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SubjectRe: [PATCH 08/11] mm: compaction: Introduce sync-light migration for use by compaction
On Fri, Jan 13, 2012 at 01:25:40PM -0800, Andrew Morton wrote:
> On Wed, 14 Dec 2011 15:41:30 +0000
> Mel Gorman <mgorman@suse.de> wrote:
>
> > This patch adds a lightweight sync migrate operation MIGRATE_SYNC_LIGHT
> > mode that avoids writing back pages to backing storage. Async
> > compaction maps to MIGRATE_ASYNC while sync compaction maps to
> > MIGRATE_SYNC_LIGHT. For other migrate_pages users such as memory
> > hotplug, MIGRATE_SYNC is used.
> >
> > This avoids sync compaction stalling for an excessive length of time,
> > particularly when copying files to a USB stick where there might be
> > a large number of dirty pages backed by a filesystem that does not
> > support ->writepages.
> >
> > ...
> >
> > --- a/include/linux/fs.h
> > +++ b/include/linux/fs.h
> > @@ -525,6 +525,7 @@ enum positive_aop_returns {
> > struct page;
> > struct address_space;
> > struct writeback_control;
> > +enum migrate_mode;
> >
> > struct iov_iter {
> > const struct iovec *iov;
> > @@ -614,7 +615,7 @@ struct address_space_operations {
> > * is false, it must not block.
> > */
> > int (*migratepage) (struct address_space *,
> > - struct page *, struct page *, bool);
> > + struct page *, struct page *, enum migrate_mode);
>
> I'm getting a huge warning spew from this with my sparc64 gcc-3.4.5.
> I'm not sure why, really.
>

Tetsuo Handa complained about the same thing using gcc 3.3 (added
to cc).

> Forward-declaring an enum in this fashion is problematic because some
> compilers (I'm unsure about gcc) use different sizeofs for enums,
> depending on the enum's value range. For example, an enum which only
> has values 0...255 can fit into a byte. (iirc, the compiler actually
> put it in a 16-bit storage).
>

Ok, I was not aware of this. Thanks for the heads-up.

> So I propose:
>
> From: Andrew Morton <akpm@linux-foundation.org>
> Subject: mm: fix warnings regarding enum migrate_mode
>
> sparc64 allmodconfig:
>
> In file included from include/linux/compat.h:15,
> from /usr/src/25/arch/sparc/include/asm/siginfo.h:19,
> from include/linux/signal.h:5,
> from include/linux/sched.h:73,
> from arch/sparc/kernel/asm-offsets.c:13:
> include/linux/fs.h:618: warning: parameter has incomplete type
>
> It seems that my sparc64 compiler (gcc-3.4.5) doesn't like the forward
> declaration of enums.
>
> Fix this by moving the "enum migrate_mode" definition into its own header
> file.
>
> Cc: Mel Gorman <mgorman@suse.de>
> Cc: Rik van Riel <riel@redhat.com>
> Cc: Andrea Arcangeli <aarcange@redhat.com>
> Cc: Minchan Kim <minchan.kim@gmail.com>
> Cc: Dave Jones <davej@redhat.com>
> Cc: Jan Kara <jack@suse.cz>
> Cc: Andy Isaacson <adi@hexapodia.org>
> Cc: Nai Xia <nai.xia@gmail.com>
> Cc: Johannes Weiner <jweiner@redhat.com>
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Acked-by: Mel Gorman <mgorman@suse.de>

--
Mel Gorman
SUSE Labs


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