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    SubjectRe: [PATCH] [13/16] HWPOISON: The high level memory error handler in the VM v3
    On Tue, Jun 02, 2009 at 02:37:20PM +0200, Nick Piggin wrote:
    > Because I don't see any difference (see my previous patch). I
    > still don't know what it is supposed to be doing differently.
    > So if you reinvent your own that looks close enough to truncate
    > to warrant a comment to say /* this is close to truncate but
    > not quite */, then yes I insist that you say exactly why it is
    > not quite like truncate ;)

    I will just delete that comment because it apparently causes so
    much confusion.

    > > > I'm suggesting that EIO is traditionally for when the data still
    > > > dirty in pagecache and was not able to get back to backing
    > > > store. Do you deny that?
    > >
    > > Yes. That is exactly the case when memory-failure triggers EIO
    > >
    > > Memory error on a dirty file mapped page.
    > But it is no longer dirty, and the problem was not that the data
    > was unable to be written back.

    Sorry I don't understand. What do you mean with "no longer dirty"

    Of course it's still dirty, just has to be discarded because it's

    > > > And I think the application might try to handle the case of a
    > > > page becoming corrupted differently. Do you deny that?
    > >
    > > You mean a clean file-mapped page? In this case there is no EIO,
    > > memory-failure just drops the page and it is reloaded.
    > >
    > > If the page is dirty we trigger EIO which as you said above is the
    > > right reaction.
    > No I mean the difference between the case of dirty page unable to
    > be written to backing sotre, and the case of dirty page becoming
    > corrupted.

    Nick, I have really a hard time following you here.

    What exactly do you want?

    A new errno? Or something else? If yes what precisely?

    I currently don't see any sane way to report this to the application
    through write(). That is because adding a new errno for something
    is incredibly hard and often impossible, and that's certainly
    the case here.

    The application can detect it if it maps the
    shared page and waits for a SIGBUS, but not through write().

    But I doubt there will be really any apps that do anything differently
    here anyways. A clever app could retry a few times if it still
    has a copy of the data, but that might even make sense on normal
    IO errors (e.g. on a SAN).

    > > > OK, given the range of errors that APIs are defined to return,
    > > > then maybe EIO is the best option. I don't suppose it is possible
    > > > to expand them to return something else?
    > >
    > > Expand the syscalls to return other errnos on specific
    > > kinds of IO error?
    > >
    > > Of course that's possible, but it has the problem that you
    > > would need to fix all the applications that expect EIO for
    > > IO error. The later I consider infeasible.
    > They would presumably exit or do some default thing, which I
    > think would be fine.

    No it's not fine if they would handle EIO. e.g. consider
    a sophisticated database which likely has sophisticated
    IO error mechanisms too (e.g. only abort the current commit)


    -- -- Speaking for myself only.

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