From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from canpmsgout01.his.huawei.com (canpmsgout01.his.huawei.com [113.46.200.216]) (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 CB6C1CA6B for ; Mon, 14 Sep 2026 03:04:05 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=113.46.200.216 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789355048; cv=none; b=Xeq3N2VaaPErS/Fry7V/WoOW7Ea/W25EqqPvTXN4kZJUUjNSC1RPH/5MOB3qJlxMFyiai9Y9qKk9NJDtANRc+EjrglYpjm/YfayBAz1VQnRoZOBHjHpx3h02Kgssh1x1F2cRKCrZ6UbHsAOA123UqZ/drDq58Lz7nuezQD8M+J8= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789355048; c=relaxed/simple; bh=f1vCNE7l71bIbNZ3My4mGjSC0gaHmeZP+zxL5LaNBHg=; h=Message-ID:Date:MIME-Version:Subject:To:CC:References:From: In-Reply-To:Content-Type; b=nozTzQclSEDu3x/ot3fGtGKUq/QNCDIjP45HCEjZAY1g3dNWEDi0Y49h78lP4YgFAC0/kMiAquWCva/j1pYNGEF06+QN7f01XKBmnkEC54wXD/J4MX/AufVDTafWC6VHtuXAyvvVozUpOLJQnTXMCqWJ1FlyJeribeSpf8n8MHY= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=huawei.com; spf=pass smtp.mailfrom=huawei.com; dkim=pass (1024-bit key) header.d=huawei.com header.i=@huawei.com header.b=AWQVK5l2; arc=none smtp.client-ip=113.46.200.216 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=huawei.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=huawei.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=huawei.com header.i=@huawei.com header.b="AWQVK5l2" dkim-signature: v=1; a=rsa-sha256; d=huawei.com; s=dkim; c=relaxed/relaxed; q=dns/txt; h=From; bh=QbEkjRvDTT9Ap9hbZxOwJdBfJpMC1NbhJsmL6dREPI0=; b=AWQVK5l2K6Dpg0gktWDL2TJvnP5Ac0hQYTpF5TUw9YTxGcbFv2O2SvIIxShFtx6UWLkKTYsAa +V/nj3qjjhqxQP4g3ap+H+yDLiMZhmRWiOeUgGhcWOG8NgkRXke+5Io8BC0dB9ayefXkhG/rV1G tIkK3g5vXP6BxnAuPXq1OWk= Received: from mail.maildlp.com (unknown [172.19.163.0]) by canpmsgout01.his.huawei.com (SkyGuard) with ESMTPS id 4hjqSM4PQQz1T4JM; Mon, 14 Sep 2026 10:52:19 +0800 (CST) Received: from kwepemr500001.china.huawei.com (unknown [7.202.194.229]) by mail.maildlp.com (Postfix) with ESMTPS id D9B3040537; Mon, 14 Sep 2026 11:03:57 +0800 (CST) Received: from [10.174.178.9] (10.174.178.9) by kwepemr500001.china.huawei.com (7.202.194.229) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.2.2562.45; Mon, 14 Sep 2026 11:03:56 +0800 Message-ID: Date: Mon, 14 Sep 2026 11:03:56 +0800 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 0/4] mm/ksm: use per-VMA locking for find_mergeable_vma() To: , , , , , , , , , , , , , , , , , , CC: , References: <20260913121716693wiN5osmShywgtitghSm7G@zte.com.cn> From: Jinjiang Tu In-Reply-To: <20260913121716693wiN5osmShywgtitghSm7G@zte.com.cn> Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 8bit X-ClientProxiedBy: kwepems100002.china.huawei.com (7.221.188.206) To kwepemr500001.china.huawei.com (7.202.194.229) 在 2026/9/13 12:17, xu.xin16@zte.com.cn 写道: >>> Patch 1 removes an unused 'vma' member from struct folio_walk. It has >>> never been used since its introduction and is pure cleanup. >>> >>> Patch 2 adds a 'walk_lock' member to struct folio_walk and extends >>> folio_walk_start() to assert the required locking mode. Existing >>> callers are converted to pass PGWALK_RDLOCK, so there is no functional >>> change. This prepares folio_walk_start() for callers that hold a >>> per-VMA read lock instead of mmap_read_lock(), which is needed by the >>> Patch 4. No functional change. >>> >>> Patch 3 tranforms the boolean 'lock_vma' into the enum 'page_walk_lock' >>> without any behavior changed, which is prepared for the Patch 4 to use >>> per-VMA locking. No functional change. >>> >>> Patch 4 introduces find_mergeable_vma_locked(), which uses the >>> universal per-VMA locking helper vma_start_read_unlocked() to look up >>> and read-lock a VM_MERGEABLE VMA without taking mmap_read_lock(). All >>> KSM call sites that previously used find_mergeable_vma() under >>> mmap_read_lock() are converted to the new helper, and the locking in >>> get_mergeable_page() is switched to PGWALK_VMA_RDLOCK_VERIFY so that >>> folio_walk_start() can verify the per-VMA lock is held. >>> >>> A microbenchmark was run to measure the time KSM takes to merge a >>> victim region under mmap_lock contention. Under interference from 4 churner >>> threads, the merge time of the per-VMA KSM-optimized kernel is >>> significantly reduced by 50%. >> Hi. >> >> During task exiting, __ksm_exit() uses mmap_write_lock() to synchronize with ksmd. >> see the comment of ksm_test_exit(). >> >> void __ksm_exit(struct mm_struct *mm) >> { >> ... >> >> if (easy_to_free) { >> mm_slot_free(mm_slot_cache, mm_slot); >> mm_flags_clear(MMF_VM_MERGE_ANY, mm); >> mm_flags_clear(MMF_VM_MERGEABLE, mm); >> mmdrop(mm); >> } else if (mm_slot) { >> mmap_write_lock(mm); >> mmap_write_unlock(mm); >> } >> >> } >> >> When ksmd currently is scanning the exiting mm, we should guarantee the mm pagetable >> still valid (i.e., mm_users > 0). However, ksm_mm_slot only holds mm_count, which only >> guarantees the mm_strcut isn't freed. So, __ksm_exit() uses mmap write lock to synchronize >> with ksmd. >> >> IIUC, vma_read_lock cannot be exclusive with mmap_write_lock(). >> > Nice catch. Thanks for pointing this out. Indeed, the original exclusion between > __ksm_exit() and ksmd relied on mmap_write_lock() blocking mmap_read_lock(), > and per-VMA read locks do not provide that exclusion. > > A possible approach to restore the necessary guarantee is to pin mm_users while > ksmd is walking the page tables: > > Before scanning a given mm, try to take a reference with mmget_not_zero(mm). > If it fails, the mm is exiting, so we skip it. > > Hold that reference for the entire duration of scanning that mm (not per-VMA), > and drop it with mmput() when done. > > On the fallback path where we need to acquire mmap_read_lock(), drop the mm_users > reference before waiting, to avoid delaying an exiting mm. We should avoid holding mm_users ref too long. Otherwise, when the task being scanned by ksmd is OOM-skilled, even though the victim task has responsed SIGKILL signal and exited, the mmaps aren't released due to mm_user > 0. We should check whether the mm_user has dropped to 1 during scanning, like what ksm_test_exit() has done. > > This directly guarantees that mm_users > 0 while ksmd is accessing the page tables, > so __mmput() cannot reach exit_mmap() and free them. It is more precise than the > old mmap_write_lock() synchronization and should not introduce noticeable delay, > since the reference is only held for the scan duration.