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    SubjectRe: ramdisk corruption problems - was: RE: pivot_root and initrd kern el panic woes
    On Thu, Dec 20, 2001 at 09:56:40PM -0800, Andrew Morton wrote:
    > Andrea Arcangeli wrote:
    > >
    > > ...
    > > actually while testing it I unfortunately found also an fs corruption
    > > bug in the ->prepare_write/commit_write/writepage/direct_IO callbacks.
    > > It's a longstanding one, since get_block born. In short, if get_block
    > > fails while mapping on a certain page during
    > > prepare_write/writepage/direct_IO (like it can happen with a full
    > > filesystem, incidentally ext2 on /dev/ram0 during my testing that is
    > > only 4M and so it overflows fast), the blocks before the ENOSPC failure
    > > will be allocated, but the i_size won't be update accordingly (no commit
    > > write, because prepare_write failed in the middle). for the
    > > generic_file_write users it's easily fixable with an hack (truncating
    > > backwards because we don't know how far we got allocating blocks when we
    > > return from prepare_write), similar hack for the direct_IO one (that
    > > commits only once at the end in function of the direct_IO generated).
    > > But for writepage is quite a pain, infact I believe the writepage blocks
    > > should be reserved in-core, to guarantee we will never fail a truncate
    > > with ENOSPC. With the shared mappings we're effectively doing allocate
    > > on flush... but with the lack of space reservation that makes it
    > > unreliable :)
    >
    > The -ac kernels handled this by zeroing out the accidentally-allocated
    > blocks at __block_prepare_write.

    actually my fix seems cleaner because it puts the "clearing" in one single
    place.

    > > So for now I did an hack to cure the other two (writepage can still
    > > corrupt the fs as said). I think the right fix (ala 2.5) is to change
    > > the API so we can use the last blocks too, but the below will cure 2.4
    > > and for writepage the right fix IMHO is to do the reservation of the
    > > space.
    >
    > This is better in a way because it reclaims the eztra few blocks. But
    > the -ac approach also works for writepage().

    yes, it can solve the metadata corruption (assuming the locking is
    right, I can as well call ->truncate within writepage but it's not
    obvious at all that it won't race because we don't hold the i_sem within
    writepage), but the data corruption still holds. I mean, there's no
    failure path to notify userspace that a certain page fault is writing
    into a page over an hole, that we don't have space to later allocate on
    disk. so to me it sounds like MAP_SHARED should preallocate the space of
    the holes so you will know that the writes into the MAP_SHARED segments
    won't be lost (current state of things will lead to silent corruption
    and pinned dirty pages in ram, aka broken allocate on flush like
    previously said).

    > Why was that code not brought across?

    Who developed that code? Can the author of the code forward port it to
    2.4.18pre and post a patch to the list so we can review? thanks,
    Avoiding the matadata corruption would be a good start at least for 2.4,
    then we should just focus on the writepage locking that could race with the
    other "create=1" get_blocks. If it doesn't race I will certainly agree
    on that approch for 2.4.

    Andrea
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