• Filipe Manana's avatar
    Btrfs: be aware of btree inode write errors to avoid fs corruption · 656f30db
    Filipe Manana authored
    While we have a transaction ongoing, the VM might decide at any time
    to call btree_inode->i_mapping->a_ops->writepages(), which will start
    writeback of dirty pages belonging to btree nodes/leafs. This call
    might return an error or the writeback might finish with an error
    before we attempt to commit the running transaction. If this happens,
    we might have no way of knowing that such error happened when we are
    committing the transaction - because the pages might no longer be
    marked dirty nor tagged for writeback (if a subsequent modification
    to the extent buffer didn't happen before the transaction commit) which
    makes filemap_fdata[write|wait]_range unable to find such pages (even
    if they're marked with SetPageError).
    So if this happens we must abort the transaction, otherwise we commit
    a super block with btree roots that point to btree nodes/leafs whose
    content on disk is invalid - either garbage or the content of some
    node/leaf from a past generation that got cowed or deleted and is no
    longer valid (for this later case we end up getting error messages like
    "parent transid verify failed on 10826481664 wanted 25748 found 29562"
    when reading btree nodes/leafs from disk).
    
    Note that setting and checking AS_EIO/AS_ENOSPC in the btree inode's
    i_mapping would not be enough because we need to distinguish between
    log tree extents (not fatal) vs non-log tree extents (fatal) and
    because the next call to filemap_fdatawait_range() will catch and clear
    such errors in the mapping - and that call might be from a log sync and
    not from a transaction commit, which means we would not know about the
    error at transaction commit time. Also, checking for the eb flag
    EXTENT_BUFFER_IOERR at transaction commit time isn't done and would
    not be completely reliable, as the eb might be removed from memory and
    read back when trying to get it, which clears that flag right before
    reading the eb's pages from disk, making us not know about the previous
    write error.
    
    Using the new 3 flags for the btree inode also makes us achieve the
    goal of AS_EIO/AS_ENOSPC when writepages() returns success, started
    writeback for all dirty pages and before filemap_fdatawait_range() is
    called, the writeback for all dirty pages had already finished with
    errors - because we were not using AS_EIO/AS_ENOSPC,
    filemap_fdatawait_range() would return success, as it could not know
    that writeback errors happened (the pages were no longer tagged for
    writeback).
    Signed-off-by: default avatarFilipe Manana <fdmanana@suse.com>
    Signed-off-by: default avatarChris Mason <clm@fb.com>
    656f30db
extent_io.c 139 KB