[RFC PATCH] mm, netfs: Provide a means of invalidation without using launder_folio

[RFC PATCH] mm, netfs: Provide a means of invalidation without using launder_folio

[David Howells]

Implement a replacement for launder_folio[1].  The key feature of
invalidate_inode_pages2() is that it locks each folio individually, unmaps
it to prevent mmap'd accesses interfering and calls the ->launder_folio()
address_space op to flush it.  This has problems: firstly, each folio is
written individually as one or more small writes; secondly, adjacent folios
cannot be added so easily into the laundry; thirdly, it's yet another op to
implement.

Here's a bit of a hacked together solution which should probably be moved
to mm/:

Use the mmap lock to cause future faulting to wait, then unmap all the
folios if we have mmaps, then, conditionally, use ->writepages() to flush
any dirty data back and then discard all pages.  The caller needs to hold a
lock to prevent ->write_iter() getting underfoot.

Note that this does not prevent ->read_iter() from accessing the file
whilst we do this since that may operate without locking.

We also have the writeback_control available and so have the opportunity to
set a flag in it to tell the filesystem that we're doing an invalidation.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Matthew Wilcox <willy@infradead.org>
cc: Miklos Szeredi <miklos@szeredi.hu>
cc: Trond Myklebust <trond.myklebust@hammerspace.com>
cc: Christoph Hellwig <hch@lst.de>
cc: Andrew Morton <akpm@linux-foundation.org>
cc: Alexander Viro <viro@zeniv.linux.org.uk>
cc: Christian Brauner <brauner@kernel.org>
cc: Jeff Layton <jlayton@kernel.org>
cc: linux-mm@kvack.org
cc: linux-fsdevel@vger.kernel.org
cc: netfs@lists.linux.dev
cc: v9fs@lists.linux.dev
cc: linux-afs@lists.infradead.org
cc: ceph-devel@vger.kernel.org
cc: linux-cifs@vger.kernel.org
cc: linux-nfs@vger.kernel.org
cc: devel@lists.orangefs.org
Link: https://lore.kernel.org/r/1668172.1709764777@warthog.procyon.org.uk/ [1]
---
 fs/netfs/misc.c       |   56 ++++++++++++++++++++++++++++++++++++++++++++++++++
 include/linux/netfs.h |    3 ++
 mm/memory.c           |    3 +-
 3 files changed, 61 insertions(+), 1 deletion(-)

diff --git a/fs/netfs/misc.c b/fs/netfs/misc.c
index bc1fc54fb724..774ce825fbec 100644
--- a/fs/netfs/misc.c
+++ b/fs/netfs/misc.c
@@ -250,3 +250,59 @@ bool netfs_release_folio(struct folio *folio, gfp_t gfp)
 	return true;
 }
 EXPORT_SYMBOL(netfs_release_folio);
+
+extern void unmap_mapping_range_tree(struct rb_root_cached *root,
+				     pgoff_t first_index,
+				     pgoff_t last_index,
+				     struct zap_details *details);
+
+/**
+ * netfs_invalidate_inode - Invalidate/forcibly write back an inode's pagecache
+ * @inode: The inode to flush
+ * @flush: Set to write back rather than simply invalidate.
+ *
+ * Invalidate all the folios on an inode, possibly writing them back first.
+ * Whilst the operation is undertaken, the mmap lock is held to prevent
+ * ->fault() from reinstalling the folios.  The caller must hold a lock on the
+ * inode sufficient to prevent ->write_iter() from dirtying more folios.
+ */
+int netfs_invalidate_inode(struct inode *inode, bool flush)
+{
+	struct address_space *mapping = inode->i_mapping;
+
+	if (!mapping || !mapping->nrpages)
+		goto out;
+
+	/* Prevent folios from being faulted in. */
+	i_mmap_lock_write(mapping);
+
+	if (!mapping->nrpages)
+		goto unlock;
+
+	/* Assume there are probably PTEs only if there are mmaps. */
+	if (unlikely(!RB_EMPTY_ROOT(&mapping->i_mmap.rb_root))) {
+		struct zap_details details = { };
+
+		unmap_mapping_range_tree(&mapping->i_mmap, 0, LLONG_MAX, &details);
+	}
+
+	/* Write back the data if we're asked to. */
+	if (flush) {
+		struct writeback_control wbc = {
+			.sync_mode	= WB_SYNC_ALL,
+			.nr_to_write	= LONG_MAX,
+			.range_start	= 0,
+			.range_end	= LLONG_MAX,
+		};
+
+		filemap_fdatawrite_wbc(mapping, &wbc);
+	}
+
+	/* Wait for writeback to complete on all folios and discard. */
+	truncate_inode_pages_range(mapping, 0, LLONG_MAX);
+
+unlock:
+	i_mmap_unlock_write(mapping);
+out:
+	return filemap_check_errors(mapping);
+}
diff --git a/include/linux/netfs.h b/include/linux/netfs.h
index 298552f5122c..40dc34ee291d 100644
--- a/include/linux/netfs.h
+++ b/include/linux/netfs.h
@@ -400,6 +400,9 @@ ssize_t netfs_buffered_write_iter_locked(struct kiocb *iocb, struct iov_iter *fr
 ssize_t netfs_unbuffered_write_iter(struct kiocb *iocb, struct iov_iter *from);
 ssize_t netfs_file_write_iter(struct kiocb *iocb, struct iov_iter *from);
 
