From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from m16.mail.126.com (m16.mail.126.com [220.197.31.9]) by smtp.subspace.kernel.org (Postfix) with ESMTP id B51A83C0B; Fri, 28 Jun 2024 01:27:01 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=220.197.31.9 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1719538025; cv=none; b=Ki56mVkyhDTOzKru3gbO1jdnxAZkbFFaIpYMYLfgpVJ9exymCJKgwJPGykSbzBvW96hsH8SHd+V8Utj3oaef3Cq6ITJPJoaURizOuPSVbd9WH1cQtkNmEiFZn+xJ7dt7qc42AUr7l26X6irPy6edG/1fT0B6Luj0/MW6V9LNOCk= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1719538025; c=relaxed/simple; bh=V38sFmASQfqGcoseLmQq14vgW3z2xQfKhNYM+Pd/RXQ=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=dezlBHKCrL17girD1r011yucgAPGGsMrFjXlYrVaMzLufD3l6uVMYWNpcKv8FqmiE8DYBYwRKhTBskPqaJ8bdA8fRRssmU4Gh4++9KoaSgbCSfxSy7onWEOzl3AGisjC7Il68iAZVqgls8GdvcV7/GeumfKxJn6sf/KKJtWgCGg= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=126.com; spf=pass smtp.mailfrom=126.com; dkim=pass (1024-bit key) header.d=126.com header.i=@126.com header.b=Rn3fwgiy; arc=none smtp.client-ip=220.197.31.9 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=126.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=126.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=126.com header.i=@126.com header.b="Rn3fwgiy" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=126.com; s=s110527; h=Message-ID:Date:MIME-Version:Subject:From: Content-Type; bh=tTOy6Kq3aNVWHj/osQBSoy/9+UEn0MO184Z7Lmt65g4=; b=Rn3fwgiy5jx0QWHgKQoup4vLNXxWu8hqTUrCjxxT7XMicWWAgC+NigrX4NebqS gLdQ7j41rHMR0rFgFFfVBPCr0HEspsoX3z/M7Rabp1D+tkZrQg+LgEa8Iux26xfP RgvQYhCAWfejscynWy7EEiidX+yNHv8IvysoS5geF3SP0= Received: from [172.21.22.210] (unknown [118.242.3.34]) by gzga-smtp-mta-g1-0 (Coremail) with SMTP id _____wDX_9kZEX5m77HCAA--.18751S2; Fri, 28 Jun 2024 09:25:48 +0800 (CST) Message-ID: <4850060d-7fc2-4869-a901-38f11058bd40@126.com> Date: Fri, 28 Jun 2024 09:25:45 +0800 Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [v2 PATCH] mm: gup: do not call try_grab_folio() in slow path To: Peter Xu , Andrew Morton Cc: Yang Shi , david@redhat.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org References: <20240627221413.671680-1-yang@os.amperecomputing.com> <20240627163242.39b0a716bd950a895c032136@linux-foundation.org> From: Ge Yang In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-CM-TRANSID:_____wDX_9kZEX5m77HCAA--.18751S2 X-Coremail-Antispam: 1Uf129KBjvJXoWxZFW5CF48Cw13Ar48Gr1UGFg_yoW5GF4xpF y3Ka9xKFWkJr10kws7tws5XFWFyrZ8JryUXws5Gr1xua98ua4xWr48X34FkF98W348Ga10 vFW2y3srZa1DZa7anT9S1TB71UUUUU7qnTZGkaVYY2UrUUUUjbIjqfuFe4nvWSU5nxnvy2 9KBjDUYxBIdaVFxhVjvjDU0xZFpf9x07jt5rxUUUUU= X-CM-SenderInfo: 51dqwwjhrrila6rslhhfrp/1tbiOhwMG2VExE332wABs0 在 2024/6/28 7:43, Peter Xu 写道: > On Thu, Jun 27, 2024 at 04:32:42PM -0700, Andrew Morton wrote: >> On Thu, 27 Jun 2024 19:19:40 -0400 Peter Xu wrote: >> >>> Yang, >>> >>> On Thu, Jun 27, 2024 at 03:14:13PM -0700, Yang Shi wrote: >>>> The try_grab_folio() is supposed to be used in fast path and it elevates >>>> folio refcount by using add ref unless zero. We are guaranteed to have >>>> at least one stable reference in slow path, so the simple atomic add >>>> could be used. The performance difference should be trivial, but the >>>> misuse may be confusing and misleading. >>> >>> This first paragraph is IMHO misleading itself.. >>> >>> I think we should mention upfront the important bit, on the user impact. >>> >>> Here IMO the user impact should be: Linux may fail longterm pin in some >>> releavnt paths when applied over CMA reserved blocks. And if to extend a >>> bit, that include not only slow-gup but also the new memfd pinning, because >>> both of them used try_grab_folio() which used to be only for fast-gup. >> >> It's still unclear how users will be affected. What do the *users* >> see? If it's a slight slowdown, do we need to backport this at all? > > The user will see the pin fails, for gpu-slow it further triggers the WARN > right below that failure (as in the original report): > > folio = try_grab_folio(page, page_increm - 1, > foll_flags); > if (WARN_ON_ONCE(!folio)) { <------------------------ here > /* > * Release the 1st page ref if the > * folio is problematic, fail hard. > */ > gup_put_folio(page_folio(page), 1, > foll_flags); > ret = -EFAULT; > goto out; > } > > For memfd pin and hugepd paths, they should just observe GUP failure on > those longterm pins, and it'll be the caller context to decide what user > can see, I think. > >> >>> >>> The patch itself looks mostly ok to me. >>> >>> There's still some "cleanup" part mangled together, e.g., the real meat >>> should be avoiding the folio_is_longterm_pinnable() check in relevant >>> paths. The rest (e.g. switch slow-gup / memfd pin to use folio_ref_add() >>> not try_get_folio(), and renames) could be good cleanups. >>> >>> So a smaller fix might be doable, but again I don't have a strong opinion >>> here. >> >> The smaller the better for backporting, of course. > > I think a smaller version might be yangge's patch, plus Yang's hugepd > "fast" parameter for the hugepd stack, then hugepd can also use > try_grab_page(). memfd-pin change can be a separate small patch perhaps > squashed. > If needed, I can submit a new version based on Yang's V1 version. > I'll leave how to move on to Yang. > > Thanks, >