Re: [RFC PATCH 3/3] overlay: Add the ability to remount volatile directories when safe

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> > I guess if we change fsync and syncfs to do nothing but return
> > error if any writeback error happened since mount we will be ok?
>
> I guess that will not be sufficient. Because overlay fsync/syncfs can
> only retrun any error which has happened so far. It is still possible
> that error happens right after this fsync call and application still
> reads back old/corrupted data.
>
> So this proposal reduces the race window but does not completely
> eliminate it.
>

That's true.

> We probably will have to sync upper/ and if there are no errors reported,
> then it should be ok to consume data back.
>
> This leads back to same issue of doing fsync/sync which we are trying
> to avoid with volatile containers. So we have two options.
>
> A. Build volatile containers should sync upper and then pack upper/ into
>   an image. if final sync returns error, throw away the container and
>   rebuild image. This will avoid intermediate fsync calls but does not
>   eliminate final syncfs requirement on upper. Now one can either choose
>   to do syncfs on upper/ or implement a more optimized syncfs through
>   overlay so that selctives dirty inodes are synced instead.
>
> B. Alternatively, live dangerously and know that it is possible that
>   writeback error happens and you read back corrupted data.
>

C. "shutdown" the filesystem if writeback errors happened and return
     EIO from any read, like some blockdev filesystems will do in face
     of metadata write errors

I happen to have a branch ready for that ;-)
https://github.com/amir73il/linux/commits/ovl-shutdown

Thanks,
Amir.



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