From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id BCB793FBA7; Thu, 2 Apr 2026 09:05:33 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1775120733; cv=none; b=OGywknzuwgyM3nvrKPHjsOCgO5X0v5aivCJQDObt1T3kqYXQGGqbZUR9s/7Gxjg8lm0A3rQ0LJHYC7Wmf1h3bAhVwFwerU0m985wbg4UZzHOJ42nm7dirAhMevafE5sR12mHUKynm9Nw/eG6TWPkaPqWyWXk+qixC/yJAiNnE/I= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1775120733; c=relaxed/simple; bh=yfHUvvgXQutH0PejSYAx1kYrHzNDD4c4DofRBzeLTh8=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=G9uT7qFVgCdVSP51DNJOr5CX16x1OYVGM4sn9Wp3NlaaO1WeqDvtzgO2YHfUp9UNHVRRR7HMFxQVIHm0epdfemwuZ6g4versTYFP9ca6YAnMF8IRskieY9PlNtbrPtwZm4ul8GqWa85WtH6gLmTOUSnf6Y2jX0L7RRixKo80GgA= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=efM43sZ2; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="efM43sZ2" Received: by smtp.kernel.org (Postfix) with ESMTPSA id D24E7C19423; Thu, 2 Apr 2026 09:05:29 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1775120733; bh=yfHUvvgXQutH0PejSYAx1kYrHzNDD4c4DofRBzeLTh8=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=efM43sZ2A/ZfqyTCE6QyXcUhTSqKNHMGoc0DNLI6pUPvzYoAPSciVsLz8B77AduZp w+XxRaaVCxnlnepcfCZi93QgTadPznyIAojrLQDjc0NEs094/tdyiDFHLPVIluVQLN dipx3TePpmSg2cPvw4jD+xaSFkoCNdM3v17oS7thDGheJQdmQohtZGWj51KOJ2wD/6 TnyzywN1U9/A1hrwLZqWF4gymmrNJb78av1IMn1MO+0ayyq6AaX/MnvziDjvFty/b8 N0oBHp4YsZYN9qenBb1q0IlcfJUAdtCvlC5pzag7GOrwMpa029pRCbadCwq0cnIC11 25jESRIAdN9bg== Date: Thu, 2 Apr 2026 10:05:27 +0100 From: "Lorenzo Stoakes (Oracle)" To: "David Hildenbrand (Arm)" Cc: Sayali Patil , Andrew Morton , Shuah Khan , linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org, Ritesh Harjani , Zi Yan , Michal Hocko , Oscar Salvador , Lorenzo Stoakes , Dev Jain , Liam.Howlett@oracle.com, linuxppc-dev@lists.ozlabs.org, Venkat Rao Bagalkote Subject: Re: [PATCH v3 08/13] selftests/mm: ensure destination is hugetlb-backed in hugepage-mremap Message-ID: <0a1befae-1037-4bbe-b976-352ef4607d2a@lucifer.local> References: <3b6ffba238a4b59de891acf896fa25d7d3193228.1774591179.git.sayalip@linux.ibm.com> <066a146b-7dbb-408e-bbc0-542bdcdd6681@linux.ibm.com> <86889304-c450-491c-8c06-48ca9c973531@kernel.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org 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: <86889304-c450-491c-8c06-48ca9c973531@kernel.org> On Thu, Apr 02, 2026 at 09:33:29AM +0200, David Hildenbrand (Arm) wrote: > On 4/1/26 22:39, Sayali Patil wrote: > > > > > > On 01/04/26 20:10, Lorenzo Stoakes (Oracle) wrote: > >> On Wed, Apr 01, 2026 at 04:21:55PM +0200, David Hildenbrand (Arm) wrote: > >> > >> OK so digging in: > >> > >> mremap -> ... -> vrm_set_new_addr() -> get_unmapped_area() -> ... (in > >> ppc arch > >> code) -> slice_get_unmapped_area(): > >> > >> unsigned long slice_get_unmapped_area(unsigned long addr, unsigned > >> long len, > >>                       unsigned long flags, unsigned int psize, > >>                       int topdown) > >> { > >>     ... > >>     /* bunch of checks */ > >> > >>     /* If we have MAP_FIXED and failed the above steps, then error out */ > >>     if (fixed) > >>         return -EBUSY; > >> > >>     ... > >> } > >> > >> Is presumably where we hit the issue. > >> > >>> > >>> That is weird. An mremap(MREMAP_FIXED) is really just an munmap() + > >>> move. > >> > >> Yeah the weird bit I guess is that we _still_ invoke > >> get_unmapped_area() but > >> with MAP_FIXED set to indicate that we want the specific address, so it's > >> subject to the above checks. > >> > >>> > >>> Are we sure this is not some actual problem in the hugetlb > >>> implementation? > >> > >> It seems the 'slices' check sees if the _target address_ has an > >> equivalent page > >> size, presumably hugetlb-mandated, and fails if they're not > >> equivalent, so this > >> change is just accounting for that. > >> > > Yes, this change accounts for that by ensuring the destination is > > created with MAP_HUGETLB so it has the same page size as the source. > > Okay, weird, so it's the right thing to do to cover all odd arch behavior. > > >> > >>> > >>> > >>> But then the test suddenly requires more hugetlb pages, no? I don't see > >>> a good reason for the MAP_POPULATE, really. It will be discarded > >>> either way. > >> > >> Yeah I'm not sure about the MAP_POPULATE being all that important here. > >> > > As far as I understand, without MAP_POPULATE, memory accesses would > > trigger userfaults, and since the test is single-threaded and has no > > background handler for the uffd, it would deadlock. MAP_POPULATE ensures > > the test runs correctly by prefaulting all pages, but please let me know > > if I’m mistaken. > > So you are saying the test would deadlock if you are not adding > MAP_POPULATE? If so, please double check if that is actually the case. > > And if it's actually the case, please carefully document that in the > patch description, and probably as a comment above the MAP_POPULATE usage. Do keep in mind MAP_POPULATE is not _guaranteed_ to work :) For guaranteed populate you need madvise(..., MADV_POPULATE_[READ/WRITE]) or to directly fault in. > > -- > Cheers, > > David Cheers, Lorenzo