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    SubjectRe: mmap over nfs leads to excessive system load
    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.

    > For vmscan->writepage, wbc->for_reclaim is set, so we know that the IO
    > should be pushed immediately. nfs_writepage() seems to dtrt here.
    > With the proposed changes, we don't need that iput() in nfs_writepage().
    > That worries me because I recall from a couple of years back that there are
    > really subtle races with doing iput() on the vmscan->writepage() path.
    > Cannot remember what they were though...

    Possibly to do with block filesystems that may trigger ->writepage()
    while inside iput_final()? NFS can't do that.


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