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    SubjectRe: [PATCH] [16/19] HWPOISON: Enable .remove_error_page for migration aware file systems
    On Tue, Aug 11, 2009 at 09:17:56AM +0200, Andi Kleen wrote:
    > On Tue, Aug 11, 2009 at 12:50:59PM +0900, Hidehiro Kawai wrote:
    > > > And application
    > > > that doesn't handle current IO errors correctly will also
    > > > not necessarily handle hwpoison correctly (it's not better and not worse)
    > >
    > > This is my main concern. I'd like to prevent re-corruption even if
    > > applications don't have good manners.
    >
    > I don't think there's much we can do if the application doesn't
    > check for IO errors properly. What would you do if it doesn't
    > check for IO errors at all? If it checks for IO errors it simply
    > has to check for them on all IO operations -- if they do
    > they will detect hwpoison errors correctly too.

    But will quite possibly do the wrong thing: ie. try to re-sync the
    same page again, or try to write the page to a new location, etc.

    This is the whole problem with -EIO semantics I brought up.


    > > That is why I suggested this:
    > > >>(2) merge this patch with new panic_on_dirty_page_cache_corruption
    >
    > You probably mean panic_on_non_anonymous_dirty_page_cache
    > Normally anonymous memory is dirty.
    >
    > > >> sysctl
    >
    > It's unclear to me this special mode is really desirable.
    > Does it bring enough value to the user to justify the complexity
    > of another exotic option? The case is relatively exotic,
    > as in dirty write cache that is mapped to a file.
    >
    > Try to explain it in documentation and you see how ridiculous it sounds; u
    > it simply doesn't have clean semantics
    >
    > ("In case you have applications with broken error IO handling on
    > your mission critical system ...")

    Not broken error handling. It is very simple: if the application is
    assuming EIO is an error with dirty data being sent to disk, rather
    than an error with the data itself (which I think may be a common
    assumption). Then it could have a problem.

    If a database for example tries to write the data to another location
    in response to EIO and then record it in a list of failed IOs before
    halting the database. Then if it restarts it might try to again try
    writing out these failed IOs (eg. give the administrator a chance to
    fix IO devices). Completely made up scenario but it is not outlandish
    and it would cause bad data corruption.

    A mission critical server will *definitely* want to panic on dirty
    page corruption, IMO, because by definition they should be able to
    tolerate panic. But if they do not know about this change to -EIO
    semantics, then it is quite possible to cause problems.



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