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 -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