+/* High-level invalidation API */
+int netfs_invalidate_inode(struct inode *inode, bool flush);
+
 /* Address operations API */
 struct readahead_control;
 void netfs_readahead(struct readahead_control *);
diff --git a/mm/memory.c b/mm/memory.c
index f2bc6dd15eb8..106f32c7d7fb 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3665,7 +3665,7 @@ static void unmap_mapping_range_vma(struct vm_area_struct *vma,
 	zap_page_range_single(vma, start_addr, end_addr - start_addr, details);
 }
 
-static inline void unmap_mapping_range_tree(struct rb_root_cached *root,
+inline void unmap_mapping_range_tree(struct rb_root_cached *root,
 					    pgoff_t first_index,
 					    pgoff_t last_index,
 					    struct zap_details *details)
@@ -3685,6 +3685,7 @@ static inline void unmap_mapping_range_tree(struct rb_root_cached *root,
 				details);
 	}
 }
+EXPORT_SYMBOL_GPL(unmap_mapping_range_tree);
 
 /**
  * unmap_mapping_folio() - Unmap single folio from processes.

Re: [RFC PATCH] mm, netfs: Provide a means of invalidation without using launder_folio

[Trond Myklebust]

On Wed, 2024-03-27 at 15:04 +0000, David Howells wrote:
> Implement a replacement for launder_folio[1].  The key feature of
> invalidate_inode_pages2() is that it locks each folio individually,
> unmaps
> it to prevent mmap'd accesses interfering and calls the -
> >launder_folio()
> address_space op to flush it.  This has problems: firstly, each folio
> is
> written individually as one or more small writes; secondly, adjacent
> folios
> cannot be added so easily into the laundry; thirdly, it's yet another
> op to
> implement.
> 
> Here's a bit of a hacked together solution which should probably be
> moved
> to mm/:
> 
> Use the mmap lock to cause future faulting to wait, then unmap all
> the
> folios if we have mmaps, then, conditionally, use ->writepages() to
> flush
> any dirty data back and then discard all pages.  The caller needs to
> hold a
> lock to prevent ->write_iter() getting underfoot.
> 
> Note that this does not prevent ->read_iter() from accessing the file
> whilst we do this since that may operate without locking.
> 
> We also have the writeback_control available and so have the
> opportunity to
> set a flag in it to tell the filesystem that we're doing an
> invalidation.
> 
> Signed-off-by: David Howells <dhowells@redhat.com>
> cc: Matthew Wilcox <willy@infradead.org>
> cc: Miklos Szeredi <miklos@szeredi.hu>
> cc: Trond Myklebust <trond.myklebust@hammerspace.com>
> cc: Christoph Hellwig <hch@lst.de>
> cc: Andrew Morton <akpm@linux-foundation.org>
> cc: Alexander Viro <viro@zeniv.linux.org.uk>
> cc: Christian Brauner <brauner@kernel.org>
> cc: Jeff Layton <jlayton@kernel.org>
> cc: linux-mm@kvack.org
> cc: linux-fsdevel@vger.kernel.org
> cc: netfs@lists.linux.dev
> cc: v9fs@lists.linux.dev
> cc: linux-afs@lists.infradead.org
> cc: ceph-devel@vger.kernel.org
> cc: linux-cifs@vger.kernel.org
> cc: linux-nfs@vger.kernel.org
> cc: devel@lists.orangefs.org
> Link:
> https://lore.kernel.org/r/1668172.1709764777@warthog.procyon.org.uk/ 
> [1]
> ---
>  fs/netfs/misc.c       |   56
> ++++++++++++++++++++++++++++++++++++++++++++++++++
>  include/linux/netfs.h |    3 ++
>  mm/memory.c           |    3 +-
>  3 files changed, 61 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/netfs/misc.c b/fs/netfs/misc.c
> index bc1fc54fb724..774ce825fbec 100644
> --- a/fs/netfs/misc.c
> +++ b/fs/netfs/misc.c
