On Fri, 2007-10-12 at 04:14 +1000, Nick Piggin wrote: > On Friday 12 October 2007 20:37, Peter Zijlstra wrote: > > On Fri, 2007-10-12 at 02:57 +1000, Nick Piggin wrote: > > > On Friday 12 October 2007 19:03, Peter Zijlstra wrote: > > > > Subject: mm: avoid dirtying shared mappings on mlock > > > > > > > > Suleiman noticed that shared mappings get dirtied when mlocked. > > > > Avoid this by teaching make_pages_present about this case. > > > > > > > > Signed-off-by: Peter Zijlstra <[email protected]> > > > > Acked-by: Suleiman Souhlal <[email protected]> > > > > > > Umm, I don't see the other piece of this thread, so I don't > > > know what the actual problem was. > > > > > > But I would really rather not do this. If you do this, then you > > > now can get random SIGBUSes when you write into the memory if it > > > can't allocate blocks or ... (some other filesystem specific > > > condition). > > > > I'm not getting this, make_pages_present() only has to ensure all the > > pages are read from disk and in memory. How is this different from a > > read-scan? > > I guess because we've mlocked a region that has PROT_WRITE access... > but actually, I suppose mlock doesn't technically require that we > can write to the memory, only that the page isn't swapped out. > > Still, it is nice to be able to have a reasonable guarantee of > writability. > > > > The pages will still be read-only due to dirty tracking, so the first > > write will still do page_mkwrite(). > > Which can SIGBUS, no? Sure, but that is no different than any other mmap'ed write. I'm not seeing how an mlocked region is special here. I agree it would be nice if mmap'ed writes would have better error reporting than SIGBUS, but such is life.
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