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    SubjectRe: [PATCH v3] introduce sys_syncfs to sync a single file system
    On Mon, Mar 14, 2011 at 01:10:42PM -0700, Andrew Morton wrote:
    > There might one day be a requirement to be able to initiate a
    > resource-management-style writeback against a whole filesystem. When
    > that happens, we'll regret not having added a "mode" argument to
    > sys_syncfs().

    I'm a bit nervous about exposing WB_SYNC_NONE to userspace, because
    its semantics are *definitely* hard to describe. For example, at the
    moment if you do a WB_SYNC_NONE writeback, the writeback code will
    clamp the amount of data written back for each inode to
    MAX_WRITEBACK_PAGES (1024) pages. Do we want to document that?
    Probably not! But if we don't document it, what can userspace expect?

    If you just issue a writeback_inodes_sb(), it's not the case that it
    will start a process that will eventually write out everything (i.e.,
    it's not the equivalent of a non-blocking data integrity sync). It
    just means, "write out some stuff".

    I could imagine userspace wanting to start a non-blocking writeout of
    all data blocking pages, and which doesn't cause queue flush / barrier
    requests. (i.e., a non-blocking-non-barrier-issuing-but-otherwise-a-
    data-integrity writeback) But that's not something that the current
    writeback machinery can do easily, at least not today.

    It wouldn't hurt to have a "flags" field which we could expand later
    --- but that can lead to portability headaches for userspace programs
    that don't know whether a particular kernel is going to support a
    particular flag or not. So it's certainly not a panacea.

    - Ted

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