Re: [PATCH v6 1/2] mm: Introduce opportunistic_compaction concept to vmscan and shrinkers

[Hillf Danton <hdanton@sina.com>]

On Tue, 23 Jun 2026 09:10:43 +1000 Dave Chinner wrote:
> On Tue, Jun 16, 2026 at 08:22:17PM -0700, Matthew Brost wrote:
> > High-order allocations using __GFP_NORETRY or __GFP_RETRY_MAYFAIL
> > are often opportunistic attempts to satisfy fragmentation-sensitive
> > allocations rather than indications of severe memory pressure. In these
> > cases, reclaim may invoke shrinkers that aggressively destroy working
> > sets even though reclaim is unlikely to materially improve the
> > allocation outcome.
> > 
> > Some shrinkers manage expensive backing or migration operations where
> > reclaim can result in substantial working set disruption despite the
> > system having sufficient free memory overall. This is particularly
> > visible in fragmentation-heavy workloads where reclaim repeatedly tears
> > down active state while kswapd attempts to satisfy higher-order
> > allocations.
> > 
> > Introduce an opportunistic_compaction hint in shrink_control that allows
> > kswapd to communicate when reclaim originates from a high-order
> > allocation context that may be fragmentation driven rather than true
> > memory pressure. Shrinkers may use this hint to avoid destructive
> > working set reclaim while still participating normally during order-0
> > or stronger reclaim conditions.
> 
> To be honest, this seems like another "push a hint through to the XE
> shrinker" mechanism under a different name. You seem so focused on
> fixing the XE reproducer that the -systemic problem- that -any-
> high-order folio demand causes is not being acknowleged.
> 
> e.g. we use high-order folios extensively in the page cache these
> days, and there are -many- cases where memory compaction driven by
> high-order demand cause significant performance regressions for page
> cache performance. To date, every single person who has wanted to
> fix the problem they are seeing has effectively attempted to -turn
> off compaction- via GFP flags.
> 
> I've even done that myself inside XFS to work around kvmalloc()
> issues with a lack of GFP_NOFAIL support and doing costly high order
> allocations that fail and trigger compaction before falling back to
> vmalloc().  However, these issues have since been fixed in the
> kvmalloc() code, such that it now does the right thing for most
> calling contexts (i.e. tries high-order kmalloc() without triggering
> compaction, then fall back to GFP_NOFAIL vmalloc()). This has made
> kvmalloc() more performant and better behaved for -all users-, not
> just XFS.
> 
> This is not sustainable - we need compaction to be robust and
> performant in the face of high-order folio demands, regardless of
> what subsystem is generating the demand.
>
Because a) compaction works only when there are enough free regular pages
available, with nothing to do with shrinker directly, b) both order and
gfp are checked in in_reclaim_compaction() in addition to checking
__GFP_NORETRY and costly_order in __alloc_pages_slowpath(), I think
individual shrinker can have more option by repeating similar check.