Re: [PATCH 00/19 V2] mm: memcontrol: charge swapin pages on instantiation

[Balbir Singh]

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.