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@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;
> 			}

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.