linux-kernel - Re: [RFC PATCH 0/5] Prototype for direct map awareness in page allocator

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Date:   Thu, 9 Mar 2023 01:59:00 +0000
From:   "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To:     "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "rppt@...nel.org" <rppt@...nel.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "song@...nel.org" <song@...nel.org>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "vbabka@...e.cz" <vbabka@...e.cz>,
        "x86@...nel.org" <x86@...nel.org>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>
Subject: Re: [RFC PATCH 0/5] Prototype for direct map awareness in page
 allocator

On Wed, 2023-03-08 at 11:41 +0200, Mike Rapoport wrote:
> From: "Mike Rapoport (IBM)" <rppt@...nel.org>
> 
> Hi,
> 
> This is a third attempt to make page allocator aware of the direct
> map
> layout and allow grouping of the pages that must be unmapped from
> the direct map.
> 
> This a new implementation of __GFP_UNMAPPED, kinda a follow up for
> this set:
> 
> https://lore.kernel.org/all/20220127085608.306306-1-rppt@kernel.org
> 
> but instead of using a migrate type to cache the unmapped pages, the
> current implementation adds a dedicated cache to serve __GFP_UNMAPPED
> allocations.

It seems a downside to having a page allocator outside of _the_ page
allocator is you don't get all of the features that are baked in there.
For example does secretmem care about numa? I guess in this
implementation there is just one big cache for all nodes.

Probably most users would want __GFP_ZERO. Would secretmem care about
__GFP_ACCOUNT? I'm sure there is more, but I guess the question is, is
the idea that these features all get built into unmapped-alloc at some
point? The alternate approach is to have little caches for each usage
like the grouped pages, which is probably less efficient when you have
a bunch of them. Or solve it just for modules like the bpf allocator.
Those are the tradeoffs for the approaches that have been explored,
right?