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[2003:cb:c708:b100:1443:78d5:341f:c97a]) by smtp.gmail.com with ESMTPSA id b21-20020a05600c4e1500b003b476cabf1csm1411944wmq.26.2022.10.20.01.59.06 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 20 Oct 2022 01:59:07 -0700 (PDT) Message-ID: <8c86678a-3bfb-3854-b1a9-ae5969e730b8@redhat.com> Date: Thu, 20 Oct 2022 10:59:05 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.3.1 Subject: Re: [PATCH v1 6/7] mm/ksm: convert break_ksm() to use walk_page_range_vma() To: linux-kernel@vger.kernel.org Cc: linux-mm@kvack.org, linux-kselftest@vger.kernel.org, Andrew Morton , Shuah Khan , Hugh Dickins , Vlastimil Babka , Peter Xu , Andrea Arcangeli , "Matthew Wilcox (Oracle)" , Jason Gunthorpe , John Hubbard , Janosch Frank References: <20220930141931.174362-1-david@redhat.com> <20220930141931.174362-7-david@redhat.com> From: David Hildenbrand Organization: Red Hat In-Reply-To: <20220930141931.174362-7-david@redhat.com> X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1666256352; a=rsa-sha256; cv=none; b=uK3CyTCp+WdAs219zLzVA0/ijN/3lw6r41f1hXqJDfMdP21jUOiLefAQEFuIFnG/krBTMf sT5UgFjREorYWz5Mx9lD7/BiXmuevO4kEpUT8ucaHyRg8JK2iF8Wrz7er6rjWgyUpIppBu rmw/ie4PQq9vYFN+kYCrNS5Fer9Z/jI= ARC-Authentication-Results: i=1; imf24.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b=NW22Exdo; spf=pass (imf24.hostedemail.com: domain of david@redhat.com designates 170.10.133.124 as permitted sender) smtp.mailfrom=david@redhat.com; dmarc=pass (policy=none) header.from=redhat.com ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1666256352; 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=VDOSs9w5teSY0v64EoGV9ODDZzj4sY8LViTNwvI0Fl4=; b=QdF4ZPj/DjqxinloIZQ4JTZlKisrrV1ypN0J9dCBnxbOMvNzTc6NTmIM5VX+BMWWCQFXMX LbApqXkoPdqK2shXPg3tg22l+qocJxK/lwI/5jBjPhZNj/Ey79SwffSSb1KPZsATqFmYwE KP3f0SFV7fjpKZzpzSjUH5ualsOTh+E= X-Stat-Signature: 7is1eta779koncqa33it5ts4qemxzf8s X-Rspamd-Queue-Id: DC7E718003C Authentication-Results: imf24.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b=NW22Exdo; spf=pass (imf24.hostedemail.com: domain of david@redhat.com designates 170.10.133.124 as permitted sender) smtp.mailfrom=david@redhat.com; dmarc=pass (policy=none) header.from=redhat.com X-Rspam-User: X-Rspamd-Server: rspam05 X-HE-Tag: 1666256351-826475 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: On 30.09.22 16:19, David Hildenbrand wrote: > FOLL_MIGRATION exists only for the purpose of break_ksm(), and > actually, there is not even the need to wait for the migration to > finish, we only want to know if we're dealing with a KSM page. > > Using follow_page() just to identify a KSM page overcomplicates GUP > code. Let's use walk_page_range_vma() instead, because we don't actually > care about the page itself, we only need to know a single property -- > no need to even grab a reference on the page. > > In my setup (AMD Ryzen 9 3900X), running the KSM selftest to test unmerge > performance on 2 GiB (taskset 0x8 ./ksm_tests -D -s 2048), this results in > a performance degradation of ~4% (old: ~5010 MiB/s, new: ~4800 MiB/s). > I don't think we particularly care for now. > > Signed-off-by: David Hildenbrand > --- > mm/ksm.c | 70 +++++++++++++++++++++++++++++++++++++++++++++++++------- > 1 file changed, 62 insertions(+), 8 deletions(-) > > diff --git a/mm/ksm.c b/mm/ksm.c > index 4d7bcf7da7c3..814c1a37c323 100644 > --- a/mm/ksm.c > +++ b/mm/ksm.c > @@ -39,6 +39,7 @@ > #include > #include > #include > +#include > > #include > #include "internal.h" > @@ -452,6 +453,60 @@ static inline bool ksm_test_exit(struct mm_struct *mm) > return atomic_read(&mm->mm_users) == 0; > } > > +int break_ksm_pud_entry(pud_t *pud, unsigned long addr, unsigned long next, > + struct mm_walk *walk) > +{ > + /* We only care about page tables to walk to a single base page. */ > + if (pud_leaf(*pud) || !pud_present(*pud)) > + return 1; > + return 0; > +} > + > +int break_ksm_pmd_entry(pmd_t *pmd, unsigned long addr, unsigned long next, > + struct mm_walk *walk) > +{ > + bool *ksm_page = walk->private; > + struct page *page = NULL; > + pte_t *pte, ptent; > + spinlock_t *ptl; > + > + /* We only care about page tables to walk to a single base page. */ > + if (pmd_leaf(*pmd) || !pmd_present(*pmd)) > + return 1; > + > + /* > + * We only lookup a single page (a) no need to iterate; and (b) > + * always return 1 to exit immediately and not iterate in the caller. > + */ > + pte = pte_offset_map_lock(walk->mm, pmd, addr, &ptl); > + ptent = *pte; > + > + if (pte_none(ptent)) > + return 1; As reported by Janosch, we fail to drop the lock here. t480s: ~/git/linux ksm_unshare $ git diff diff --git a/mm/ksm.c b/mm/ksm.c index 26aec41b127c..94f8e114c89f 100644 --- a/mm/ksm.c +++ b/mm/ksm.c @@ -435,7 +435,7 @@ int break_ksm_pmd_entry(pmd_t *pmd, unsigned long addr, unsigned long next, pte = pte_offset_map_lock(walk->mm, pmd, addr, &ptl); if (pte_none(*pte)) - return 1; + goto out_unlock; if (!pte_present(*pte)) { swp_entry_t entry = pte_to_swp_entry(*pte); @@ -451,6 +451,7 @@ int break_ksm_pmd_entry(pmd_t *pmd, unsigned long addr, unsigned long next, } if (page && PageKsm(page)) *ksm_page = true; +out_unlock: pte_unmap_unlock(pte, ptl); return 1; } -- Thanks, David / dhildenb