Re: [PATCH 2/9] mm: arm ready for split ptlock

[Nicolas Pitre]

On Sat, 22 Oct 2005, Russell King wrote:

> 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.

Going through a kernel buffer will simply double the overhead.  Let's 
suppose it should not be a big enough issue to stop the patch from being 
merged though (and it looks cleaner that way). However I'd like for the 
WARN_ON((unsigned long)frame & 7) to remain as both the kernel and user 
buffers should be 64-bit aligned.

> > 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.

I don't see how standard COW could not happen.  The only difference with 
a true write fault as if we used put_user() is that we bypassed the data 
abort vector and the code to get the FAR value.  Or am I missing 
something?


Nicolas
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