Re: [PATCH v2] mm/page_alloc: fix defrag_mode for non-reclaimable allocations

["Vlastimil Babka (SUSE)" <vbabka@kernel.org>]

On 5/26/26 19:51, Johannes Weiner wrote:
> On Tue, May 26, 2026 at 03:13:09PM +0200, Vlastimil Babka (SUSE) wrote:
> 
> As you may remember it was developed to help with higher order / THP
> success rates under pressure.
> 
> The impetus for actually deploying it was that we saw issues with
> avalanches of large page cache folios vacuuming up the higher-order
> chunks; this (ironically) also led to failures on the network side.
> 
> It's kind of a structural problem. We have real preproduction buffers
> for order-0 pages through the watermarks. But for higher orders we
> only ensure there is at least one page. That easily fails under even
> mild competition.
> 
> Since we wanted to roll defrag_mode for THP in multi-tenant systems
> anyway, we figured we might as well take the plunge now and battle
> test the feature this way.

Great!

> defrag_mode fixes *that* issue, by preproducing watermark buffers in
> contiguous pageblocks - making everything up to that order more
> readily available. I'm still hoping to make it the default eventually,
> which was the plan with the original huge page allocator series. As we
> keep leaning into higher order requests more and more, and especially
> grow the non-optional ones, we kind of need non-optional preproduction
> guarantees for higher orders as well.
> 
> But there are bugs like this one, and we're still figuring out some
> overreclaim issues with it in production as well. So I'm glad it's
> optional for the time being ;-)

Right :) thanks for sharing!