Re: [PATCH v2 1/2] mm/vmstat, mm/memcontrol: add _monotonic vmstat readers

[Johannes Weiner]

On Mon, Jul 13, 2026 at 09:34:16AM -0700, Usama Arif wrote:
> lruvec_page_state(), node_page_state(), and global_node_page_state()
> all clamp negative reads to zero on CONFIG_SMP so that a transient
> per-CPU delta skew presents as zero pages rather than
> as a garbage unsigned value. This is the right behaviour for
> non-monotonic page-count readers.
> 
> It is however incorrect for callers that snapshot a monotonically-
> incremented event counter and compute a delta from two samples.
> Once the underlying signed long wraps past LONG_MAX, the clamped read
> drops to zero while the previously-recorded snapshot still holds the
> pre-wrap value; the unsigned subtraction then underflows into a
> ~2^31 spurious delta for 32-bit architecture and corrupts the
> caller's accumulator.
> 
> Add non-clamping siblings that return the underlying state value
> cast to unsigned long:
> 
>   global_node_page_state_monotonic()
>   node_page_state_monotonic()
>   lruvec_page_state_monotonic()
> 
> With both samples read via the _monotonic variant, unsigned modular
> subtraction stays correct across a signed-long wraparound as long
> as the true growth between two samples fits in unsigned long
> (< 2^32 on 32-bit, < 2^64 on 64-bit); the 32-bit bound is the
> practically-reachable one that motivates this helper.
> 
> The variants are only safe for monotonically-incremented counters.
> Non-monotonic page-count readers must keep using the existing
> clamped helpers so transient negative reads still present as zero.
> 
> This is a prerequisite for the following patch which
> replaces the producer-side anon_cost/file_cost accumulators with a
> read-side accumulator in prepare_scan_control() that samples
> monotonic per-LRU vmstat counters (PGROTATE_*, PGRECLAIM_PAGEOUT_*,
> WORKINGSET_RESTORE_*) via lruvec_page_state_monotonic() and folds
> the unsigned modular delta into a per-lruvec cost_accum[].
> 
> Signed-off-by: Usama Arif <usama.arif@xxxxxxxxx>

Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>

It's unfortunate that we mix state counts with monotonic events in
node_stat_item. We have vm_event_state for monotonics, but they aren't
tracked per-node (which we need here and for other places in vmscan),
and they can't easily be made so because there are "global" events in
there that don't easily map to a specific node.

So this seems like the best solution for now.