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[2003:cb:c703:7600:a8ea:29ce:7ee3:dd41]) by smtp.gmail.com with ESMTPSA id d18-20020a5d6452000000b002366553eca7sm1896548wrw.83.2022.11.30.08.31.53 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 30 Nov 2022 08:31:53 -0800 (PST) Message-ID: <02f5ff51-c78c-ba87-e627-560b52090cbc@redhat.com> Date: Wed, 30 Nov 2022 17:31:52 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.5.0 Subject: Re: [PATCH 03/10] mm/hugetlb: Document huge_pte_offset usage To: Peter Xu Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, James Houghton , Jann Horn , Andrew Morton , Andrea Arcangeli , Rik van Riel , Nadav Amit , Miaohe Lin , Muchun Song , Mike Kravetz References: <20221129193526.3588187-1-peterx@redhat.com> <20221129193526.3588187-4-peterx@redhat.com> From: David Hildenbrand Organization: Red Hat In-Reply-To: X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; 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t=1669825917; a=rsa-sha256; cv=none; b=N+RuvdON3RcRR9z25zthRFu9jQO3kTjQRwmZYhPSc2wx5WvzNABsDmRWK6owg9FvKCsZs4 vs+LnV4E26+jy4RxJpxrCPJSDmp6umti0P9dqIIcfTYRw7OXTa6erV+0lGcRre9bxLHUUl 8Lg98GSA9Lcm7k4oBH7zavE69LHjCSs= X-Stat-Signature: ftrkmj4s54dbdfu7tewo15k74hb6s8zc X-Rspamd-Queue-Id: B94ACA001E Authentication-Results: imf25.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b="FDhwRcC/"; spf=pass (imf25.hostedemail.com: domain of david@redhat.com designates 170.10.129.124 as permitted sender) smtp.mailfrom=david@redhat.com; dmarc=pass (policy=none) header.from=redhat.com X-Rspamd-Server: rspam06 X-Rspam-User: X-HE-Tag: 1669825917-745375 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 30.11.22 17:25, Peter Xu wrote: > On Wed, Nov 30, 2022 at 05:11:36PM +0100, David Hildenbrand wrote: >> On 30.11.22 17:09, Peter Xu wrote: >>> On Wed, Nov 30, 2022 at 11:24:34AM +0100, David Hildenbrand wrote: >>>> On 29.11.22 20:35, Peter Xu wrote: >>>>> huge_pte_offset() is potentially a pgtable walker, looking up pte_t* for a >>>>> hugetlb address. >>>>> >>>>> Normally, it's always safe to walk a generic pgtable as long as we're with >>>>> the mmap lock held for either read or write, because that guarantees the >>>>> pgtable pages will always be valid during the process. >>>>> >>>>> But it's not true for hugetlbfs, especially shared: hugetlbfs can have its >>>>> pgtable freed by pmd unsharing, it means that even with mmap lock held for >>>>> current mm, the PMD pgtable page can still go away from under us if pmd >>>>> unsharing is possible during the walk. >>>>> >>>>> So we have two ways to make it safe even for a shared mapping: >>>>> >>>>> (1) If we're with the hugetlb vma lock held for either read/write, it's >>>>> okay because pmd unshare cannot happen at all. >>>>> >>>>> (2) If we're with the i_mmap_rwsem lock held for either read/write, it's >>>>> okay because even if pmd unshare can happen, the pgtable page cannot >>>>> be freed from under us. >>>>> >>>>> Document it. >>>>> >>>>> Signed-off-by: Peter Xu >>>>> --- >>>>> include/linux/hugetlb.h | 32 ++++++++++++++++++++++++++++++++ >>>>> 1 file changed, 32 insertions(+) >>>>> >>>>> diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h >>>>> index 551834cd5299..81efd9b9baa2 100644 >>>>> --- a/include/linux/hugetlb.h >>>>> +++ b/include/linux/hugetlb.h >>>>> @@ -192,6 +192,38 @@ extern struct list_head huge_boot_pages; >>>>> pte_t *huge_pte_alloc(struct mm_struct *mm, struct vm_area_struct *vma, >>>>> unsigned long addr, unsigned long sz); >>>>> +/* >>>>> + * huge_pte_offset(): Walk the hugetlb pgtable until the last level PTE. >>>>> + * Returns the pte_t* if found, or NULL if the address is not mapped. >>>>> + * >>>>> + * Since this function will walk all the pgtable pages (including not only >>>>> + * high-level pgtable page, but also PUD entry that can be unshared >>>>> + * concurrently for VM_SHARED), the caller of this function should be >>>>> + * responsible of its thread safety. One can follow this rule: >>>>> + * >>>>> + * (1) For private mappings: pmd unsharing is not possible, so it'll >>>>> + * always be safe if we're with the mmap sem for either read or write. >>>>> + * This is normally always the case, IOW we don't need to do anything >>>>> + * special. >>>> >>>> Maybe worth mentioning that hugetlb_vma_lock_read() and friends already >>>> optimize for private mappings, to not take the VMA lock if not required. >>> >>> Yes we can. I assume this is not super urgent so I'll hold a while to see >>> whether there's anything else that needs amending for the documents. >>> >>> Btw, even with hugetlb_vma_lock_read() checking SHARED for a private only >>> code path it's still better to not take the lock at all, because that still >>> contains a function jump which will be unnecesary. >> >> IMHO it makes coding a lot more consistent and less error-prone when not >> care about whether to the the lock or not (as an optimization) and just >> having this handled "automatically". >> >> Optimizing a jump out would rather smell like a micro-optimization. > > Or we can move the lock helpers into the headers, too. Ah, yes. -- Thanks, David / dhildenb