On Fri, May 08, 2020 at 02:30:47PM -0400, Johannes Weiner wrote: > This patch series reworks memcg to charge swapin pages directly at > swapin time, rather than at fault time, which may be much later, or > not happen at all. > > Changes in version 2: > - prevent double charges on pre-allocated hugepages in khugepaged > - leave shmem swapcache when charging fails to avoid double IO (Joonsoo) > - fix temporary accounting bug by switching rmap<->commit (Joonsoo) > - fix double swap charge bug in cgroup1/cgroup2 code gating > - simplify swapin error checking (Joonsoo) > - mm: memcontrol: document the new swap control behavior (Alex) > - review tags > > The delayed swapin charging scheme we have right now causes problems: > > - Alex's per-cgroup lru_lock patches rely on pages that have been > isolated from the LRU to have a stable page->mem_cgroup; otherwise > the lock may change underneath him. Swapcache pages are charged only > after they are added to the LRU, and charging doesn't follow the LRU > isolation protocol. > > - Joonsoo's anon workingset patches need a suitable LRU at the time > the page enters the swap cache and displaces the non-resident > info. But the correct LRU is only available after charging. > > - It's a containment hole / DoS vector. Users can trigger arbitrarily > large swap readahead using MADV_WILLNEED. The memory is never > charged unless somebody actually touches it. > > - It complicates the page->mem_cgroup stabilization rules > > In order to charge pages directly at swapin time, the memcg code base > needs to be prepared, and several overdue cleanups become a necessity: > > To charge pages at swapin time, we need to always have cgroup > ownership tracking of swap records. We also cannot rely on > page->mapping to tell apart page types at charge time, because that's > only set up during a page fault. > > To eliminate the page->mapping dependency, memcg needs to ditch its > private page type counters (MEMCG_CACHE, MEMCG_RSS, NR_SHMEM) in favor > of the generic vmstat counters and accounting sites, such as > NR_FILE_PAGES, NR_ANON_MAPPED etc. Could you elaborate on what this means and the implications of this on user space programs? > > To switch to generic vmstat counters, the charge sequence must be > adjusted such that page->mem_cgroup is set up by the time these > counters are modified. > > The series is structured as follows: > > 1. Bug fixes > 2. Decoupling charging from rmap > 3. Swap controller integration into memcg > 4. Direct swapin charging > Thanks, Balbir Singh.
