From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-qv1-f46.google.com (mail-qv1-f46.google.com [209.85.219.46]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id D153937C0F3 for ; Thu, 16 Apr 2026 20:14:08 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.219.46 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1776370453; cv=none; b=bLL3uOWWyvA6BelKFV9W0HYlML0dtTiLw9dWbSikk96Y9Byp34KQH6VqHFfwsmwOUCTdoOmBgmQQ9UBHfBHcFNqTsZMtMB5pz4o3MhxxlgrMBXwkDTqvJTTd+WdIWBMhLOj0rGW9O2G6UOBipy/qzvBcJWkGgVQ6JMMqSope2D0= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1776370453; c=relaxed/simple; bh=e7Fg9iUXgdaJVBpIOwk6ko7gURjRziHIJE6o8vDP+M8=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=cOfUDZ6UV2TdBRgRHyDkX/+YdUsSc5Bs/Dez+DgIU/aAxt4BBDtyNEp4SvEwzKIXjr2St+NpfYWTMNznc5JU2JcKwk6DqqZ0EoDWdzHkeJeFMDPmYDyv82AHa4/Wt81kqPlb3lHzPrloEauKYh4RZ7mSeogtrL0VSBs99qIi0DE= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=gourry.net; spf=pass smtp.mailfrom=gourry.net; dkim=pass (2048-bit key) header.d=gourry.net header.i=@gourry.net header.b=p1uxz9FL; arc=none smtp.client-ip=209.85.219.46 Authentication-Results: smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=gourry.net Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=gourry.net Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=gourry.net header.i=@gourry.net header.b="p1uxz9FL" Received: by mail-qv1-f46.google.com with SMTP id 6a1803df08f44-89fc4147f2eso100346366d6.3 for ; Thu, 16 Apr 2026 13:14:08 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gourry.net; s=google; t=1776370446; x=1776975246; darn=lists.linux.dev; h=in-reply-to:content-transfer-encoding:content-disposition :mime-version:references:message-id:subject:cc:to:from:date:from:to :cc:subject:date:message-id:reply-to; bh=VzpFkDx1hfZFcFbwqFMS/1j2riFwDbWAqbQBRsqDt2o=; b=p1uxz9FLQi5KlHpjvhKh5+sBUmZBEJM+RRyVgGjxzf0kEyQ8fM7IMuQT7Ds4pAmZNE +roPOiAMoXfPDWcEXEWuSZhP75wbX0BS+agqZw1a0k1CN4jQQEaBNleDczWJ6b91n6u6 IEpaHfOL967gDRzulZP35/Mjomt1thBeY2W8sYcrl8SII07wb5WAIeK1/8VbbfO8v97B jTH3WfZLbRt6bkoRacZRm5sDpe7HLm/sgnzoIp6BmKSYizhVu5bQ67etxibMZHIMZHB6 j735Wh3vVgzJhDIiZCqUkSqY79ao0i2L9EOQga1MMpfFPdtgKBbAVKkwUAo3fywqNgHm lnAg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20251104; t=1776370446; x=1776975246; h=in-reply-to:content-transfer-encoding:content-disposition :mime-version:references:message-id:subject:cc:to:from:date:x-gm-gg :x-gm-message-state:from:to:cc:subject:date:message-id:reply-to; bh=VzpFkDx1hfZFcFbwqFMS/1j2riFwDbWAqbQBRsqDt2o=; b=qTXn0hQ260kB7oTAtU4UDLngRhbNJMyeKp8i8T1qs86y8gNOeHk+ackqSqBP6nOqbn PTF+lgWqJeq2D8o8wys5S5H/szl8vwA/JnXoLv7VNH8eGcpTeeQ/3/tAaMovn4r2Mi9m 6fnXsdxSJsfkHW7lDc5HG2pnFIlze8dyRL50wQA5hM62Tu5IwRYo6MxmPgnLxuk8iCc+ TBMERp/qxvb+TswgAEbVgywSNT8/nbfOsDRsza9vkTkD7iWEaGQ5uknCE3+A6XaK6QFf KaW0/bKigYsO57erbCG25UFVh2RnPJvWC5m9JMfnEz7O/5E0A71I82gECz6mbMd8Gd9t LS4g== X-Forwarded-Encrypted: i=1; AFNElJ9Lc/0Z7EiY2Keym/1o4NDipdR3gMIWiUmDzoUinA8hbLdnu/sUmcfvpj0y8JEONk2jJAFJGGo=@lists.linux.dev X-Gm-Message-State: AOJu0Yy3dV4hgnFj4ZZgueGfWvbpTd/PZ3r97cSTKuKHnlj4kPD7YvLy peH/W31JaG/X0F65Uxnh9HAZsT5JQVsmJrFgUkOr3bVxwOVvgJUW4z6ffkV1KW/hxwk= X-Gm-Gg: AeBDiet9PuZBcJ2XDEu0PJs5Wt9fAYo7qlcdfRzyyy6ol/yYh0MH7avBfkfZAHFWwes m7/yh4rEWbbLZ2SGeO2XU+QJQaNlG7W+ObBm+wLqGPwt8GEn1s7CfLwYzNW1GlcZJh4vsjaqnWv POA/VpeyLTGL9ieRldgAeAMkx9c+4qRf/6kdpPstjwp7u8w7hdnIK/1ri5IcSNbgYSvQMTHqQRQ vrSXH9tVsPpXqVo0ABOrgHsrkRAXexrqSM7aVvmsNefHHa0X4fK7glcnD8TNdAAy8UAFM5gKKM8 43inZwMDq0/OBWwq9sTvw/9v3MAqaI1JbAum/y3hLoP7ww3DNri0t9t/klUySy28WWO1QfB6+CR XAMCy2uiIz0Rg4wzgmTSNUFVYB6tlPdNpzfKs6GnUOJ7+fxBURvv0+jxoWyhRQnULwm4v7b6tbN K7f6HuOkbjMjEdYEvRO0UQt7AZLZ+Jp/ZE65xPamHuS+V/Ah6FGDfWQ+1NEe8= X-Received: by 2002:a0c:f09a:0:b0:8ae:6347:8c8b with SMTP id 6a1803df08f44-8b025ef029fmr11410336d6.37.1776370446430; Thu, 16 Apr 2026 13:14:06 -0700 (PDT) Received: from gourry-fedora-PF4VCD3F ([2607:fb90:ea1b:4643:9f36:3ffb:fc60:9518]) by smtp.gmail.com with ESMTPSA id 6a1803df08f44-8ae6cb9ee20sm43873996d6.26.2026.04.16.13.14.03 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 