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

[Minchan Kim <minchan@kernel.org>]

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@linux.intel.com>
> > > 
> > > 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@linux.intel.com>
> > > ---
> > >  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

> 
> (Of course, we could separate ramfs's mapping_unevictable case from
> the Mlocked case; but I'd prefer to continue to treat them the same.)

Fair enough.

> 
> The other reason it looks as if ramfs pages cannot be migrated, is
> that it does not set a suitable ->migratepage method, so would be
> handled by fallback_migrate_page(), whose PageDirty test will end
> up failing the migration with -EBUSY or -EINVAL - if I read it
> correctly.

True.

> 
> Perhaps other such reasons would surface once those are fixed.
> But until ramfs pages can be migrated, they should not be allocated
> with __GFP_MOVABLE.  (I've been writing about the migratability of
> small pages: I expect you have the migratability of THPages in flux.)

Agreed.

> 
> Hugh
> 
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-- 
Kind regards,
Minchan Kim

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