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    SubjectRe: 2.4.18 no timestamp update on modified mmapped files
    Alan Cox wrote:
    > > On Tue, 11 Jun 2002, Andrew Morton wrote:
    > > >
    > > > I think it's too late to fix this in 2.4. If we did, a person
    > > > could develop and test an application on 2.4.21, ship it, then
    > > > find that it fails on millions of 2.4.17 machines.
    > >
    > > Oh, please reconsider that! Doesn't loss of modification time
    > > approach data loss? Surely we'll continue to fix any data loss
    > > issues in 2.4, and be grateful if you fixed this mmap modtime loss.
    > It doesnt approach data loss, when doing incremental backups it
    > *is* data loss. Ditto with rsync --newer

    A more serious form of data loss occurs when an application has a shared
    mapping over a sparse file. If the filesystem is out of space when
    the VM decides to write back some pages, your data simply gets dropped
    on the floor. Even a subsequent msync() won't tell you that you have
    a shiny new bunch of zeroes in your file.

    It's not simple to fix. Approaches might be:

    1: Map the page to disk at fault time, generate SIGBUS on
    ENOSPC (the standards don't seem to address this issue, and
    this is a non-standard overload of SIGBUS).

    2: Resurrect delayed-allocation patches, use their reservation
    API to generate the SIGBUS.

    3: Record the fact that there has been a data loss in the mapping
    and return that information to a subsequent msync(). (I have
    most-of-a-patch for this. It's fairly murky).

    4: Dirty the page again if writepage() failed. This fills the machine
    up with unfreeable pages, but emitting ENOSPC messages into the
    logs may be acceptable - the operator makes some space, the data
    gets written and the messages stop.

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