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

[IBM]

Ritesh Harjani (IBM) <ritesh.list@xxxxxxxxx> writes:

> Matthew Wilcox <willy@xxxxxxxxxxxxx> 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;
> 			}
>

Hi Salvatore,

Did you get a chance to test the above two options (shared by Matthew
and me)? And were you able to recover the performance back with those?

So, in a longer run, as Dave suggested, we might need to fix this by
maybe considering removing compaction in the direct reclaim path. But I
guess for fixing it in older kernel releases, we might need a quick fix
,so maybe worth trying the above suggested changes, perhaps.

Also, I am somehow not able to hit this problem at my end (even after
creating a bit of memory fragmentation). So please also feel free to
share the steps, if you have a setup to re-create it easily.

-ritesh