16 Apr 2026 13:14:05 -0700 (PDT) Date: Thu, 16 Apr 2026 16:14:01 -0400 From: Gregory Price To: Joanne Koong Cc: John Groves , "Darrick J. Wong" , Miklos Szeredi , Bernd Schubert , John Groves , Dan Williams , Bernd Schubert , Alison Schofield , John Groves , Jonathan Corbet , Shuah Khan , Vishal Verma , Dave Jiang , Matthew Wilcox , Jan Kara , Alexander Viro , David Hildenbrand , Christian Brauner , Randy Dunlap , Jeff Layton , Amir Goldstein , Jonathan Cameron , Stefan Hajnoczi , Josef Bacik , Bagas Sanjaya , Chen Linxuan , James Morse , Fuad Tabba , Sean Christopherson , Shivank Garg , Ackerley Tng , Aravind Ramesh , Ajay Joshi , "venkataravis@micron.com" , "linux-doc@vger.kernel.org" , "linux-kernel@vger.kernel.org" , "nvdimm@lists.linux.dev" , "linux-cxl@vger.kernel.org" , "linux-fsdevel@vger.kernel.org" , djbw@kernel.org Subject: Re: [PATCH V10 00/10] famfs: port into fuse Message-ID: References: <38744253-efa3-41c5-a491-b177a4a4c835@bsbernd.com> <20260414185740.GA604658@frogsfrogsfrogs> Precedence: bulk X-Mailing-List: nvdimm@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: On Thu, Apr 16, 2026 at 08:56:46AM -0700, Joanne Koong wrote: > On Tue, Apr 14, 2026 at 5:10 PM John Groves wrote: > > > > There is a FUSE_DAX_FMAP capability that the kernel may advertise or not > > at init time; this capability "is" the famfs GET_FMAP AND GET_DAXDEV > > commands. In the future, if we find a way to use BPF (or some other > > mechanism) to avoid needing those fuse messages, the kernel could be updated > > to NEVER advertise the FUSE_DAX_FMAP capability. All of the famfs-specific > > code could be taken out of kernels that never advertise that capability. > > I’m not sure the capability bit can be used like that (though I am > hoping it can!). As I understand it, once the kernel advertises a > capability, it must continue supporting it in future kernels else > userspace programs that rely on it will break. > FUSE_DAX_FMAP is already conditional on CONFIG_FUSE_DAX, the kernel is not required to continue advertising FUSE_DAX_FMAP in perpetuity. Setting CONFIG_FUSE_DAX=n does not mean userland "is broken", this would only be the case if FUSE_DAX_FMAP was advertised but not actually supported. If DAX were removed from the kernel (unlikely, but stick with me) this would be equivalent to permanently changing CONFIG_FUSE_DAX to always off, and there would be no squabbles over whether that particular change broke userland (there would be much strife over removing dax). While not a deprecation method, this is what capability bits are designed for. Same as cpuid capability bits - just because the bit is there doesn't mean a processor is required to support it in perpetuity. They're only required to support it if the bit is turned on. --- I think the focus here needs to be on whether this interface ACTUALLY needs to be more generic - and whether that is actually FEASIBLE. It's not like this is a new problem - and there are real design reasons why John chose this route. The additional overhead is not trivial for FAMFS - FAMFS is not doing i/o. He already has data showing fuse caused a performance hit due to overhead on open - his concern of overhead on fault being catastrophic is grounded in data. For others it's an age old problem of self-describing protocols (parsing vs giant inflexible binary blobs, pick your poison). It's extremely unlikely we will find a one-size-fits-all solution that doesn't eventually run right back into this same problem. I worry that this discussion is going to turn towards implementing a solution grounded in parsing arbitrary formats and how to store them, and that is completely detached from why FAMFS went this route in the first place. I question whether the actual issue here lies in the interface APPEARING more general purpose than it actually is - and therefore inviting attempts to over-genericize it. Is there a world here where this is solved by a name change and a capability bit? I think so. ~Gregory