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[2003:cb:c72a:bc00:9a2d:8a48:ef51:96fb]) by smtp.gmail.com with ESMTPSA id x6-20020adff0c6000000b0033cfc035940sm14071979wro.34.2024.02.20.09.31.06 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 20 Feb 2024 09:31:07 -0800 (PST) Message-ID: Date: Tue, 20 Feb 2024 18:31:05 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v7 10/10] fork: Use __mt_dup() to duplicate maple tree in dup_mmap() From: David Hildenbrand To: Peng Zhang , Liam.Howlett@oracle.com, corbet@lwn.net, akpm@linux-foundation.org, willy@infradead.org, brauner@kernel.org, surenb@google.com, michael.christie@oracle.com, mjguzik@gmail.com, mathieu.desnoyers@efficios.com, npiggin@gmail.com, peterz@infradead.org, oliver.sang@intel.com, mst@redhat.com Cc: maple-tree@lists.infradead.org, linux-mm@kvack.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org References: <20231027033845.90608-1-zhangpeng.00@bytedance.com> <20231027033845.90608-11-zhangpeng.00@bytedance.com> <6058742c-26e5-4600-85ad-0a21d8fd2e42@redhat.com> Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: A8F992001D X-Rspam-User: X-Stat-Signature: p31pyhyzcz9i8yjeuiqxdkum8m9hzx36 X-Rspamd-Server: rspam01 X-HE-Tag: 1708450272-115200 X-HE-Meta: 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 zhplJp57 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 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: List-Subscribe: List-Unsubscribe: On 20.02.24 18:24, David Hildenbrand wrote: > On 27.10.23 05:38, Peng Zhang wrote: >> In dup_mmap(), using __mt_dup() to duplicate the old maple tree and then >> directly replacing the entries of VMAs in the new maple tree can result >> in better performance. __mt_dup() uses DFS pre-order to duplicate the >> maple tree, so it is efficient. >> >> The average time complexity of __mt_dup() is O(n), where n is the number >> of VMAs. The proof of the time complexity is provided in the commit log >> that introduces __mt_dup(). After duplicating the maple tree, each element >> is traversed and replaced (ignoring the cases of deletion, which are rare). >> Since it is only a replacement operation for each element, this process is >> also O(n). >> >> Analyzing the exact time complexity of the previous algorithm is >> challenging because each insertion can involve appending to a node, pushing >> data to adjacent nodes, or even splitting nodes. The frequency of each >> action is difficult to calculate. The worst-case scenario for a single >> insertion is when the tree undergoes splitting at every level. If we >> consider each insertion as the worst-case scenario, we can determine that >> the upper bound of the time complexity is O(n*log(n)), although this is a >> loose upper bound. However, based on the test data, it appears that the >> actual time complexity is likely to be O(n). >> >> As the entire maple tree is duplicated using __mt_dup(), if dup_mmap() >> fails, there will be a portion of VMAs that have not been duplicated in >> the maple tree. To handle this, we mark the failure point with >> XA_ZERO_ENTRY. In exit_mmap(), if this marker is encountered, stop >> releasing VMAs that have not been duplicated after this point. >> >> There is a "spawn" in byte-unixbench[1], which can be used to test the >> performance of fork(). I modified it slightly to make it work with >> different number of VMAs. >> >> Below are the test results. The first row shows the number of VMAs. >> The second and third rows show the number of fork() calls per ten seconds, >> corresponding to next-20231006 and the this patchset, respectively. The >> test results were obtained with CPU binding to avoid scheduler load >> balancing that could cause unstable results. There