From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by smtp.lore.kernel.org (Postfix) with ESMTP id D734BC83F1A for ; Tue, 22 Jul 2025 20:07:26 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 5BBBD6B0093; Tue, 22 Jul 2025 16:07:26 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 593456B0096; Tue, 22 Jul 2025 16:07:26 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 4CFC66B0098; Tue, 22 Jul 2025 16:07:26 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0016.hostedemail.com [216.40.44.16]) by kanga.kvack.org (Postfix) with ESMTP id 3DC846B0093 for ; Tue, 22 Jul 2025 16:07:26 -0400 (EDT) Received: from smtpin16.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay05.hostedemail.com (Postfix) with ESMTP id CD9BA5932D for ; Tue, 22 Jul 2025 20:07:25 +0000 (UTC) X-FDA: 83692985250.16.A50A80C Received: from nyc.source.kernel.org (nyc.source.kernel.org [147.75.193.91]) by imf18.hostedemail.com (Postfix) with ESMTP id 261D21C000F for ; Tue, 22 Jul 2025 20:07:24 +0000 (UTC) Authentication-Results: imf18.hostedemail.com; dkim=pass header.d=linux-foundation.org header.s=korg header.b=OZ95R17l; spf=pass (imf18.hostedemail.com: domain of akpm@linux-foundation.org designates 147.75.193.91 as permitted sender) smtp.mailfrom=akpm@linux-foundation.org; dmarc=none ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1753214844; a=rsa-sha256; cv=none; b=SjAy5nLe//o0jeWKQ5ljhewBS6EUAlUDiRS1zMMxrOX//CcWFIMQo7PmWGLJzk9AC5gFTT bn9qtQeDvyM8+TeJ68zQnrLLC0K4RztlxmVgGT5rL8gqrLLonMPM7ANqpfiPL2vc9S2cRG Dn0A2XsfyuBnTf5XRDtWEpdm8+3ONbI= ARC-Authentication-Results: i=1; imf18.hostedemail.com; dkim=pass header.d=linux-foundation.org header.s=korg header.b=OZ95R17l; spf=pass (imf18.hostedemail.com: domain of akpm@linux-foundation.org designates 147.75.193.91 as permitted sender) smtp.mailfrom=akpm@linux-foundation.org; dmarc=none ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1753214844; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=Erlzn8t+joNvTF8iKzi+gIAbF5whSV5OaTPfr6f/hnE=; b=aHleKg0e3tLrxLcOotDgzip14e9YMJkrDUpqvHifyMvp3/h9OcQXl8nm7jI9OvyqYWIObH Jhmfa2feucIn5++NMgjeJOa0jl7NSXxv+VDp7JWV/VHeLld6W8fn0XZwFtEpXx+Ejt1i6e B/2ha7L0ELaZthhr7j9EdSv7RXsVYL0= Received: from smtp.kernel.org (transwarp.subspace.kernel.org [100.75.92.58]) by nyc.source.kernel.org (Postfix) with ESMTP id 52999A563D5; Tue, 22 Jul 2025 20:07:23 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 1F4B1C4CEEB; Tue, 22 Jul 2025 20:07:22 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linux-foundation.org; s=korg; t=1753214842; bh=A9U10LdyLRNTz78nkxw/9fL65JMOLX7O4NaZFljiv6s=; h=Date:From:To:Cc:Subject:In-Reply-To:References:From; b=OZ95R17ljE7FLT7YxbnUbAL7zPf2GHea6OsnFTItLRRuwg8bfXeNCg+KkVV9PSB6+ Ln6BhMG6sYRxWR0tTHq+NkHiVGCQPDy5X9NX9HwmcVAf4DVjGHt0pUK96NXlTpGtQP oP5DcJl4eR1LKWv4rd+hL02r3116GUoKCW5eRqnY= Date: Tue, 22 Jul 2025 13:07:21 -0700 From: Andrew Morton To: Francois Dugast Cc: balbirs@nvidia.com, airlied@gmail.com, apopple@nvidia.com, baohua@kernel.org, baolin.wang@linux.alibaba.com, dakr@kernel.org, david@redhat.com, donettom@linux.ibm.com, jane.chu@oracle.com, jglisse@redhat.com, kherbst@redhat.