Re: [PATCH 1/1] iomap: avoid compaction for costly folio order allocation

Ritesh Harjani
2026-04-04T17:24:50Z

Matthew Wilcox <willy@infradead.org> writes:

> On Fri, Apr 03, 2026 at 07:35:34PM +0000, Salvatore Dipietro wrote:
>> Commit 5d8edfb900d5 ("iomap: Copy larger chunks from userspace")
>> introduced high-order folio allocations in the buffered write
>> path. When memory is fragmented, each failed allocation triggers
>> compaction and drain_all_pages() via __alloc_pages_slowpath(),
>> causing a 0.75x throughput drop on pgbench (simple-update) with
>> 1024 clients on a 96-vCPU arm64 system.
>>
>> Strip __GFP_DIRECT_RECLAIM from folio allocations in
>> iomap_get_folio() when the order exceeds PAGE_ALLOC_COSTLY_ORDER,
>> making them purely opportunistic.
>
> If you look at __filemap_get_folio_mpol(), that's kind of being tried
> already:
>
> if (order > min_order)
> alloc_gfp |= __GFP_NORETRY | __GFP_NOWARN;
>
> * %__GFP_NORETRY: The VM implementation will try only very lightweight
> * memory direct reclaim to get some memory under memory pressure (thus
> * it can sleep). It will avoid disruptive actions like OOM killer. The
> * caller must handle the failure which is quite likely to happen under
> * heavy memory pressure. The flag is suitable when failure can easily be
> * handled at small cost, such as reduced throughput.
>
> which, from the description, seemed like the right approach. So either
> the description or the implementation should be updated, I suppose?
>
> Now, what happens if you change those two lines to:
>
> if (order > min_order) {
> alloc_gfp &= ~__GFP_DIRECT_RECLAIM;
> alloc_gfp |= __GFP_NOWARN;
> }

Hi Matthew,

Shouldn't we try this instead? This would still allows us to keep
__GFP_NORETRY and hence light weight direct reclaim/compaction for
atleast the non-costly order allocations, right?

if (order > min_order) {
alloc_gfp |= __GFP_NOWARN;
if (order > PAGE_ALLOC_COSTLY_ORDER)
alloc_gfp &= ~__GFP_DIRECT_RECLAIM;
else
alloc_gfp |= __GFP_NORETRY;
}

-ritesh

>
> Do you recover the performance?