are still some >> fluctuations in the test results, but at least they are better than the >> original performance. >> >> 21 121 221 421 821 1621 3221 6421 12821 25621 51221 >> 112100 76261 54227 34035 20195 11112 6017 3161 1606 802 393 >> 114558 83067 65008 45824 28751 16072 8922 4747 2436 1233 599 >> 2.19% 8.92% 19.88% 34.64% 42.37% 44.64% 48.28% 50.17% 51.68% 53.74% 52.42% >> >> [1] https://github.com/kdlucas/byte-unixbench/tree/master >> >> Signed-off-by: Peng Zhang >> Suggested-by: Liam R. Howlett >> Reviewed-by: Liam R. Howlett >> --- >> include/linux/mm.h | 11 +++++++++++ >> kernel/fork.c | 40 +++++++++++++++++++++++++++++----------- >> mm/internal.h | 11 ----------- >> mm/memory.c | 7 ++++++- >> mm/mmap.c | 9 ++++++--- >> 5 files changed, 52 insertions(+), 26 deletions(-) >> >> diff --git a/include/linux/mm.h b/include/linux/mm.h >> index 14d5aaff96d0..e9111ec5808c 100644 >> --- a/include/linux/mm.h >> +++ b/include/linux/mm.h >> @@ -996,6 +996,17 @@ static inline int vma_iter_bulk_alloc(struct vma_iterator *vmi, >> return mas_expected_entries(&vmi->mas, count); >> } >> >> +static inline int vma_iter_clear_gfp(struct vma_iterator *vmi, >> + unsigned long start, unsigned long end, gfp_t gfp) >> +{ >> + __mas_set_range(&vmi->mas, start, end - 1); >> + mas_store_gfp(&vmi->mas, NULL, gfp); >> + if (unlikely(mas_is_err(&vmi->mas))) >> + return -ENOMEM; >> + >> + return 0; >> +} >> + >> /* Free any unused preallocations */ >> static inline void vma_iter_free(struct vma_iterator *vmi) >> { >> diff --git a/kernel/fork.c b/kernel/fork.c >> index 1e6c656e0857..1552ee66517b 100644 >> --- a/kernel/fork.c >> +++ b/kernel/fork.c >> @@ -650,7 +650,6 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm, >> int retval; >> unsigned long charge = 0; >> LIST_HEAD(uf); >> - VMA_ITERATOR(old_vmi, oldmm, 0); >> VMA_ITERATOR(vmi, mm, 0); >> >> uprobe_start_dup_mmap(); >> @@ -678,16 +677,22 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm, >> goto out; >> khugepaged_fork(mm, oldmm); >> >> - retval = vma_iter_bulk_alloc(&vmi, oldmm->map_count); >> - if (retval) >> + /* Use __mt_dup() to efficiently build an identical maple tree. */ >> + retval = __mt_dup(&oldmm->mm_mt, &mm->mm_mt, GFP_KERNEL); >> + if (unlikely(retval)) >> goto out; >> >> mt_clear_in_rcu(vmi.mas.tree); >> - for_each_vma(old_vmi, mpnt) { >> + for_each_vma(vmi, mpnt) { >> struct file *file; >> >> vma_start_write(mpnt); > > We used to call vma_start_write() on the *old* VMA, to prevent any kind of page faults in > the old MM while we are duplicating PTEs (and COW-share pages). > > See > > commit fb49c455323ff8319a123dd312be9082c49a23a5 > Author: Suren Baghdasaryan > Date: Sat Jul 8 12:12:12 2023 -0700 > > fork: lock VMAs of the parent process when forking > > When forking a child process, the parent write-protects anonymous pages > and COW-shares them with the child being forked using copy_present_pte(). > > We must not take any concurrent page faults on the source vma's as they > are being processed, as we expect both the vma and the pte's behind it > to be stable. For example, the anon_vma_fork() expects the parents > vma->anon_vma to not change during the vma copy. > > > Unless I am missing something, we now call vma_start_write() on the *new* VMA? > > If that is the case, this is broken and needs fixing; likely, going over all > VMAs in the old_mm and calling vma_start_write(). > > But maybe there is some magic going on that I am missing :) ... likely the magic is that the new tree links the same VMAs (we are not duplicating the VMAs before vm_area_dup()), so we are indeed locking the MM in the old_mm (that is temporarily linked into the new MM). If that's the case, all good :) -- Cheers, David / dhildenb