Messages in this thread | | | Date | Wed, 02 May 2007 11:42:45 +1000 | From | Nick Piggin <> | Subject | Re: 2.6.22 -mm merge plans: mm-more-rmap-checking |
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Hugh Dickins wrote: > On Mon, 30 Apr 2007, Andrew Morton wrote: > >>... >> mm-more-rmap-checking.patch >>... >> >>Misc MM things. Will merge. > > > Would Nick mind very much if I ask you to drop this one? > You did CC me ages ago, but I've only just run across it. > It's a small matter, but I'd prefer it dropped for now.
I guess I would prefer it to go under CONFIG_DEBUG_VM. Speaking of which, it would be nice to be able to turn that on unconditionally in -rc1. Although I may have put a few too many things under it, so it might slow down too much...
>>>Re-introduce rmap verification patches that Hugh removed when he removed >>>PG_map_lock. PG_map_lock actually isn't needed to synchronise access to >>>anonymous pages, because PG_locked and PTL together already do. >>> >>>These checks were important in discovering and fixing a rare rmap corruption >>>in SLES9. > > > It introduces some silly checks which were never in mainline, > nor so far as I can tell in SLES9: I'm thinking of those > + BUG_ON(address < vma->vm_start || address >= vma->vm_end);
Yes, but IIRC I put that in because there was another check in SLES9 that I actually couldn't put in, but used this one instead because it also caught the bug we saw.
> There are few callsites for these rmap functions, I don't think > they need to be checking their arguments in that way. > > It also changes the inline page_dup_rmap (a single atomic increment) > into a bugchecking out-of-line function: do we really want to slow > down fork in that way, for 2.6.22 to fix a rare corruption in SLES9?
This was actually a rare corruption that is also in 2.6.21, and as few rmap callsites as we have, it was never noticed until the SLES9 bug check was triggered.
> What I really like about the patch is Nick's observation that my > /* else checking page index and mapping is racy */ > is no longer true: a change we made to the do_swap_page sequence > some while ago has indeed cured that raciness, and I'm happy to > reintroduce the check on mapping and index in page_add_anon_rmap, > and his BUG_ON(!PageLocked(page)) there (despite BUG_ONs falling > out of fashion very recently).
Hmm, I didn't notice the do_swap_page change, rather just derived its safety by looking at the current state of the code (which I guess must have been post-do_swap_page change)...
Do you have a pointer to the patch, for my interest?
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