Re: [PATCH v7 04/14] mm/shmem: Support memfile_notifier

[Chao Peng]

On Tue, Jul 12, 2022 at 08:02:34PM +0200, Gupta, Pankaj wrote:
> On 7/6/2022 10:20 AM, Chao Peng wrote:
> > From: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
> > 
> > Implement shmem as a memfile_notifier backing store. Essentially it
> > interacts with the memfile_notifier feature flags for userspace
> > access/page migration/page reclaiming and implements the necessary
> > memfile_backing_store callbacks.
> > 
> > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> > Signed-off-by: Chao Peng <chao.p.peng@xxxxxxxxxxxxxxx>
> > ---
> >   include/linux/shmem_fs.h |   2 +
> >   mm/shmem.c               | 109 ++++++++++++++++++++++++++++++++++++++-
> >   2 files changed, 110 insertions(+), 1 deletion(-)
...

> > +#ifdef CONFIG_MIGRATION
> > +static int shmem_migrate_page(struct address_space *mapping,
> > +			      struct page *newpage, struct page *page,
> > +			      enum migrate_mode mode)
> > +{
> > +	struct inode *inode = mapping->host;
> > +	struct shmem_inode_info *info = SHMEM_I(inode);
> > +
> > +	if (info->memfile_node.flags & MEMFILE_F_UNMOVABLE)
> > +		return -EOPNOTSUPP;
> > +	return migrate_page(mapping, newpage, page, mode);
> 
> Wondering how well page migrate would work for private pages
> on shmem memfd based backend?

>From high level:
  - KVM unset MEMFILE_F_UNMOVABLE bit to indicate it capable of
    migrating a page.
  - Introduce new 'migrate' callback(s) to memfile_notifier_ops for KVM
    to register.
  - The callback is hooked to migrate_page() here.
  - Once page migration requested, shmem calls into the 'migrate'
    callback(s) to perform additional steps for encrypted memory (For
    TDX we will call TDH.MEM.PAGE.RELOCATE).

Chao
> 
> > +}
> > +#endif
> > +
> >   const struct address_space_operations shmem_aops = {
> >   	.writepage	= shmem_writepage,
> >   	.dirty_folio	= noop_dirty_folio,
> > @@ -3814,7 +3872,7 @@ const struct address_space_operations shmem_aops = {
> >   	.write_end	= shmem_write_end,
> >   #endif
> >   #ifdef CONFIG_MIGRATION
> > -	.migratepage	= migrate_page,
> > +	.migratepage	= shmem_migrate_page,
> >   #endif
> >   	.error_remove_page = shmem_error_remove_page,
> >   };
> > @@ -3931,6 +3989,51 @@ static struct file_system_type shmem_fs_type = {
> >   	.fs_flags	= FS_USERNS_MOUNT,
> >   };