Re: [PATCH v5 03/13] mm/shmem: Support memfile_notifier

[Chao Peng]

On Fri, Mar 11, 2022 at 10:08:22AM +1100, Dave Chinner wrote:
> On Thu, Mar 10, 2022 at 10:09:01PM +0800, Chao Peng wrote:
> > From: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
> > 
> > It maintains a memfile_notifier list in shmem_inode_info structure and
> > implements memfile_pfn_ops callbacks defined by memfile_notifier. It
> > then exposes them to memfile_notifier via
> > shmem_get_memfile_notifier_info.
> > 
> > We use SGP_NOALLOC in shmem_get_lock_pfn since the pages should be
> > allocated by userspace for private memory. If there is no pages
> > allocated at the offset then error should be returned so KVM knows that
> > the memory is not private memory.
> > 
> > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> > Signed-off-by: Chao Peng <chao.p.peng@xxxxxxxxxxxxxxx>
> > ---
> >  include/linux/shmem_fs.h |  4 +++
> >  mm/shmem.c               | 76 ++++++++++++++++++++++++++++++++++++++++
> >  2 files changed, 80 insertions(+)
> > 
> > diff --git a/include/linux/shmem_fs.h b/include/linux/shmem_fs.h
> > index 2dde843f28ef..7bb16f2d2825 100644
> > --- a/include/linux/shmem_fs.h
> > +++ b/include/linux/shmem_fs.h
> > @@ -9,6 +9,7 @@
> >  #include <linux/percpu_counter.h>
> >  #include <linux/xattr.h>
> >  #include <linux/fs_parser.h>
> > +#include <linux/memfile_notifier.h>
> >  
> >  /* inode in-kernel data */
> >  
> > @@ -28,6 +29,9 @@ struct shmem_inode_info {
> >  	struct simple_xattrs	xattrs;		/* list of xattrs */
> >  	atomic_t		stop_eviction;	/* hold when working on inode */
> >  	unsigned int		xflags;		/* shmem extended flags */
> > +#ifdef CONFIG_MEMFILE_NOTIFIER
> > +	struct memfile_notifier_list memfile_notifiers;
> > +#endif
> >  	struct inode		vfs_inode;
> >  };
> >  
> > diff --git a/mm/shmem.c b/mm/shmem.c
> > index 9b31a7056009..7b43e274c9a2 100644
> > --- a/mm/shmem.c
> > +++ b/mm/shmem.c
> > @@ -903,6 +903,28 @@ static struct folio *shmem_get_partial_folio(struct inode *inode, pgoff_t index)
> >  	return page ? page_folio(page) : NULL;
> >  }
> >  
> > +static void notify_fallocate(struct inode *inode, pgoff_t start, pgoff_t end)
> > +{
> > +#ifdef CONFIG_MEMFILE_NOTIFIER
> > +	struct shmem_inode_info *info = SHMEM_I(inode);
> > +
> > +	memfile_notifier_fallocate(&info->memfile_notifiers, start, end);
> > +#endif
> > +}
> 
> *notify_populate(), not fallocate.  This is a notification that a
> range has been populated, not that the fallocate() syscall was run
> to populate the backing store of a file.
> 
> i.e.  fallocate is the name of a userspace filesystem API that can
> be used to manipulate the backing store of a file in various ways.
> It can both populate and punch away the backing store of a file, and
> some operations that fallocate() can run will do both (e.g.
> FALLOC_FL_ZERO_RANGE) and so could generate both
> notify_invalidate() and a notify_populate() events.

Yes, I fully agreed fallocate syscall has both populating and hole
punching semantics so notify_fallocate can be misleading since we
actually mean populate here.

> 
> Hence "fallocate" as an internal mm namespace or operation does not
> belong anywhere in core MM infrastructure - it should never get used
> anywhere other than the VFS/filesystem layers that implement the
> fallocate() syscall or use it directly.

Will use your suggestion through the series where applied. Thanks for
your suggestion.

Chao
> 
> Cheers,
> 
> Dave.
> 
> -- 
> Dave Chinner
> david@xxxxxxxxxxxxx