On Tue, Apr 14, 2009 at 05:11:14PM -0700, Andrew Morton wrote:
> On Sun, 12 Apr 2009 15:16:05 +0800
> Wu Fengguang <fengguang.wu@xxxxxxxxx> wrote:
>
> > [This patch may not necessarily be merged, but at least we should
> > be aware of the problem.]
> >
> > When user space requests past-EOF data, do_generic_file_read() will
> > issue a bonus readpage call, which may be unfavorable.
> >
> > do_generic_file_read:
> > -> find_page:
> > -> find_get_page() = NULL
> > -> page_cache_sync_readahead()
> > -> find_get_page() = NULL
> > -> no_cached_page:
> > -> readpage:
> > -> nfs_readpage() = error
> > -> readpage_error:
> >
> > Reported-by: Xu Chenfeng <xcf@xxxxxxxxxxx>
> > Signed-off-by: Wu Fengguang <fengguang.wu@xxxxxxxxx>
> > ---
> > mm/filemap.c | 5 +++++
> > 1 file changed, 5 insertions(+)
> >
> > --- mm.orig/mm/filemap.c
> > +++ mm/mm/filemap.c
> > @@ -1269,6 +1269,11 @@ readpage_error:
> > goto out;
> >
> > no_cached_page:
> > + isize = i_size_read(inode);
> > + end_index = (isize - 1) >> PAGE_CACHE_SHIFT;
> > + if (unlikely(!isize || index > end_index))
> > + goto out;
> > +
> > /*
> > * Ok, it wasn't cached, so we need to create a new
> > * page..
>
> Is this a problem which needs to be solved? userspace does something
> silly and the kernel behaves a bit suboptimally?
>
> If thats the only problem here then it's not worth adding fastpath
> cycles to fix it?
>
Currently, it is the only problem I have found. It is caused by 'dd' which tries to access
the page > end_index. However, as linux-based operating systems have
many zero-size configure files, 'isize == 0' could be a more general
case. Without this fastpath, 'no_cached_page' will create many unusable
pages.
I add the following lines and boot a small system in kvm.
if (unlikely(!isize || index > end_index))
+ {
+ printk(KERN_DEBUG "over read: %s %lu/%lu\n",
+ filp->f_path.dentry->d_name.name, index, end_index);
goto out;
+ }
The 'over read' message in boot time:
Apr 6 23:57:43 kvm kernel: [ 4.334520] over read: locale 0/18446744073709551615
Apr 6 23:57:43 kvm kernel: [ 4.350895] over read: locale 0/18446744073709551615
Apr 6 23:57:43 kvm kernel: [ 4.365575] over read: locale 0/18446744073709551615
Apr 6 23:57:43 kvm kernel: [ 4.695195] over read: mtab 0/18446744073709551615
Apr 6 23:57:43 kvm kernel: [ 5.326886] over read: ifstate 0/18446744073709551615
Apr 6 23:57:43 kvm kernel: [ 5.326912] over read: ifstate 0/18446744073709551615
Apr 6 23:57:43 kvm kernel: [ 5.396599] over read: portmap_mapping 0/18446744073709551615
Apr 6 23:57:43 kvm kernel: [ 6.076429] over read: random-seed 1/0
Apr 6 23:57:43 kvm kernel: [ 6.152477] over read: utmp 0/18446744073709551615
Apr 6 23:57:43 kvm kernel: [ 6.152492] over read: utmp 0/18446744073709551615
Apr 6 23:57:43 kvm kernel: [ 6.367410] over read: etab 0/18446744073709551615
Apr 6 23:57:43 kvm kernel: [ 6.426226] over read: xtab.tmp 0/18446744073709551615
Apr 6 23:57:43 kvm kernel: [ 6.426229] over read: xtab 0/18446744073709551615
Apr 6 23:57:43 kvm kernel: [ 6.533658] over read: locale 0/18446744073709551615
Apr 6 23:57:43 kvm kernel: [ 6.550716] over read: locale 0/18446744073709551615
Apr 6 23:57:43 kvm kernel: [ 6.565761] over read: locale 0/18446744073709551615
Apr 6 23:57:47 kvm kernel: [ 10.122621] over read: environment 0/18446744073709551615
Apr 6 23:57:47 kvm kernel: [ 10.122751] over read: locale 0/18446744073709551615
Apr 6 23:58:49 kvm kernel: [ 72.388823] over read: etab.tmp 0/18446744073709551615
Apr 6 23:59:17 kvm kernel: [ 6.184260] over read: locale 0/18446744073709551615
Apr 6 23:59:17 kvm kernel: [ 6.221483] over read: locale 0/18446744073709551615
Apr 6 23:59:17 kvm kernel: [ 6.260493] over read: locale 0/18446744073709551615
Apr 6 23:59:17 kvm kernel: [ 7.185390] over read: mtab 0/18446744073709551615
Apr 6 23:59:17 kvm kernel: [ 8.960259] over read: ifstate 0/18446744073709551615
Apr 6 23:59:17 kvm kernel: [ 8.960322] over read: ifstate 0/18446744073709551615
Apr 6 23:59:17 kvm kernel: [ 9.155492] over read: portmap_mapping 0/18446744073709551615
Apr 6 23:59:17 kvm kernel: [ 10.922985] over read: random-seed 1/0
Apr 6 23:59:17 kvm kernel: [ 11.151462] over read: utmp 0/18446744073709551615
Apr 6 23:59:17 kvm kernel: [ 11.151497] over read: utmp 0/18446744073709551615
Apr 6 23:59:17 kvm kernel: [ 11.708856] over read: etab 0/18446744073709551615
Apr 6 23:59:17 kvm kernel: [ 11.810557] over read: xtab.tmp 0/18446744073709551615
Apr 6 23:59:17 kvm kernel: [ 11.810561] over read: xtab 0/18446744073709551615
Apr 6 23:59:17 kvm kernel: [ 11.954552] over read: locale 0/18446744073709551615
Apr 6 23:59:17 kvm kernel: [ 11.975360] over read: locale 0/18446744073709551615
Apr 6 23:59:17 kvm kernel: [ 11.994969] over read: locale 0/18446744073709551615
Apr 6 23:59:22 kvm kernel: [ 16.132338] over read: environment 0/18446744073709551615
Apr 6 23:59:22 kvm kernel: [ 16.132471] over read: locale 0/18446744073709551615
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