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

[Hugh Dickins]

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).

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.

(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.)

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.

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.)

Hugh
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