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has observable overhead when the buffer pages are not resident or not mapped. The overhead comes at least from two factors, one is the additional work needed to detect the situation, prepare and execute the rollbacks. Another is the consequence of the i/o splitting into the batches of the held pages, causing filesystems see series of the smaller i/o requests instead of the single large request. Note that expected case of the resident i/o buffer does not expose these issues. Provide a prefaulting for the userspace i/o buffers, disabled by default. I am careful of not enabling prefaulting by default for now, since it would be detrimental for the applications which speculatively pass extra-large buffers of anonymous memory to not deal with buffer sizing (if such apps exist). Found and tested by: bde, emaste Sponsored by: The FreeBSD Foundation MFC after: 1 week
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