Re: [PATCHv2, RFC 20/30] ramfs: enable transparent huge page cache
From: Simon Jeons
Date: Fri Apr 05 2013 - 02:47:41 EST
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?
(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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