Re: [PATCHv2, RFC 20/30] ramfs: enable transparent huge page cache

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On Fri, Apr 05, 2013 at 02:47:25PM +0800, Simon Jeons wrote:
> Hi Minchan,
> On 04/03/2013 09:11 AM, Minchan Kim wrote:
> >On Tue, Apr 02, 2013 at 03:15:23PM -0700, Hugh Dickins wrote:
> >>On Tue, 2 Apr 2013, Kirill A. Shutemov wrote:
> >>>Kirill A. Shutemov wrote:
> >>>>From: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
> >>>>
> >>>>ramfs is the most simple fs from page cache point of view. Let's start
> >>>>transparent huge page cache enabling here.
> >>>>
> >>>>For now we allocate only non-movable huge page. It's not yet clear if
> >>>>movable page is safe here and what need to be done to make it safe.
> >>>>
> >>>>Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> >>>>---
> >>>>  fs/ramfs/inode.c |    6 +++++-
> >>>>  1 file changed, 5 insertions(+), 1 deletion(-)
> >>>>
> >>>>diff --git a/fs/ramfs/inode.c b/fs/ramfs/inode.c
> >>>>index c24f1e1..da30b4f 100644
> >>>>--- a/fs/ramfs/inode.c
> >>>>+++ b/fs/ramfs/inode.c
> >>>>@@ -61,7 +61,11 @@ struct inode *ramfs_get_inode(struct super_block *sb,
> >>>>  		inode_init_owner(inode, dir, mode);
> >>>>  		inode->i_mapping->a_ops = &ramfs_aops;
> >>>>  		inode->i_mapping->backing_dev_info = &ramfs_backing_dev_info;
> >>>>-		mapping_set_gfp_mask(inode->i_mapping, GFP_HIGHUSER);
> >>>>+		/*
> >>>>+		 * TODO: what should be done to make movable safe?
> >>>>+		 */
> >>>>+		mapping_set_gfp_mask(inode->i_mapping,
> >>>>+				GFP_TRANSHUGE & ~__GFP_MOVABLE);
> >>>Hugh, I've found old thread with the reason why we have GFP_HIGHUSER here, not
> >>>GFP_HIGHUSER_MOVABLE:
> >>>
> >>>http://lkml.org/lkml/2006/11/27/156
> >>>
> >>>It seems the origin reason is not longer valid, correct?
> >>Incorrect, I believe: so far as I know, the original reason remains
> >>valid - though it would only require a couple of good small changes
> >>to reverse that - or perhaps you have already made these changes?
> >>
> >>The original reason is that ramfs pages are not migratable,
> >>therefore they should be allocated from an unmovable area.
> >>
> >>As I understand it (and I would have preferred to run a test to check
> >>my understanding before replying, but don't have time for that), ramfs
> >>pages cannot be migrated for two reasons, neither of them a good reason.
> >>
> >>One reason (okay, it wouldn't have been quite this way in 2006) is that
> >>ramfs (rightly) calls mapping_set_unevictable(), so its pages will fail
> >>the page_evictable() test, so they will be marked PageUnevictable, so
> >>__isolate_lru_page() will refuse to isolate them for migration (except
> >>for CMA).
> >True.
> >
> >>I am strongly in favour of removing that limitation from
> >>__isolate_lru_page() (and the thread you pointed - thank you - shows Mel
> >>and Christoph were both in favour too); and note that there is no such
> >>restriction in the confusingly similar but different isolate_lru_page().
> >>
> >>Some people do worry that migrating Mlocked pages would introduce the
> >>occasional possibility of a minor fault (with migration_entry_wait())
> >>on an Mlocked region which never faulted before.  I tend to dismiss
> >>that worry, but maybe I'm wrong to do so: maybe there should be a
> >>tunable for realtimey people to set, to prohibit page migration from
> >>mlocked areas; but the default should be to allow it.
> >I agree.
> >Just FYI for mlocked page migration
> >
> >I tried migratioin of mlocked page and Johannes and Mel had a concern
> >about that.
> >http://lkml.indiana.edu/hypermail/linux/kernel/1109.0/00175.html
> >
> >But later, Peter already acked it and I guess by reading the thread that
> >Hugh was in favour when page migration was merged first time.
> >
> >http://marc.info/?l=linux-mm&m=133697873414205&w=2
> >http://marc.info/?l=linux-mm&m=133700341823358&w=2
> >
> >Many people said mlock means memory-resident, NOT pinning so it could
> >allow minor fault while Mel still had a concern except CMA.
> >http://marc.info/?l=linux-mm&m=133674219714419&w=2
> 
> How about add a knob?

Maybe, volunteering?

-- 
Kind regards,
Minchan Kim

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