Messages in this thread | | | Date | Sat, 22 Oct 2005 18:02:40 +0100 | From | Russell King <> | Subject | Re: [PATCH 2/9] mm: arm ready for split ptlock |
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On Sat, Oct 22, 2005 at 05:22:20PM +0100, Hugh Dickins wrote: > Signal handling's preserve and restore of iwmmxt context currently > involves reading and writing that context to and from user space, while > holding page_table_lock to secure the user page(s) against kswapd. If > we split the lock, then the structure might span two pages, secured by > different locks. That would be manageable; but it seems simpler just > to read into and write from a kernel stack buffer, copying that out and > in without locking (the structure is 160 bytes in size, and here we're > near the top of the kernel stack). Or would the overhead be noticeable?
Please contact Nicolas Pitre about that - that was my suggestion, but ISTR apparantly the overhead is too high.
> arm_syscall's cmpxchg emulation use pte_offset_map_lock, instead of > pte_offset_map and mm-wide page_table_lock; and strictly, it should now > also take mmap_sem before descending to pmd, to guard against another > thread munmapping, and the page table pulled out beneath this thread.
Now that I look at it, it's probably buggy - if the page isn't already dirty, it will modify without the COW action. Again, please contact Nicolas about this.
> Updated two comments in fault-armv.c. adjust_pte is interesting, since > its modification of a pte in one part of the mm depends on the lock held > when calling update_mmu_cache for a pte in some other part of that mm. > This can't be done with a split page_table_lock (and we've already taken > the lowest lock in the hierarchy here): so we'll have to disable split > on arm, unless CONFIG_CPU_CACHE_VIPT to ensures adjust_pte never used.
Well, adjust_pte is extremely critical to ensure correct cache behaviour (and therefore data integrity) so if split ptlock is incompatible with this, split ptlock loses.
As far as adjust_pte being called, it's only called for VIVT caches, which means the configuration has to do if VIVT, disable split ptlock.
-- Russell King Linux kernel 2.6 ARM Linux - http://www.arm.linux.org.uk/ maintainer of: 2.6 Serial core - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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