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 S0/66Yzp vClZyP2NVNz+Ga3zNWambHllDhbgI44a0spjKDdB2+c7nSAz1e54OYx/B/BC0AlIfbC9ib5tNWk2fBOnp6PNvTtdLTM6Wu3osmCB4TW22/LOc1q7jR+u517aK7NKHeJpE9b13GLIPiJbtl1uf0ykMvsGlVPpipXvgCox3LkAR0Wrs4WKV36oeGnXH0Z5vraeezhu+XJT6URqQHREuarEs6iUVlGtPPokQDACZhGF5ZpirK02F+tXNFjAtf6Hsto8Llj5LSxrHVWMMvaheoSLWpfn4OAZMTS/7vqqdSoSMP1b48mX7vc7Nnub3GdybqbiL/8lp9YHBxrrFrQXNWYuEGirQDu+djXZ3zbyGJMKjDDoA+OtRTKJ0K4Dk1sYu/WwRds2bjjwr9V9BjUpefboMWQjiDIm+TJS6FIMnOstxWfIY3AY66T4usqN0Vzfg5cRI4TSRsmoot0/K0XE= 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 10/20/25 8:58 AM, Jan Kara wrote: > On Mon 20-10-25 15:59:07, Matthew Wilcox wrote: >> On Mon, Oct 20, 2025 at 03:59:33PM +0200, Jan Kara wrote: >>> The idea was to bounce buffer the page we are writing back in case we spot >>> a long-term pin we cannot just wait for - hence bouncing should be rare. >>> But in this more general setting it is challenging to not bounce buffer for >>> every IO (in which case you'd be basically at performance of RWF_DONTCACHE >>> IO or perhaps worse so why bother?). Essentially if you hand out the real >>> page underlying the buffer for the IO, all other attemps to do IO to that >>> page have to block - bouncing is no longer an option because even with >>> bouncing the second IO we could still corrupt data of the first IO once we >>> copy to the final buffer. And if we'd block waiting for the first IO to >>> complete, userspace could construct deadlock cycles - like racing IO to >>> pages A, B with IO to pages B, A. So far I'm not sure about a sane way out >>> of this... >> >> There isn't one. We might have DMA-mapped this page earlier, and so a >> device could write to it at any time. Even if we remove PTE write >> permissions ... > > True but writes through DMA to the page are guarded by holding a page pin > these days so we could in theory block getting another page pin or mapping Do you mean, "setting up to do DMA is guarded by holding a FOLL_LONGTERM page pin"? Or something else (that's new to me)? thanks, John Hubbard > the page writeably until the pin is released... if we can figure out a > convincing story for dealing with long-term pins from RDMA and dealing with > possible deadlocks created by this. > > Honza