linux-kernel - Re: Should I use kmap or kmap_atomic to map user pages that will be written in a loop ?

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Sun, 26 Jul 2009 23:52:09 +0200
From:	Laurent Pinchart <laurent.pinchart@...net.be>
To:	Jonathan Corbet <corbet@....net>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Should I use kmap or kmap_atomic to map user pages that will be written in a loop ?

Hi Jonathan,

first of all thanks for your answer.

On Sunday 26 July 2009 23:26:55 Jonathan Corbet wrote:
> On Sat, 25 Jul 2009 23:41:47 +0200
>
> Laurent Pinchart <laurent.pinchart@...net.be> wrote:
> > Pages will be written to from the kernel in USB interrupt context. I can
> > then either kmap_atomic() pages before copying data and kunmap_atomic()
> > them right after, or kmap() them once at the beginning of the video
> > stream and keep them mapped until the end.
>
> Video buffers can be big, and the streaming interface requires at least
> two of them.  That's a lot of kmap'd pages.  It seems to me that
> kmap_atomic() is the way to go for something like this.

Ok thanks.

> But, then, these are user-space buffers, and you're seemingly buffering
> the data through kernel space buffers first?  It seems like using
> copy_to_user() in a workqueue (or a threaded interrupt handler) might be
> a more straightforward way to go, unless I'm missing something.

I receive data from the USB subsystem in URB buffers, which are small kernel 
buffers. As I have to strip headers from those buffers, I can't initialize the 
URBs to copy data directly to the userspace buffers, so there's at least one 
memcpy operation involved :-S

I could indeed append the URBs to a list in the callback called from interrupt 
context, and process them from a threaded interrupt handler. Would it make 
much difference ?

Regards,

Laurent Pinchart

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/