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Tue, 17 Mar 2026 11:08:44 -0400 X-MC-Unique: jb3R1-qHPtq-hR_gdhRPhA-1 X-Mimecast-MFC-AGG-ID: jb3R1-qHPtq-hR_gdhRPhA_1773760123 Received: from mx-prod-int-08.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-08.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.111]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mx-prod-mc-03.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id B05501956055; Tue, 17 Mar 2026 15:08:42 +0000 (UTC) Received: from fedora (unknown [10.22.80.202]) by mx-prod-int-08.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 4B2A41800576; Tue, 17 Mar 2026 15:08:40 +0000 (UTC) Date: Tue, 17 Mar 2026 11:08:37 -0400 From: Audra Mitchell To: "David Hildenbrand (Arm)" Cc: linux-kselftest@vger.kernel.org, akpm@linux-foundation.org, lorenzo.stoakes@oracle.com, Liam.Howlett@oracle.com, vbabka@suse.cz, rppt@kernel.org, surenb@google.com, mhocko@suse.com, shuah@kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] selftests/mm: Fix soft-dirty kselftest supported check Message-ID: References: <20260218184210.206466-1-audra@redhat.com> MIME-Version: 1.0 In-Reply-To: X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.111 X-Mimecast-MFC-PROC-ID: AgeK1_8YEz45HDJKdAdxHgtQy6My2ywrdohM4IiZ-98_1773760123 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=us-ascii Content-Disposition: inline X-Stat-Signature: 1e7oo4cfzffafh4q4cgtqm4nmf9it4gz X-Rspam-User: X-Rspamd-Queue-Id: 3C5D3C0014 X-Rspamd-Server: rspam12 X-HE-Tag: 1773760131-646357 X-HE-Meta: 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 I+EuliC4 E5wr+7SkLwiJCWeyuhldMWvTwCGUFJ06lHJOIvNwlxz0RfGH9QrpXUC6+O4SG6Zod4hZ+MLJztCDH0AZUasqRhWxL5694LlYH/g/DolP//xqpADlfInRVz4Im0zJQsWDF03zl0TDjUZol/aRY+m2OFQdF+846IqbWM7hi4rx58X0X8JGlh65QeuDlHG8n44Ht8Fc49/mo++Jr4hVAUdqLtwMlZWNX2wm6uSdKrMeTiiVTRsscxittOvOS8CI2dxQBm0BKQFKVzRdiYjgPJMnI5idZxKxIisxGx3sk1IDLNlia2CcBE/SisSrlBInp10eOVfW9 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: Sorry! I missed this email so never responded! On Tue, Feb 24, 2026 at 05:15:14PM +0100, David Hildenbrand (Arm) wrote: > On 2/18/26 19:42, Audra Mitchell wrote: > > On architectures with separate user address space, such as s390 or > > those without an MMU, the call to __access_ok will return true. > > Where is this __access_ok() you mention here? Somewhere in > fs/proc/task_mmu.c? > > Where in the soft-dirty test is that triggered? > > I'm wondering whether the soft-dirty test should be adjusted, but I did > not yet understand from where this behavior is triggered. The problem arises when we are checking to see what features/categories are supported. The call chain for the soft-dirty program goes: main() ->test_simple() ->pagemap_is_softdirty() ->page_entry_is() ->pagemap_scan_supported() ->__pagemap_scan_get_categories() ->ioctl() We enter the kernel with an ioctl, expecting to have an EFAULT returned (see the comment from pagemap_scan_get_categories(): /* Provide an invalid address in order to trigger EFAULT. */ ret = __pagemap_scan_get_categories(fd, start, (struct page_region *) ~0UL); Once we enter the kernel, we will check the arguments passed which includes the call to access_ok: do_pagemap_cmd() ->do_pagemap_scan() ->pagemap_scan_get_args() ->access_ok() Here is the path within pagemap_scan_get_args where we expect to fail return the EFAULT: if (arg->vec && !access_ok((void __user *)(long)arg->vec, size_mul(arg->vec_len, sizeof(struct page_region)))) return -EFAULT; However, if CONFIG_ALTERNATE_USER_ADDRESS_SPACE is enabled or if CONFIG_MMU is NOT enabled, then we just return true: if (IS_ENABLED(CONFIG_ALTERNATE_USER_ADDRESS_SPACE) || !IS_ENABLED(CONFIG_MMU)) return true; The intent appears to be just getting the categories available to us and verifying that we have the feature available for testing. However, this corner case means the soft-dirty test will fail with the following: # -------------------- # running ./soft-dirty # -------------------- # TAP version 13 # 1..15 # Bail out! PAGEMAP_SCAN succeeded unexpectedly # # Totals: pass:0 fail:0 xfail:0 xpass:0 skip:0 error:0 # [FAIL] not ok 1 soft-dirty # exit=1 # SUMMARY: PASS=0 SKIP=0 FAIL=1 1..1 Since the intent is just to validate that the features are available to us for testing, I think we can just modify the check so that we don't fail if we return 0. Let me know what you think, or if you have more questions! > Do we have a Fixes: tag? I always hesistate to add a Fixes tag on situations like this since this is a corner case that was not considered by the original author. If we need a fixes tag, then it would be: Fixes: 600bca580579 ("selftests/mm: check that PAGEMAP_SCAN returns correct categories") Thanks a bunch! -- Audra Mitchell