Re: [PATCH] Revert "perf/aux: Allocate non-contiguous AUX pages by default"

[Leo Yan <leo.yan@arm.com>]

On Wed, Nov 04, 2026 at 12:59:13PM +0000, Will Deacon wrote:

[...]

> > Sorry for the late reply, and sorry that the commit message in
> > 18049c8cff9c ("perf/aux: Allocate non-contiguous AUX pages by default")
> > was imprecise about "exacerbating memory fragmentation". You and Dev are
> > right that grouping same-lifetime pages into higher-order allocations
> > does not itself worsen fragmentation, and the TLB reduction numbers for
> > TRBE and SPE with large vmap() mappings make total sense given TRBE's
> > tiny TLB.
> > 
> > However, a straight revert will reintroduce a severe production issue we
> > hit on Android devices, so I'd like to share what actually happened and
> > suggest a way to get large mappings for SPE/TRBE without bringing back
> > the regression.
> 
> [...]
> 
> Thanks, Yabin. This just hardens my stance [1] that the allocation decision
> should be driven by userspace and not second-guessed by the kernel. So the
> revert is _not_ the right approach, in my view.

If a userspace interface is needed, what remaining reclaim or allocation
policy should userspace control? I am not clear how userspace control
would fix the PSI reclaim stalls described in Yabin's reply.

Alternatively, we could optimize the reported reclaim cost in the kernel
by clearing __GFP_RECLAIM for optional higher-order attempts, while
preserving the existing policy at order 0 and for
PERF_PMU_CAP_AUX_NO_SG PMUs.

If testing confirms this works, combining this mitigation with a revert
of 18049c8cff9c would be a reasonable way to restore higher-order
allocations for SPE/TRBE without adding a userspace interface.

Thanks,
Leo