lkml.org 
[lkml]   [2012]   [May]   [1]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
From
Date
SubjectRe: [PATCH] Describe race of direct read and fork for unaligned buffers
On Tue, May 1, 2012 at 11:11 AM, Jeff Moyer <jmoyer@redhat.com> wrote:
> KOSAKI Motohiro <kosaki.motohiro@gmail.com> writes:
>
>>> Hello,
>>>
>>> Thank you revisit this. But as far as my remember is correct, this issue is NOT
>>> unaligned access issue. It's just get_user_pages(_fast) vs fork race issue. i.e.
>>> DIRECT_IO w/ multi thread process should not use fork().
>>
>> The problem is, fork (and its COW logic) assume new access makes cow break,
>> But page table protection can't detect a DMA write. Therefore DIO may override
>> shared page data.
>
> Hm, I've only seen this with misaligned or multiple sub-page-sized reads
> in the same page.  AFAIR, aligned, page-sized I/O does not get split.
> But, I could be wrong...

If my remember is correct, the reproducer of past thread is misleading.

dma_thread.c in
http://lkml.indiana.edu/hypermail/linux/kernel/0903.1/01498.html has
align parameter. But it doesn't only change align. Because of, every
worker thread read 4K (pagesize), then
- when offset is page aligned
-> every page is accessed from only one worker
- when offset is not page aligned
-> every page is accessed from two workers

But I don't remember why two threads are important things. hmm.. I'm
looking into the code a while.
Please don't 100% trust me.
--
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/

\
 
 \ /
  Last update: 2012-05-01 18:01    [W:0.239 / U:0.228 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site