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Shutemov" , Yang Shi , David Hildenbrand , Jiaqi Yan , "linux-kernel@vger.kernel.org" Subject: Re: [PATCH v5 3/3] mm/khugepaged: maintain page cache uptodate flag Message-ID: References: <20230307052036.1520708-1-stevensd@google.com> <20230307052036.1520708-4-stevensd@google.com> <866d1a75-d462-563-dfd7-1aa2971a285b@google.com> <6F9B0A6A-B7F1-4E25-AB15-8F3782D2EF83@fb.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <6F9B0A6A-B7F1-4E25-AB15-8F3782D2EF83@fb.com> X-Rspam-User: X-Rspamd-Queue-Id: 9E204A002B X-Rspamd-Server: rspam01 X-Stat-Signature: u9h66t166jj4r1s1fn3ro1xya4ktqizf X-HE-Tag: 1679664682-485333 X-HE-Meta: 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 yjdp3ma/ DR/AYQPYX0+ayoLFN3FELFRGAqmR8/tvi89qXKmtXsCYVI8EX4Cjy/9v6Cqps7TfdfNPK51L0R2xujXA4pC0uxVES2iLt0Or5yK1A/+cVmiFxvaZkZzni5KRz1WYsdVvAwKuGbtXOTlYBmfiD35oLJdi/dklBvUGVL+zp8iQdbTBmARAvX0Ogt/hX68SYHrfQwXH12I8HdTj12KZKUyUFfiB7UgkQ3hGZRkip+7jRKyEot3eKJstBappOQv9zvGZzyyZb 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 Fri, Mar 24, 2023 at 06:03:37AM +0000, Song Liu wrote: > > > > On Mar 23, 2023, at 8:30 PM, Matthew Wilcox wrote: > > [...] > > > > > The Uptodate flag check needs to be done by the caller; the > > find_get_page() family return !uptodate pages. > > > > But find_get_page() does not advertise itself as NMI-safe. And I > > think it's wrong to try to make it NMI-safe. Most of the kernel is > > not NMI-safe. I think it's incumbent on the BPF people to get the > > information they need ahead of taking the NMI. NMI handlers are not > > supposed to be doing a huge amount of work! I don't really understand > > why it needs to do work in NMI context; surely it can note the location of > > the fault and queue work to be done later (eg on irq-enable, task-switch > > or return-to-user) > > The use case here is a profiler (similar to perf-record). Parsing the > build id in side the NMI makes the profiler a lot simpler. Otherwise, > we will need some post processing for each sample. Simpler for you, maybe. But this is an NMI! It's not supposed to be doing printf-formatting or whatever, much less poking around in the file cache. Like perf, it should record a sample and then convert that later. Maybe it can defer to a tasklet, but i think scheduling work is a better option. > OTOH, it is totally fine if build_id_parse() fails some time, say < 5%. > The profiler output is still useful in such cases. > > I guess the next step is to replace find_get_page() with a NMI-safe > version? No, absolutely not. Stop doing so much work in an NMI.