Messages in this thread | | | Date | Sun, 21 Feb 2010 20:45:37 +0900 (JST) | From | KOSAKI Motohiro <> | Subject | Re: [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 :-)
sorry for the delay. I had little machine trouble. Reviewed-and-Tested-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
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