> @@ -250,3 +250,59 @@ bool netfs_release_folio(struct folio *folio,
> gfp_t gfp)
>  	return true;
>  }
>  EXPORT_SYMBOL(netfs_release_folio);
> +
> +extern void unmap_mapping_range_tree(struct rb_root_cached *root,
> +				     pgoff_t first_index,
> +				     pgoff_t last_index,
> +				     struct zap_details *details);
> +
> +/**
> + * netfs_invalidate_inode - Invalidate/forcibly write back an
> inode's pagecache
> + * @inode: The inode to flush
> + * @flush: Set to write back rather than simply invalidate.
> + *
> + * Invalidate all the folios on an inode, possibly writing them back
> first.
> + * Whilst the operation is undertaken, the mmap lock is held to
> prevent
> + * ->fault() from reinstalling the folios.  The caller must hold a
> lock on the
> + * inode sufficient to prevent ->write_iter() from dirtying more
> folios.
> + */
> +int netfs_invalidate_inode(struct inode *inode, bool flush)
> +{
> +	struct address_space *mapping = inode->i_mapping;
> +
> +	if (!mapping || !mapping->nrpages)
> +		goto out;
> +
> +	/* Prevent folios from being faulted in. */
> +	i_mmap_lock_write(mapping);
> +
> +	if (!mapping->nrpages)
> +		goto unlock;
> +
> +	/* Assume there are probably PTEs only if there are mmaps.
> */
> +	if (unlikely(!RB_EMPTY_ROOT(&mapping->i_mmap.rb_root))) {
> +		struct zap_details details = { };
> +
> +		unmap_mapping_range_tree(&mapping->i_mmap, 0,
> LLONG_MAX, &details);
> +	}
> +
> +	/* Write back the data if we're asked to. */
> +	if (flush) {
> +		struct writeback_control wbc = {
> +			.sync_mode	= WB_SYNC_ALL,
> +			.nr_to_write	= LONG_MAX,
> +			.range_start	= 0,
> +			.range_end	= LLONG_MAX,
> +		};
> +
> +		filemap_fdatawrite_wbc(mapping, &wbc);
> +	}
> +
> +	/* Wait for writeback to complete on all folios and discard.
> */
> +	truncate_inode_pages_range(mapping, 0, LLONG_MAX);
> +
> +unlock:
> +	i_mmap_unlock_write(mapping);
> +out:
> +	return filemap_check_errors(mapping);
> +}
> diff --git a/include/linux/netfs.h b/include/linux/netfs.h
> index 298552f5122c..40dc34ee291d 100644
> --- a/include/linux/netfs.h
> +++ b/include/linux/netfs.h
> @@ -400,6 +400,9 @@ ssize_t netfs_buffered_write_iter_locked(struct
> kiocb *iocb, struct iov_iter *fr
>  ssize_t netfs_unbuffered_write_iter(struct kiocb *iocb, struct
> iov_iter *from);
>  ssize_t netfs_file_write_iter(struct kiocb *iocb, struct iov_iter
> *from);
>  
> +/* High-level invalidation API */
> +int netfs_invalidate_inode(struct inode *inode, bool flush);
> +
>  /* Address operations API */
>  struct readahead_control;
>  void netfs_readahead(struct readahead_control *);
> diff --git a/mm/memory.c b/mm/memory.c
> index f2bc6dd15eb8..106f32c7d7fb 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -3665,7 +3665,7 @@ static void unmap_mapping_range_vma(struct
> vm_area_struct *vma,
>  	zap_page_range_single(vma, start_addr, end_addr -
> start_addr, details);
>  }
>  
> -static inline void unmap_mapping_range_tree(struct rb_root_cached
> *root,
> +inline void unmap_mapping_range_tree(struct rb_root_cached *root,
>  					    pgoff_t first_index,
>  					    pgoff_t last_index,
>  					    struct zap_details
> *details)
> @@ -3685,6 +3685,7 @@ static inline void
> unmap_mapping_range_tree(struct rb_root_cached *root,
>  				details);
>  	}
>  }
> +EXPORT_SYMBOL_GPL(unmap_mapping_range_tree);
>  
>  /**
>   * unmap_mapping_folio() - Unmap single folio from processes.
> 

