Re: [PATCH 1/2] Allow GUP to fail instead of waiting on a page.

[Gleb Natapov <gleb@redhat.com>]

On Tue, Feb 01, 2011 at 04:42:40PM -0800, Andrew Morton wrote:
> On Tue,  1 Feb 2011 13:21:46 +0200
> Gleb Natapov <gleb@redhat.com> wrote:
> 
> > GUP user may want to try to acquire a reference to a page if it is already
> > in memory, but not if IO, to bring it in, is needed. For example KVM may
> > tell vcpu to schedule another guest process if current one is trying to
> > access swapped out page. Meanwhile, the page will be swapped in and the
> > guest process, that depends on it, will be able to run again.
> > 
> > This patch adds FAULT_FLAG_RETRY_NOWAIT (suggested by Linus) and
> > FOLL_NOWAIT follow_page flags. FAULT_FLAG_RETRY_NOWAIT, when used in
> > conjunction with VM_FAULT_ALLOW_RETRY, indicates to handle_mm_fault that
> > it shouldn't drop mmap_sem and wait on a page, but return VM_FAULT_RETRY
> > instead.
> >
> > ...
> >
> > +#define FOLL_NOWAIT	0x20	/* return if disk transfer is needed */
> 
> The comment is a little misleading.  Or incomplete.
> 
> For both swap-backed and file-backed pages, the code will initiate the
> disk transfer and will then return without waiting for it to complete. 
> This (important!) information isn't really presented in either the
> changelog or the code itself.
> 
> This?
> 
Yes, this is better. Thanks you. I see that the patch below is in your queue
already. Should I re-spin my patch with improved comment anyway?

> --- a/include/linux/mm.h~mm-allow-gup-to-fail-instead-of-waiting-on-a-page-fix
> +++ a/include/linux/mm.h
> @@ -1537,7 +1537,8 @@ struct page *follow_page(struct vm_area_
>  #define FOLL_GET	0x04	/* do get_page on page */
>  #define FOLL_DUMP	0x08	/* give error on hole if it would be zero */
>  #define FOLL_FORCE	0x10	/* get_user_pages read/write w/o permission */
> -#define FOLL_NOWAIT	0x20	/* return if disk transfer is needed */
> +#define FOLL_NOWAIT	0x20	/* if a disk transfer is needed, start the IO
> +				 * and return without waiting upon it */
>  #define FOLL_MLOCK	0x40	/* mark page as mlocked */
>  #define FOLL_SPLIT	0x80	/* don't return transhuge pages, split them */
>  
> _

--
			Gleb.