Messages in this thread | | | Date | Tue, 30 Dec 2008 07:35:24 +0100 | From | Nick Piggin <> | Subject | Re: early fixmap causes kmap breakage |
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On Mon, Dec 29, 2008 at 10:22:12PM -0800, Eric W. Biederman wrote: > Nick Piggin <npiggin@suse.de> writes: > > > On Mon, Dec 29, 2008 at 03:17:31PM -0800, Andrew Morton wrote: > >> On Thu, 18 Dec 2008 22:15:43 +0100 > >> Nick Piggin <npiggin@suse.de> wrote: > >> > >> > Hi, > >> > > >> > I've debugged a problem where i386+pae systems with more than a few CPUs > >> > blow up at boot in the kmap_atomic code. > >> > >> ping? > > > > No further progress here, I'm waiting on input for how to fix this > > "nicely". Meantime, clearing the early fixmap pte I guess works, but > > you lose a page... is it possible to put it into .initdata or is > > there some issue with that? (I guess on a PAE kernel, 4K isn't a > > big deal). > > > > > >> > The problem is that the kmap_atomic pte pages all need to be contiguous > >> > memory because the pte is calculated via the first kmap pte page + an > >> > offset (so as not to have to walk the page tables every time). > >> > > >> > The fixmap setup code crudely allocates contiguous pte pages, which is fine, > >> > but if it finds an already populated pmd entry, then it will not switch it > >> > to a new, contiguous pte page. So the early fixmap introduces a discontig > >> > page table right in the middle of the kmap atomic fixmaps. > > Where is this?
Where is what?
> >> > Commenting out the eaarly fixmap setup in head_32.S gets everything working > >> > properly. What would be the best way to fix this? Could we put the early > >> > fixmap page table in initdata, and then have the fixmap setup proper first > >> > clear its corresponding pmd entry? > > Why would we want or need to?
So the fixmap allocator allocates contiguous pagetables. Wasn't my description clear enough?
> >> How come users/testers aren't reporting this? > > > > Because apparently nobody tests 32-bit PAE systems with more than a couple > > of CPUs anymore. This bug comes from HW vendor doing testing of SLES11. > > Hmm. > > I have taken a quick skim and I am not seeming the part of the code you are > talking about. Is the problem code in mainline? > > I'm guessing it has something to do with reserve_top_address() being called > with a bad value in the normal case. But I don't see it being called > at all in the normal case.
Yes I think so. one_page_table_init will not allocate a new pte page if one already exists, so it will not be contiguous with previous/subsequent pages with the early fixmap pte there. page_table_range_init explains why that is a problem (this code seems terribly fragile and prone to breakage but I don't want to expend theeffort on it -- still, I have a patch to panic the kernel if it breaks its guarantee of contiguous page tables, which makes breakage easily visible).
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