Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Fri, 24 Mar 2023 09:37:31 +0100
From:   Michal Hocko <mhocko@suse.com>
To:     Mike Rapoport <rppt@kernel.org>
Cc:     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>,
        Song Liu <song@kernel.org>,
        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()
Message-ID: <ZB1hS9lBabp1K7XN@dhcp22.suse.cz>
References: <20230308094106.227365-1-rppt@kernel.org>
 <20230308094106.227365-2-rppt@kernel.org>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20230308094106.227365-2-rppt@kernel.org>
Precedence: bulk

On Wed 08-03-23 11:41:02, Mike Rapoport wrote:
> From: "Mike Rapoport (IBM)" <rppt@kernel.org>
> 
> When set_memory or set_direct_map APIs used to change attribute or
> permissions for chunks of several pages, the large PMD that maps these
> pages in the direct map must be split. Fragmenting the direct map in such
> manner causes TLB pressure and, eventually, performance degradation.
> 
> To avoid excessive direct map fragmentation, add ability to allocate
> "unmapped" pages with __GFP_UNMAPPED flag that will cause removal of the
> allocated pages from the direct map and use a cache of the unmapped pages.
> 
> This cache is replenished with higher order pages with preference for
> PMD_SIZE pages when possible so that there will be fewer splits of large
> pages in the direct map.
> 
> The cache is implemented as a buddy allocator, so it can serve high order
> allocations of unmapped pages.

Why do we need a dedicated gfp flag for all this when a dedicated
allocator is used anyway. What prevents users to call unmapped_pages_{alloc,free}?
-- 
Michal Hocko
SUSE Labs