Subject: [PATCH] [ARM] Do not call flush_cache_user_range with mmap_semheld

From: [email-masked] (Jamie Lokier)
Date: Thu, 6 May 2010 17:01:53 +0100

Catalin Marinas wrote:
> The flush_cache_user_range operation cannot actually damage the data. If
> the application is so badly written that one of its threads remaps a
> page range while another thread writes to it and flushes the caches,
> then it deserves the memory corruption.

It may deserve corruption, but doing corruption silently is cruel.

Moreover, calling mprotect(PROT_READ) in one thread while another
thread is writing to the same regions is a valid, and used, garbage
collector dirty-tracking technique.  (Page faults provide the info,
and the fault handler uses PROT_WRITE to let the faulting thread
continue on each tracked page.)

Is it possible to percolate EFAULT to the right places when the cache
flush faults?

> Personally, I would go even further and remove the find_vma() call (of
> course with an access_ok() call to make sure the address isn't a kernel
> one). I actually did some tests but the performance improvement was too
> significant to be worth arguing the case on the list. But the app I was
> using was a simple test where the vma tree was small. Things may be
> different for a fully featured Java VM for example.

Seems a reasonable thing to do, and a fully-featured use-lots-of-VMAs
app sounds like the sort of app which wants to flush part of caches
quickly.

-- Jamie