Le quintidi 25 thermidor, an CCXIII, Linus Torvalds a écrit : > We do need to support /dev/mem for X, but even that might go away some > day. If I may interfere, I would like to point another use of /dev/mem: long-running memory tests that do not alter the service much. I have written a small package which does such tests. The principle is to write pseudo-random data in a page, and later read it back and compare it to what it should be. For such a test, /dev/mem can be used two ways: - to access memory pages that have been disabled by mem=/memmap= command lines arguments; - to look for the physical address of an userland-allocated and mlock()ed page (by writing a known pattern in it). (Well, I am not even sure that the physical address of a mlock()ed page can not change.) With the increasing cheapness but lack of reliability of noname RAM, such tests become very useful for home and even professional usage. I could easyly imagine a fully automated distribution that would enable such a test tool for a month after the first installation, for example. It can be noted that those tests do not require the full features of /dev/mem, and especially not the security threatening ones. It would be sufficient to have some sort of API to: - map a particular physical page of memory in a process' address space, optionnally reallocating a previous usage of that page, and failing if it can not be done; - ask the physical address of a maped page. The second can be done with /proc/kcore, but it depends on internal structures of the kernel. I believe it could easily be done with a /proc/$PID/maps-like file. The first could be implemented as a flag to mmap (maybe MAP_PHYSICAL) which would cause the addr argument to be the physical address of the requested page. It may seem a lot of trouble for some very specific need, but I have remarked that bad memory bits are an increasing problem, and ECC memory and motherboards supporting it are not easy to find for non high-grade professional hardware. A well-integrated software workaround would be really useful I believe. Regads, -- Nicolas George
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