On Mon, Feb 14, 2022 at 02:48:05PM -0800, Andrew Morton wrote: > On Mon, 14 Feb 2022 16:49:21 -0500 Johannes Weiner <hannes@xxxxxxxxxxx> wrote: > > > Once upon a time, all swapins counted toward memory pressure[1]. Then > > Joonsoo introduced workingset detection for anonymous pages and we > > gained the ability to distinguish hot from cold swapins[2][3]. But we > > failed to update swap_readpage() accordingly, and now we account > > partial memory pressure in the swapin path of cold memory. > > > > Not for all situations - which adds more inconsistency: paths using > > the conventional submit_bio() and lock_page() route will not see much > > pressure - unless storage itself is heavily congested and the bio > > submissions stall. ZRAM and ZSWAP do most of the work directly from > > swap_readpage() and will see all swapins reflected as pressure. > > > > Restore consistency by making all swapin stall accounting conditional > > on the page actually being part of the workingset. > > Does this have any known runtime effects? If not, can we > hazard a guess? hm, how about this paragrah between "not for all situations" and "restore consistency": IOW, a workload doing cold swapins could see little to no pressure reported with on-disk swap, but potentially high pressure with a zram or zswap backend. That confuses any psi-based health monitoring, load shedding, proactive reclaim, or userspace OOM killing schemes that might be in place for the workload.