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org, lyude@redhat.com, matthew.brost@intel.com, peterx@redhat.com, ryan.roberts@arm.com, shuah@kernel.org, simona@ffwll.ch, wangkefeng.wang@huawei.com, willy@infradead.org, ziy@nvidia.com Subject: Re: [PATCH] mm/hmm: Do not fault in device private pages owned by the caller Message-Id: <20250722130721.9169d564eeeb28807e18635a@linux-foundation.org> In-Reply-To: <20250722193445.1588348-1-francois.dugast@intel.com> References: <9ae3e014-c7d0-4d58-af0e-925bcd9e4cfd@nvidia.com> <20250722193445.1588348-1-francois.dugast@intel.com> X-Mailer: Sylpheed 3.8.0beta1 (GTK+ 2.24.33; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-Rspam-User: X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: 261D21C000F X-Stat-Signature: 58dbqy5iytgs3u3ag65niigr9stwztup X-HE-Tag: 1753214844-555866 X-HE-Meta: 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 BXG3FPU0 jw4il4COEbyz4EQplwpDMU0WRJ69QXDTYYt8sm9G5rhT6Fi4deLEt+VXCM2XAmDhNIgC5C0B4zo2Z8yIt5ZUV+oNCDbEtfNXI9S95dH3vtRynTI/VDxBBZojntIgH5x/61Q8cd8+oeGd5kFSPwmR9eu7hVY1k4TMRgssTh6FuhS/UTYWFU2F99sVAgD2EXnO5atb8YiR9RisZ/VL7+/mLbEMJvSPbly//ihOTW1j71jGWjz8= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Tue, 22 Jul 2025 21:34:45 +0200 Francois Dugast wrote: > When the PMD swap entry is device private and owned by the caller, > skip the range faulting and instead just set the correct HMM PFNs. > This is similar to the logic for PTEs in hmm_vma_handle_pte(). Please always tell us why a patch does something, not only what it does. > For now, each hmm_pfns[i] entry is populated as it is currently done > in hmm_vma_handle_pmd() but this might not be necessary. A follow-up > optimization could be to make use of the order and skip populating > subsequent PFNs. I infer from this paragraph that this patch is a performance optimization? Have its effects been measured? > --- a/mm/hmm.c > +++ b/mm/hmm.c > @@ -355,6 +355,31 @@ static int hmm_vma_walk_pmd(pmd_t *pmdp, > } > > if (!pmd_present(pmd)) { > + swp_entry_t entry = pmd_to_swp_entry(pmd); > + > + /* > + * Don't fault in device private pages owned by the caller, > + * just report the PFNs. > + */ Similarly, this tells us "what" it does, which is fairly obvious from the code itself. What is not obvious from the code is the "why". > + if (is_device_private_entry(entry) && > + pfn_swap_entry_folio(entry)->pgmap->owner == > + range->dev_private_owner) { > + unsigned long cpu_flags = HMM_PFN_VALID | > + hmm_pfn_flags_order(PMD_SHIFT - PAGE_SHIFT); > + unsigned long pfn = swp_offset_pfn(entry); > + unsigned long i; > + > + if (is_writable_device_private_entry(entry)) > + cpu_flags |= HMM_PFN_WRITE; > + > + for (i = 0; addr < end; addr += PAGE_SIZE, i++, pfn++) { > + hmm_pfns[i] &= HMM_PFN_INOUT_FLAGS; > + hmm_pfns[i] |= pfn | cpu_flags; > + } > + > + return 0; > + } > + > if (hmm_range_need_fault(hmm_vma_walk, hmm_pfns, npages, 0)) > return -EFAULT; > return hmm_pfns_fill(start, end, range, HMM_PFN_ERROR);