Re: your mail

Wu Fengguang
2010-06-22T02:59:46Z

> - use tagging also for WB_SYNC_NONE writeback - there's problem with an
> interaction with wbc->nr_to_write. If we tag all dirty pages, we can
> spend too much time tagging when we write only a few pages in the end
> because of nr_to_write. If we tag only say nr_to_write pages, we may
> not have enough pages tagged because some pages are written out by
> someone else and so we would have to restart and tagging would become

This could be addressed by ignoring nr_to_write for the WB_SYNC_NONE
writeback triggered by sync(). write_cache_pages() already ignored
nr_to_write for WB_SYNC_ALL.

> essentially useless. So my option is - switch to tagging for WB_SYNC_NONE
> writeback if we can get rid of nr_to_write. But that's a story for
> a different patch set.

Besides introducing overheads, it will be a policy change in which the
system loses control to somehow "throttle" writeback of huge files.

So it may be safer to enlarge nr_to_write instead of canceling it totally.

Thanks,
Fengguang