Messages in this thread | | | Date | Thu, 14 Jun 2012 10:27:55 -0400 | From | Rik van Riel <> | Subject | Re: bugs in page colouring code |
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On 06/14/2012 09:20 AM, Russell King - ARM Linux wrote: > On Wed, Jun 13, 2012 at 03:29:36PM -0400, Rik van Riel wrote: >> COLOUR_ALIGN_DOWN can use the pgoff % shm_align_mask either positively >> or negatively, depending on the address initially found by >> get_unmapped_area >> >> static inline unsigned long COLOUR_ALIGN_DOWN(unsigned long addr, >> unsigned long pgoff) >> { >> unsigned long base = addr& ~shm_align_mask; >> unsigned long off = (pgoff<< PAGE_SHIFT)& shm_align_mask; >> >> if (base + off<= addr) >> return base + off; >> >> return base - off; >> } > > Yes, that is bollocks code, introduced by this commit: > > commit 7dbaa466780a754154531b44c2086f6618cee3a8 > Author: Rob Herring<rob.herring@calxeda.com> > Date: Tue Nov 22 04:01:07 2011 +0100
It's not just ARM that has this bug. It appears to be cut'n'pasted from other architectures (MIPS? SPARC?).
>> The fix would be to return an address that is a whole shm_align_mask >> lower: (((base - shm_align_mask)& ~shm_align_mask) + off > > Yes, agreed.
I will make sure the arch-independent colouring code does that.
> This brings up the question: should a MAP_PRIVATE mapping see updates > to the backing file made via a shared mapping and/or writing the file > directly? After all, a r/w MAP_PRIVATE mapping which has been CoW'd > won't see the updates. > > So I'd argue that a file mapped MAP_SHARED must be mapped according to > the colour rules, but a MAP_PRIVATE is free not to be so.
OK, fair enough.
>> Secondly, MAP_FIXED never checks for page colouring alignment. >> I assume the cache aliasing on AMD Bulldozer is merely a performance >> issue, and we can simply ignore page colouring for MAP_FIXED? >> That will be easy to get right in an architecture-independent >> implementation. > > There's a whole bunch of issues with MAP_FIXED, specifically address > space overflow has been discussed previously, and resulted in this patch: > > [PATCH 0/6] get rid of extra check for TASK_SIZE in get_unmapped_area
Turns out, get_unmapped_area_prot (the function that calls arch_get_unmapped_area) checks for these overflows, so we should be fine.
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