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    SubjectRe: invalidate_inode_pages in 2.5.32/3
    Trond Myklebust wrote:
    >
    > >>>>> " " == Andrew Morton <akpm@zip.com.au> writes:
    >
    > > I'm not sure what semantics we really want for this. If we
    > > were to "invalidate" a mapped page then it would become
    > > anonymous, which makes some sense.
    >
    > > But if we want to keep the current "don't detach mapped pages
    > > from pagecache" semantics then we should test page->pte.direct
    > > rather than page_count(page). Making that 100% reliable would
    > > be tricky because of the need for locking wrt concurrent page
    > > faults.
    >
    > I believe that Linus is very much in favour of this latter
    > approach. We had the 'anonymize page' semantics under 2.2.x, but they
    > were changed in the 2.4.0-pre series.

    hm.

    > The problem is that NFS can clear your page cache at any
    > moment. Things like out-of-order RPC calls etc. can trigger it. If
    > your knee-jerk response is to anonymize all those pages that are
    > referenced, you might end up with page aliasing (well - in practice
    > not, since we do protect against that but Linus wasn't convinced).

    Oh. I thought this was a "purely theoretical" discussion because
    it only pertains to directory data. You seem to be saying that
    NFS is doing this for S_ISREG pagecache also?

    Again: what do you _want_ to do? Having potentially incorrect pagecache
    mapped into process memory space is probably not the answer to that ;)

    Should we be forcibly unmapping the pages from pagetables? That would
    result in them being faulted in again, and re-read.

    > > The mapping's releasepage must try to clear away whatever is
    > > held at ->private. If that was successful then releasepage()
    > > must clear PG_private, decrement
    > > page-> count and return non-zero. If the info at ->private is not
    > > freeable, releasepage returns zero. ->releasepage() may not
    > > sleep in
    > > 2.5.
    >
    > Interesting. Our 'private data' in this case would be whether or not
    > there is some pending I/O operation on the page. We don't keep it in
    > page->private, but I assume that's less of a problem ;-)
    > It's unrelated to the topic we're discussing, but having looked at it
    > I was thinking that we might want to use it in order to replace the
    > NFS 'flushd' daemon. Currently the latter is needed mainly in order
    > to ensure that readaheads actually do complete in a timely fashion
    > (i.e. before we run out of memory). Since try_to_release_page() is
    > called in shrink_cache(), I was thinking that we might pass that buck
    > on to releasepage()

    That might work. It's a bit of a hassle that ->releasepage() must
    be nonblocking, but generally it just wants to grab locks, decrement
    refcounts and free things.

    > (btw: there's currently a bug w.r.t. that'. If I understand you
    > correctly, the releasepage() thing is unrelated to page->buffers, but
    > the call in shrink_cache() is masked by an 'if (page->buffers))

    That would be in a 2.4 kernel? In 2.4, page->buffers can only
    contain buffers. If it contains anything else the kernel will
    die.

    > Extending that idea, we might also be able to get rid of
    > nfs_try_to_free_pages(), if we also make releasepage() call the
    > necessary routines to flush dirty pages too...

    ->releasepage() should never succeed against a dirty page. In fact
    the VM shouldn't even bother calling it - there's a minor efficiency
    bug there.

    If your mapping has old, dirty pages then the VM will call your
    ->vm_writeback to write some of them back. Or it will repeatedly
    call ->writepage if you don't define ->vm_writeback. That's the
    place to clean the pages.
    -
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