lkml.org 
[lkml]   [2007]   [May]   [16]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: [rfc] optimise unlock_page
    On Wed, May 16, 2007 at 06:21:09PM +0100, Hugh Dickins wrote:
    > On Sun, 13 May 2007, Nick Piggin wrote:
    > > On Fri, May 11, 2007 at 02:15:03PM +0100, Hugh Dickins wrote:
    > >
    > > > But again I wonder just what the gain has been, once your double
    > > > unmap_mapping_range is factored in. When I suggested before that
    > > > perhaps the double (well, treble including the one in truncate.c)
    > > > unmap_mapping_range might solve the problem you set out to solve
    > > > (I've lost sight of that!) without pagelock when faulting, you said:
    > > >
    > > > > Well aside from being terribly ugly, it means we can still drop
    > > > > the dirty bit where we'd otherwise rather not, so I don't think
    > > > > we can do that.
    > > >
    > > > but that didn't give me enough information to agree or disagree.
    > >
    > > Oh, well invalidate wants to be able to skip dirty pages or have the
    > > filesystem do something special with them first. Once you have taken
    > > the page out of the pagecache but still mapped shared, then blowing
    > > it away doesn't actually solve the data loss problem... only makes
    > > the window of VM inconsistency smaller.
    >
    > Right, I think I see what you mean now, thanks: userspace
    > must not for a moment be allowed to write to orphaned pages.

    Yep.


    > Whereas it's not an issue for the privately COWed pages you added
    > the second unmap_mapping_range for: because it's only truncation
    > that has to worry about them, so they're heading for SIGBUS anyway.
    >
    > Yes, and the page_mapped tests in mm/truncate.c are just racy
    > heuristics without the page lock you now put into faulting.

    Yes.
    -
    To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
    the body of a message to majordomo@vger.kernel.org
    More majordomo info at http://vger.kernel.org/majordomo-info.html
    Please read the FAQ at http://www.tux.org/lkml/

    \
     
     \ /
      Last update: 2007-05-16 19:41    [W:0.021 / U:0.920 seconds]
    ©2003-2016 Jasper Spaans. hosted at Digital OceanAdvertise on this site