Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1768877Ab2KOT4K (ORCPT ); Thu, 15 Nov 2012 14:56:10 -0500 Received: from mail-yh0-f46.google.com ([209.85.213.46]:38009 "EHLO mail-yh0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1768751Ab2KOT4J (ORCPT ); Thu, 15 Nov 2012 14:56:09 -0500 Date: Thu, 15 Nov 2012 11:56:05 -0800 (PST) From: Hugh Dickins X-X-Sender: hugh@eggly.anvils To: Jaegeuk Hanse cc: Dave Jones , Andrew Morton , Johannes Weiner , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] tmpfs: fix shmem_getpage_gfp VM_BUG_ON In-Reply-To: <50A49C46.9040406@gmail.com> Message-ID: References: <20121025023738.GA27001@redhat.com> <20121101191052.GA5884@redhat.com> <20121101232030.GA25519@redhat.com> <20121102014336.GA1727@redhat.com> <20121106135402.GA3543@redhat.com> <50A30ADD.9000209@gmail.com> <50A49C46.9040406@gmail.com> User-Agent: Alpine 2.00 (LNX 1167 2008-08-23) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1290 Lines: 31 Offtopic... On Thu, 15 Nov 2012, Jaegeuk Hanse wrote: > > Another question. Why the function shmem_fallocate which you add to kernel > need call shmem_getpage? Because shmem_getpage(_gfp) is where shmem's page lookup and allocation complexities are handled. I assume the question behind your question is: why does shmem actually allocate pages for its fallocate, instead of just reserving the space? I did play with just reserving the space, with more special entries in the radix_tree to note the reservations made. It should be doable for the vm_enough_memory and sbinfo->used_blocks reservations. What absolutely deterred me from taking that path was the mem_cgroup case: shmem and swap and memcg are not easy to get working right together, and nobody would thank me for complicating memcg just for shmem_fallocate. By allocating pages, the pre-existing memcg code just works; if we used reservations instead, we would have to track their memcg charges in some additional new way. I see no justification for that complication. Hugh -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/