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From:   Kent Overstreet <kent.overstreet@linux.dev>
To:     Song Liu <song@kernel.org>
Cc:     Mike Rapoport <rppt@kernel.org>, linux-mm@kvack.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Rick Edgecombe <rick.p.edgecombe@intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Vlastimil Babka <vbabka@suse.cz>, linux-kernel@vger.kernel.org,
        x86@kernel.org
Subject: Re: [RFC PATCH 1/5] mm: intorduce __GFP_UNMAPPED and unmapped_alloc()
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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.