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

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

[Ritesh Harjani]

Let's cc: linux-mm too.

Salvatore Dipietro <dipiets@amazon.it> writes:

> 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

Isn't it the right thing to do i.e. run compaction, when memory is
fragmented? 

> 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.
>

I think removing the __GFP_DIRECT_RECLAIM flag unconditionally at the
caller may cause -ENOMEM. Note that it is the __filemap_get_folio()
which retries with smaller order allocations, so instead of changing the
callers, shouldn't this be fixed in __filemap_get_folio() instead?

Maybe in there too, we should keep the reclaim flag (if passed by
caller) at least for <= PAGE_ALLOC_COSTLY_ORDER + 1

Thoughts?

-ritesh

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

[Matthew Wilcox]

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;
			}

Do you recover the performance?

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

[Ritesh Harjani]

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?

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

[Matthew Wilcox]

On Sat, Apr 04, 2026 at 10:17:33PM +0530, Ritesh Harjani wrote:
> 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;
> 			}

Uhh ... maybe?  I'd want someone more familiar with the page allocator
than I am to say whether that's the right approach.  If it is, that
seems too complex, and maybe we need a better approach to the page
allocator flags.

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

[Ritesh Harjani]

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

> 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;
> 			}
>

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

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

[Salvatore Dipietro]

I have submitted a v2 of the patch based on Ritesh's suggestion.
https://lore.kernel.org/linux-mm/20260420161404.642-1-dipiets@amazon.it/T/#u

Salvatore



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

[Matthew Wilcox]

On Mon, Apr 20, 2026 at 04:33:28PM +0000, Salvatore Dipietro wrote:
> I have submitted a v2 of the patch based on Ritesh's suggestion.
> https://lore.kernel.org/linux-mm/20260420161404.642-1-dipiets@amazon.it/T/#u

... but without linking back to this thread, so nobody who was exposed
to that thread for the first time knows about this one.  That's poor form.

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

[Ritesh Harjani]

Matthew Wilcox <willy@infradead.org> writes:

> On Mon, Apr 20, 2026 at 04:33:28PM +0000, Salvatore Dipietro wrote:
>> I have submitted a v2 of the patch based on Ritesh's suggestion.
>> https://lore.kernel.org/linux-mm/20260420161404.642-1-dipiets@amazon.it/T/#u
>
> ... but without linking back to this thread, so nobody who was exposed
> to that thread for the first time knows about this one.  That's poor form.

Yup.
Also, given the Maintainers (willy, Christoph, Dave) shown their
dis-interest in taking the patch in it's current form, the right way is
to get back with performance data with both the approaches (which we
were discussing) and first get the consensus from everyone, before
proposing this as a patch :).

Having said that, we do care if a genuine performance issue gets
reported. In that context, I wanted to understand your setup a bit from
memory fragmentation perspective. Are you trying to simulate memory
fragmentation and then benchmarking? Or was this problem hitting when
you run simply run the reproduction steps mentioned in your cover
letter?


BTW - I was following the other thread too where PREEMPT_LAZY problem
was getting discussed. And from what I understood, you mentioned [1]
enabling THP on the system made that problem go away. Also it looks like
enabling THP is the right thing to do for this kind of workload. Does
that also mean enabling THP fixed this problem too? Do you still hit
memory fragmentation and/or similar throughput drop w/o this fix after
you enable THP? It will be good to know those details too please.

[1]: https://lore.kernel.org/all/20260403191942.21410-1-dipiets@amazon.it/T/#md88ca4258766e897e432df85874d197db476c7d1

-ritesh

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

[Vlastimil Babka]

On 4/6/26 00:43, Dave Chinner wrote:
> 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.
>> 
>> Fixes: 5d8edfb900d5 ("iomap: Copy larger chunks from userspace")
>> Cc: stable@vger.kernel.org
>> Signed-off-by: Salvatore Dipietro <dipiets@amazon.it>

BTW, backporting perf regressions fixes to 6.6, when they are only reported
at the time 7.0 is released, might be too risky. There will likely be a
different workload that will regress as a result, no matter what we do.

>> ---
>>  fs/iomap/buffered-io.c | 15 ++++++++++++++-
>>  1 file changed, 14 insertions(+), 1 deletion(-)
>> 
>> diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
>> index 92a831cf4bf1..cb843d54b4d9 100644
>> --- a/fs/iomap/buffered-io.c
>> +++ b/fs/iomap/buffered-io.c
>> @@ -715,6 +715,7 @@ EXPORT_SYMBOL_GPL(iomap_is_partially_uptodate);
>>  struct folio *iomap_get_folio(struct iomap_iter *iter, loff_t pos, size_t len)
>>  {
>>  	fgf_t fgp = FGP_WRITEBEGIN | FGP_NOFS;
>> +	gfp_t gfp;
>>  
>>  	if (iter->flags & IOMAP_NOWAIT)
>>  		fgp |= FGP_NOWAIT;
>> @@ -722,8 +723,20 @@ struct folio *iomap_get_folio(struct iomap_iter *iter, loff_t pos, size_t len)
>>  		fgp |= FGP_DONTCACHE;
>>  	fgp |= fgf_set_order(len);
>>  
>> +	gfp = mapping_gfp_mask(iter->inode->i_mapping);
>> +
>> +	/*
>> +	 * If the folio order hint exceeds PAGE_ALLOC_COSTLY_ORDER,
>> +	 * strip __GFP_DIRECT_RECLAIM to make the allocation purely
>> +	 * opportunistic.  This avoids compaction + drain_all_pages()
>> +	 * in __alloc_pages_slowpath() that devastate throughput
>> +	 * on large systems during buffered writes.
>> +	 */
>> +	if (FGF_GET_ORDER(fgp) > PAGE_ALLOC_COSTLY_ORDER)
>> +		gfp &= ~__GFP_DIRECT_RECLAIM;
> 
> Adding these "gfp &= ~__GFP_DIRECT_RECLAIM" hacks everywhere
> we need to do high order folio allocation is getting out of hand.
> 
> Compaction improves long term system performance, so we don't really
> just want to turn it off whenever we have demand for high order
> folios.
> 
> We should be doing is getting rid of compaction out of the direct
> reclaim path - it is -clearly- way too costly for hot paths that use
> large allocations, especially those with fallbacks to smaller
> allocations or vmalloc.
> 
> Instead, memory reclaim should kick background compaction and let it
> do the work. If the allocation path really, really needs high order
> allocation to succeed, then it can direct the allocation to retry
> until it succeeds and the allocator itself can wait for background
> compaction to make progress.
> 
> For code that has fallbacks to smaller allocations, then there is no
> need to wait for compaction - we can attempt fast smaller allocations
> and continue that way until an allocation succeeds....

So, should we do a LSF/MM session?

But I think in any case, the page allocator needs to know which allocations
do have the fallback. __GFP_NORETRY exists for this. Here it wasn't tried at
all, in v2 [1] it was, but not alone. I'd start from __GFP_NORETRY alone,
and then we can look at tweaking what it does if it's currently insufficient.

We could have a helper to encapsulate this "turn this allocation to a
lightweight fallbackable one", which would add __GFP_NORETRY. It probably
already exists somewhere but not gfp.h. But I'm not sure we can simply
change GFP_KERNEL to start failing more for non-costly orders. We've
discussed that a lot in the past :)

[1] https://lore.kernel.org/all/20260420161404.642-1-dipiets@amazon.it/

> -Dave.