vmscan->releasepage path, none
    > > >> > of that applies.
    > > >>
    > > >> Just out of interest, what ensures that the mapping is still around for
    > > >> the 'spin_unlock_irq(&mapping->tree_lock);' in __remove_mapping()?
    > > >
    > > > Nothing, afacit.
    > >
    > > No, we're good.
    > >
    > > Module unload has to go through a "stop_machine()" cycle, and that in
    > > turn requires an idle period for everything. And just a preemption
    > > reschedule isn't enough for that.
    > >
    > > So what is sufficient is that
    > >
    > > - we had the page locked and on the mapping
    > >
    > > This implies that we had an inode reference to the module, and the
    > > page lock means that the inode reference cannot go away (because it
    > > will involve invalidate-pages etc)
    > >
    > > - we're not sleeping after __remove_mapping, so unload can't happen afterwards.
    > >
    > > A _lot_ of the module races depend on that latter thing. We have
    > > almost no cases that are strictly about actual reference counts etc.
    > >
    >
    > OK, the stop_machine() plugs a lot of potential race-vs-module-unload
    > things. But Trond is referring to races against vmscan inode reclaim,
    > unmount, etc.
    >

    Yes, that was my question.

    However, Linus' explanation appears to answer one of Hugh's objections:
    we can apparently use a preempt-disable() in order to ensure that the
    module containing the mapping->a_ops is not unloaded until after the
    ->freepage() call is complete.

    That would imply that ->freepage() cannot sleep, but I don't think that
    is too nasty a restriction.

    --
    Trond Myklebust
    Linux NFS client maintainer

    NetApp
    Trond.Myklebust@netapp.com
    www.netapp.com



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