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SubjectRe: [PATCH] mm: Make copy_from_user() in migrate.c statically predictable
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> On 02/18/2010 03:02 PM, Linus Torvalds wrote:
> >
> > Hmm. When making simplifications like this, I would really suggest you
> > also move the declaration of the variable itself into the block where it
> > is now used, rather than leaving it be function-wide.
> >
> > Yes, it's used in the final condition of the for-loop, but that whole loop
> > is just screwy. The 'err' handling is insane. Sometimes 'err' is a return
> > value form copy_to/from_user, and sometimes it's a errno. The two are
> > _not_ the same thing, they don't even have the same type!
> >
> > And 'i' is totally useless too.
> >
> > So that whole loop should be rewritten.
> >
>
> OK, I was trying to make the minimal set of changes given the late -rc
> status.
>
> > I don't even have page migration enabled, so I haven't even compile-tested
> > this, but wouldn't something like this work? It's smaller, gets rid of two
> > pointless variables, and looks simpler to me. Hmm?
>
> The code definitely looks cleaner, and it's a much more standard
> "chunked data loop" form. Weirdly enough, though, gcc 4.4.2 can't
> figure out the copy_from_user() that way... despite having the same
> min() structure as my code.
>
> However, if I change it to:
>
> chunk_nr = nr_pages;
> if (chunk_nr > DO_PAGES_STAT_CHUNK_NR)
> chunk_nr = DO_PAGES_STAT_CHUNK_NR;
>
> ... then it works!
>
> Overall, it looks like gcc is rather fragile with regards to its ability
> to constant-propagate. It's probably no coincidence that chunked loops
> is the place where we really have problems with this kind of stuff.
>
> Updated patch, which compile-tests for me, attached.

hehe, I'm ESPer. I think you hope I do runtime-test, plz wait 12 hour :-)





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