From: jreiser@bitwagon•com (John Reiser)
To: linux-arm-kernel@lists•infradead.org
Subject: ARM cacheflush syscall with range that spans multiple vma
Date: Mon, 10 Jun 2013 13:16:57 -0700 [thread overview]
Message-ID: <51B63439.7090000@bitwagon.com> (raw)
In-Reply-To: <20130610090947.GC6580@mudshark.cambridge.arm.com>
On 06/10/2013 02:09 AM -0700, Will Deacon wrote:
> On Mon, Jun 10, 2013 at 09:59:48AM +0100, Russell King - ARM Linux wrote:
>> On Sun, Jun 09, 2013 at 05:05:24PM -0700, John Reiser wrote:
>>> Why does the ARM cacheflush syscall stop after the lowest vma
>>> which intersects the user-requested range? The range could
>>> span more than one vma having contiguous addresses, such as
>>> two files MAP_SHARED into adjacent pages; or even a region
>>> that contains holes (pages not present.)
>>
>> Because you're not supposed to use it on large ranges because it's
>> an expensive operation.
>
> I posted some patches to address this recently. Obviously it's still
> expensive, but it makes the syscall restartable so that you can't DoS the
> system.
>
> git://git.kernel.org/pub/scm/linux/kernel/git/will/linux.git cacheflush
>
> There's WIP code there for a new iovec-based syscall too.
Please merge those patches soon.
My "app" is user-mode execve() of a compressed ET_EXEC, so UPX must flush
all of the re-generated .text, which can be a megabyte or more. Thus I flush
one page per syscall, or write all of .text to a temporary file
(achieves cache flush because DMA accesses only memory, not cache),
or heuristically flush by "sweeping" 1/2 MB of consecutive words (thus
generating deliberate collisions and evictions.) Each of those sucks.
It is *EXTREMELY* discouraging that cacheflush() misbehaves so badly.
*PLEASE* return an error status when you decide not to honor the API!
--
next prev parent reply other threads:[~2013-06-10 20:16 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-06-10 0:05 ARM cacheflush syscall with range that spans multiple vma John Reiser
2013-06-10 8:59 ` Russell King - ARM Linux
2013-06-10 9:09 ` Will Deacon
2013-06-10 20:16 ` John Reiser [this message]
2013-06-10 23:42 ` Russell King - ARM Linux
2013-06-11 10:11 ` Will Deacon
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