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SubjectRe: [PATCH] mm: remove global locks from mm/highmem.c
On Tue, Jan 30, 2007 at 05:11:32PM -0800, Andrew Morton wrote:
> On Wed, 31 Jan 2007 11:44:36 +1100
> David Chinner <dgc@sgi.com> wrote:
>
> > On Mon, Jan 29, 2007 at 06:15:57PM -0800, Andrew Morton wrote:
> > > We still don't know what is the source of kmap() activity which
> > > necessitated this patch btw. AFAIK the busiest source is ext2 directories,
> > > but perhaps NFS under certain conditions?
> > >
> > > <looks at xfs_iozero>
> > >
> > > ->prepare_write no longer requires that the caller kmap the page.
> >
> > Agreed, but don't we (xfs_iozero) have to map it first to zero it?
> >
> > I think what you are saying here, Andrew, is that we can
> > do something like:
> >
> > page = grab_cache_page
> > ->prepare_write(page)
> > kaddr = kmap_atomic(page, KM_USER0)
> > memset(kaddr+offset, 0, bytes)
> > flush_dcache_page(page)
> > kunmap_atomic(kaddr, KM_USER0)
> > ->commit_write(page)
> >
> > to avoid using kmap() altogether?
>
> Yup. Even better, use clear_highpage().

For even more goodness, clearmem_highpage_flush() does exactly
the right thing for partial page zeroing ;)

Thanks, Andrew, I've added a patch to my QA tree with this mod.

Cheers,

Dave.
--
Dave Chinner
Principal Engineer
SGI Australian Software Group
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