Re: [PATCH v5] mm/page_alloc: avoid direct compaction for costly __GFP_NORETRY allocations

[Salvatore Dipietro]

On Fri, 11 Sep 2026 09:32:00 -0700 Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:

> does any of this look real?

The mechanism seems real, but it only shows up with vm.defrag_mode=1,
which is off by default.

With defrag_mode=1, a costly __GFP_NORETRY allocation that used to fail
cleanly can now succeed by fragmenting another migratetype's pageblock,
which seems like the wrong trade for a caller whose premise is a cheap
lower-order fallback. Johannes, do we want to keep it strict?
If so, we could add a condition like below. It is equally
applicable to v4 and v5. Happy to send it separately or in v6.

	/*
	 * Reclaim/compaction cannot run, so defrag_mode's strategy
	 * of enforcing ALLOC_NOFRAGMENT cannot be fulfilled. Allow
	 * fallbacks rather than failing the allocation outright.
	 * Not for costly __GFP_NORETRY: those have a cheap lower
	 * order fallback, so failing beats fragmenting.
	 */
	if (defrag_mode && (alloc_flags & ALLOC_NOFRAGMENT) &&
	    !(costly_order && (gfp_mask & __GFP_NORETRY)) &&
	    (gfp_mask & __GFP_KSWAPD_RECLAIM)) {
		alloc_flags &= ~ALLOC_NOFRAGMENT;
		goto retry;
	}


Salvatore





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