linux-kernel - Re: [PATCH v3 3/3] mm/vmscan: don't demote if there is not enough free memory in the lower memory tier

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Message-ID: <aXlNvgS1CsU0JoE_@gourry-fedora-PF4VCD3F>
Date: Tue, 27 Jan 2026 18:43:58 -0500
From: Gregory Price <gourry@...rry.net>
To: Bing Jiao <bingjiao@...gle.com>
Cc: Akinobu Mita <akinobu.mita@...il.com>, linux-cxl@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	akpm@...ux-foundation.org, axelrasmussen@...gle.com,
	yuanchu@...gle.com, weixugc@...gle.com, hannes@...xchg.org,
	david@...nel.org, mhocko@...nel.org, zhengqi.arch@...edance.com,
	shakeel.butt@...ux.dev, lorenzo.stoakes@...cle.com,
	Liam.Howlett@...cle.com, vbabka@...e.cz, rppt@...nel.org,
	surenb@...gle.com
Subject: Re: [PATCH v3 3/3] mm/vmscan: don't demote if there is not enough
 free memory in the lower memory tier

On Tue, Jan 27, 2026 at 11:28:59PM +0000, Bing Jiao wrote:
> Hi, Gregory, hope you are doing well.
> 
> I observed that during the allocation of a large folio,
> alloc_migration_target() cleans __GFP_RECLAIM but subsequently applies
> GFP_TRANSHUGE. Given that GFP_TRANSHUGE includes __GFP_DIRECT_RECLAIM,
> I am wondering if this triggers a form of reclamation that should be
> avoided during demotion.
> 
> struct folio *alloc_migration_target(struct folio *src, unsigned long private)
> ...
> 	if (folio_test_large(src)) {
> 		/*
> 		 * clear __GFP_RECLAIM to make the migration callback
> 		 * consistent with regular THP allocations.
> 		 */
> 		gfp_mask &= ~__GFP_RECLAIM;
> 		gfp_mask |= GFP_TRANSHUGE;
> 		order = folio_order(src);
> 	}
> 
> #define GFP_TRANSHUGE	(GFP_TRANSHUGE_LIGHT | __GFP_DIRECT_RECLAIM)
> 

I think the answer is that the demotion code is a mess and no one
actually knows what it's doing.  We probably need to rework this
entirely because we've now found at least 2 paths which clean and then
add reclaim.

~Gregory