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    SubjectRe: mmap over nfs leads to excessive system load
    Trond Myklebust <trond.myklebust@fys.uio.no> wrote:
    >
    > On Wed, 2005-11-16 at 17:23 -0500, Trond Myklebust wrote:
    > > On Wed, 2005-11-16 at 14:10 -0800, Andrew Morton wrote:
    > >
    > > > > How is the filesystem supposed to distinguish between the cases
    > > > > "VM->writepage()", and "VM->writepages->mpage_writepages->writepage()"?
    > > > >
    > > >
    > > > Via the writeback_control, hopefully.
    > > >
    > > > For write_one_page(), sync_mode==WB_SYNC_ALL, so NFS should start the I/O
    > > > immediately (it appears to not do so).
    > >
    > > Sorry, but so does filemap_fdatawrite(). WB_SYNC_ALL clearly does not
    > > discriminate between a writepages() and a single writepage() situation,
    > > whatever the original intention was.
    >
    > IMHO, the correct way to distinguish between the two would be to use the
    > wbc->nr_to_write field. If all the instances of writepage() were to set
    > that field to '1', then the filesystems could do the right thing.

    yes, except ->writepages is supposed to decrement nr_to_write as it proceeds,
    so it'll end up at `1' by accident on the last go around the loop.

    I think a separate boolean is better - it's just a single bit.

    > As it is, you have shrink_list() that sets it to the value
    > "SWAP_CLUSTER_MAX" for no apparent reason...

    How weird. That's presumably wrong, but I'd need to check the changelogs
    to doublecheck. ugh, 264 of them.
    -
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