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

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Message-ID: <ZGZ5f00iemeG3GpC@moria.home.lan>
Date:   Thu, 18 May 2023 15:16:15 -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 12:03:03PM -0700, Song Liu wrote:
> > Second, bitmap based solution cannot handle > 2MiB allocation cleanly,
> > while tree based solution can. For example, if a big driver uses 3MiB, the
> > tree based allocator can allocate 4MiB for it, and use the rest 1MiB for
> > smaller allocations.

2 MB is just a good default size for the initial unmapped allocations if
the main consideration is avoiding mapping fragmentation. The approach
can work with larger allocations if required.