(no subject)

Jan Kara
2010-06-16T16:39:20Z

Hello,

here is the fourth version of the writeback livelock avoidance patches
for data integrity writes. To quickly summarize the idea: we tag dirty
pages at the beginning of write_cache_pages with a new TOWRITE tag and
then write only tagged pages to avoid parallel writers to livelock us.
See changelogs of the patches for more details.
I have tested the patches with fsx and a test program I wrote which
checks that if we crash after fsync, the data is indeed on disk.
If there are no more concerns, can these patches get merged?

Honza

Changes since last version:
- tagging function was changed to stop after given amount of pages to
avoid keeping tree_lock and irqs disabled for too long
- changed names and updated comments as Andrew suggested
- measured memory impact and reported it in the changelog

Things suggested but not changed (I want to avoid going in circles ;):
- 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
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.
- implement function for clearing several tags (TOWRITE, DIRTY) at once
- IMHO not worth it because we would save only conversion of page index
to radix tree offsets. The rest would have to be separate anyways. And
the interface would be incosistent as well...
- use __lookup_tag to implement radix_tree_range_tag_if_tagged - doesn't
quite work because __lookup_tag returns only leaf nodes so we'd have to
implement tree traversal anyways to tag also internal nodes.