Messages in this thread |  | | From | (Linus Torvalds) | Subject | Re: Security question: "Text file busy" overwriting executables but not shared libraries? | Date | Sat, 13 Oct 2001 22:27:30 +0000 (UTC) |
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In article <Pine.LNX.4.33.0110131219520.8900-100000@penguin.transmeta.com>, Linus Torvalds <torvalds@transmeta.com> wrote: > >The explicit flag is probably a good idea also because of usage patterns >(PAGE_COPY is a slowdown _if_ the file is actually written to or even >mapped shared).
Actually, I missed the obvious case: quite often when you do a "read()", the reader itself will end up writing to the area read into. In which case doing the PAGE_COPY would also slow down measurably, due to the extra overhead of the copy-on-write fault (which not just does the copy that we tried to avoid, but will take a fault and more VM locks).
So if we want to do this optimization, we _definitely_ want it to be explicitly controlled by a flag, like O_DIRECT is. There are just too many cases where it's a pessimization, and while the user can often tell before-hand, the kernel simply cannot.
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