linux-kernel - Re: [RFC PATCH 1/5] mm: intorduce __GFP_UNMAPPED and unmapped

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Date:   Thu, 18 May 2023 16:13:41 -0400
From:   Kent Overstreet <kent.overstreet@...ux.dev>
To:     Song Liu <song@...nel.org>
Cc:     Mike Rapoport <rppt@...nel.org>, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Rick Edgecombe <rick.p.edgecombe@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vlastimil Babka <vbabka@...e.cz>, linux-kernel@...r.kernel.org,
        x86@...nel.org
Subject: Re: [RFC PATCH 1/5] mm: intorduce __GFP_UNMAPPED and unmapped_alloc()

On Thu, May 18, 2023 at 01:03:28PM -0700, Song Liu wrote:
> If we use compound_order(), we will round up to power of 2 for all
> allocations. Does this mean we will use 4MiB for a 2.1MiB allocation?

Yes. This means we lose on average 33% - I believe, someone with better
statistics might correct me - to internal fragmentation. But the buddy
allocator will be better at avoiding and dealing with external
fragmentation over time.