Re: [PATCH v6] mm/page_alloc: avoid direct reclaim and compaction for costly __GFP_NORETRY allocations

[Andrew Morton]

On Fri, 2 Oct 2026 10:23:47 +0200 "Vlastimil Babka (SUSE)" <vbabka@xxxxxxxxxx> wrote:

> On 10/2/26 03:29, Andrew Morton wrote:
> > On Thu, 1 Oct 2026 08:21:51 +0000 Salvatore Dipietro <dipiets@xxxxxxxxx> wrote:
> > 
> >> Commit 5d8edfb900d5 ("iomap: Copy larger chunks from userspace")
> >> introduced high-order folio allocations in the iomap buffered write
> >> path.  When memory is fragmented, each failed costly-order allocation
> >> enters __alloc_pages_slowpath() which runs direct reclaim, direct
> >> compaction and drain_all_pages(), causing a 0.38x throughput drop on
> >> PostgreSQL pgbench (simple-update) with 1024 clients on a 96-vCPU arm64
> >> system.
> >> 
> >> The root issue is that direct reclaim and direct compaction are too
> >> expensive for hot allocation paths that have fallbacks to smaller
> >> allocations.  __filemap_get_folio_mpol() already marks higher-order
> >> allocations with __GFP_NORETRY | __GFP_NOWARN, signalling that the
> >> caller can handle failure.  However, the page allocator still enters the
> >> full blocking slowpath for costly orders with __GFP_NORETRY, which is
> >> unnecessarily aggressive when the caller will simply retry at a lower
> >> order.
> >> 
> 
> Since it expands the impact of a pre-existing bug of nonblocking allocation
> eating reserves [1] it shouldn't proceed to mainline without a fix for that
> (Johannes posted it there but the review discussion got stalled).
> 
> > Thanks, I'll update mm-hotfixes to this version.
> 
> I've also argued for not handling it as a hotfix [2]
> 
> [1] https://lore.kernel.org/all/9f415dc7-adad-4161-b20d-7c3173f50ff3@xxxxxxxxxx/
> 
> [2] https://lore.kernel.org/all/c6e2c1e1-a285-406e-a2b6-c09e05df91d8@xxxxxxxxxx/
> 

Thanks.  I moved this to tail of mm-unstable, removed its cc:stable and
added a note indicating that changes are expected.