Re: [PATCH mm-new v1 2/2] mm: vmscan: skip anon folios zswap would refuse when writeback is off

[Yosry Ahmed]

On Fri, Oct 30, 2026 at 2:53 AM Jiayuan Chen <jiayuan.chen@xxxxxxxxx> wrote:
>
> A memcg with memory.zswap.writeback=0 can only swap into zswap. Once
> zswap stops taking its pages, because the pool is at its limit or the
> memcg is at memory.zswap.max, swap_writeout() has nowhere to put a
> dirty folio and returns AOP_WRITEPAGE_ACTIVATE.
>
> shrink_folio_list() only learns that at the very end. Every folio it
> picks gets a swap slot, is unmapped, fails in swap_writeout() and goes
> back to the active list, to be aged and tried again. Each round costs
> a slot, an rmap walk, a TLB flush and a minor fault for the owner, and
> frees nothing. The folio also stays in the swap cache holding its slot.
>
> The obvious place for a zswap check is can_reclaim_anon_pages(), but
> it cannot go there. Zero-filled folios never reach zswap:
> swap_writeout() marks them in the swap table and returns before
> zswap_store() is called. They need no pool space, they are common, and
> a memcg-wide "no anon" decision would keep them in memory for as long
> as zswap is full. With zswap full they must still be reclaimable.

I really wonder if we should revisit not swapping zero-filled pages at
all, and instead mapping them to the zero page read-only (and whatever
the equivalent for shmem could be), because yes, the right place is
can_reclaim_anon_pages(). I don't remember what happened to that
proposal, Usama do you remember?