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    SubjectRe: Question regarding concurrent accesses through block device and fs
    Nick Piggin <> writes:

    > On Monday 02 March 2009 01:42:55 Francis Moreau wrote:


    > OK, the "buffercache", the cache of block device contents, is normally
    > thought of as metadata when it is being used by the filesystem (eg.
    > usually via bread() etc), or data when it is being read/written from
    > userspace via /dev/<blockdevice>.
    > In the former case, the buffer.c/filesystem code together know when a
    > metadata buffer is unused (because the filesystem has deallocated it),
    > so unmap_underlying_metadata will work there.
    > And it is insane to have a mounted filesystem and have userspace working
    > on the same block device, so unmap_underlying_metadata doesn't have to
    > care about that case. (IIRC some filesystem tools can do this, but there
    > are obviously a lot of tricks to it)

    Thanks for clarifying this.


    > Depends on the filesystem. Many do just use the buffercache as a
    > writeback cache for their metadata, and are happy to just let the
    > dirty page flushers write it out when it suits them

    I guess you're talking about the pdflush threads here.

    This is the case where I can't find when the metadata are actually
    written back to the disk by the flushers. I looked at
    writback_inodes() but I fail to find this out.

    Could you point out the place in the code where this happen ?

    > (or when there are explicit sync instructions given).

    yes I see where this happens in these cases.

    > Most of the time, these filesystems don't really know or care when
    > exactly their metadata is under writeback.

    This sounds very weird to me but I need to learn how things work
    before doing any serious comments.


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