Messages in this thread |  | | Date | Sat, 13 Oct 2001 12:23:47 -0700 (PDT) | From | Linus Torvalds <> | Subject | Re: Security question: "Text file busy" overwriting executables but not shared libraries? |
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On Sat, 13 Oct 2001, Jamie Lokier wrote: > > In fact it was proposed here on this list years ago, and I think you > argued against it (TLB flush costs). The costs and kernel > infrastructure have changed and maybe the idea could be revisited now.
It's still not entirely unlikely that doing VM mappings is simply more expensive than just doing a memcpy. The TLB invalidate is only part of the issue - you also have the page table walk, the VM lock, and the fact that PAGE_COPY itself ends up being overhead.
Which is why the PAGE_COPY kind of read() optimization is _probably_ only worth it if the user asks for it directly (or automatically only for large reads together with single-threaded applications).
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).
Linus
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