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([2a01:599:908:c963:af9b:c64c:dfd8:5999]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-35dd066eb35sm9016009f8f.115.2024.06.03.09.05.58 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 03 Jun 2024 09:05:59 -0700 (PDT) Message-ID: <35866f91-7d96-462a-aa0a-ac8a6b8cbcf8@redhat.com> Date: Mon, 3 Jun 2024 18:05:37 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v11 09/12] mm: implement LUF(Lazy Unmap Flush) defering tlb flush when folios get unmapped To: Dave Hansen , Byungchul Park Cc: Byungchul Park , linux-kernel@vger.kernel.org, linux-mm@kvack.org, kernel_team@skhynix.com, akpm@linux-foundation.org, ying.huang@intel.com, vernhao@tencent.com, mgorman@techsingularity.net, hughd@google.com, willy@infradead.org, peterz@infradead.org, luto@kernel.org, tglx@linutronix.de, mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com, rjgolo@gmail.com References: <20240531092001.30428-1-byungchul@sk.com> <20240531092001.30428-10-byungchul@sk.com> <26dc4594-430b-483c-a26c-7e68bade74b0@redhat.com> <20240603093505.GA12549@system.software.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Stat-Signature: pho4wezyymorboxhci1gktochf5b66wa X-Rspamd-Queue-Id: 286CC4001C X-Rspam-User: X-Rspamd-Server: rspam10 X-HE-Tag: 1717430763-765158 X-HE-Meta: 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 Z69xyh1W 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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 03.06.24 15:23, Dave Hansen wrote: > On 6/3/24 02:35, Byungchul Park wrote: > ...> In luf's point of view, the points where the deferred flush should be >> performed are simply: >> >> 1. when changing the vma maps, that might be luf'ed. >> 2. when updating data of the pages, that might be luf'ed. > > It's simple, but the devil is in the details as always. > >> All we need to do is to indentify the points: >> >> 1. when changing the vma maps, that might be luf'ed. >> >> a) mmap and munmap e.i. fault handler or unmap_region(). >> b) permission to writable e.i. mprotect or fault handler. >> c) what I'm missing. > > I'd say it even more generally: anything that installs a PTE which is > inconsistent with the original PTE. That, of course, includes writes. > But it also includes crazy things that we do like uprobes. Take a look > at __replace_page(). > > I think the page_vma_mapped_walk() checks plus the ptl keep LUF at bay > there. But it needs some really thorough review. > > But the bigger concern is that, if there was a problem, I can't think of > a systematic way to find it. Fully agreed! > >> 2. when updating data of the pages, that might be luf'ed. >> >> a) updating files through vfs e.g. file_end_write(). >> b) updating files through writable maps e.i. 1-a) or 1-b). >> c) what I'm missing. > > Filesystems or block devices that change content without a "write" from > the local system. Network filesystems and block devices come to mind. > I honestly don't know what all the rules are around these, but they > could certainly be troublesome. > > There appear to be some interactions for NFS between file locking and > page cache flushing. > > But, stepping back ... > > I'd honestly be a lot more comfortable if there was even a debugging LUF > mode that enforced a rule that said: > > 1. A LUF'd PTE can't be rewritten until after a luf_flush() occurs I was playing with the idea of using a PTE marker. Then it's clear for munmap/mremap/page faults that there is an outstanding flush required. the alternative might be a VMA flag, but that's harder to actually enforce an invariant. > 2. A LUF'd page's position in the page cache can't be replaced until > after a luf_flush() That's the most tricky bit. I think these are the VFS concerns like 1) Page migration/reclaim ends up freeing the old page. TLB not flushed. 2) write() to the new page / write from other process to the new page 3) CPU reads stale content from old page PTE markers can't handle that. -- Cheers, David / dhildenb