This is hardly a drop-in replacement for launder_page. The whole point
of using invalidate_inode_pages2() was that it only requires taking the
page locks, allowing us to use it in contexts such as
nfs_release_file().

The above use of truncate_inode_pages_range() will require any caller
to grab several locks in order to prevent data loss through races with
write system calls.

-- 
Trond Myklebust
Linux NFS client maintainer, Hammerspace
trond.myklebust@hammerspace.com

Re: [RFC PATCH] mm, netfs: Provide a means of invalidation without using launder_folio

[Matthew Wilcox]

On Wed, Mar 27, 2024 at 03:56:50PM +0000, Trond Myklebust wrote:
> On Wed, 2024-03-27 at 15:04 +0000, David Howells wrote:
> > Implement a replacement for launder_folio[1].  The key feature of
> > invalidate_inode_pages2() is that it locks each folio individually,
> > unmaps
> > it to prevent mmap'd accesses interfering and calls the -
> > >launder_folio()
> > address_space op to flush it.  This has problems: firstly, each folio
> > is
> > written individually as one or more small writes; secondly, adjacent
> > folios
> > cannot be added so easily into the laundry; thirdly, it's yet another
> > op to
> > implement.
> 
> This is hardly a drop-in replacement for launder_page. The whole point
> of using invalidate_inode_pages2() was that it only requires taking the
> page locks, allowing us to use it in contexts such as
> nfs_release_file().
> 
> The above use of truncate_inode_pages_range() will require any caller
> to grab several locks in order to prevent data loss through races with
> write system calls.

I don't understand why you need launder_folio now
that you have a page_mkwrite implementation (your commit
e3db7691e9f3dff3289f64e3d98583e28afe03db used this as justification).
Other filesystems (except the network filesystems that copied the NFS
implementation) don't implement launder_folio.

[RFC PATCH v2] mm, netfs: Provide a means of invalidation without using launder_folio

[David Howells]

mm, netfs: Provide a means of invalidation without using launder_folio

Implement a replacement for launder_folio.  The key feature of
invalidate_inode_pages2() is that it locks each folio individually, unmaps
it to prevent mmap'd accesses interfering and calls the ->launder_folio()
address_space op to flush it.  This has problems: firstly, each folio is
written individually as one or more small writes; secondly, adjacent folios
cannot be added so easily into the laundry; thirdly, it's yet another op to
implement.

Instead, use the invalidate lock to cause anyone wanting to add a folio to
the inode to wait, then unmap all the folios if we have mmaps, then,
conditionally, use ->writepages() to flush any dirty data back and then
discard all pages.

The invalidate lock prevents ->read_iter(), ->write_iter() and faulting
through mmap all from adding pages for the duration.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Matthew Wilcox <willy@infradead.org>
cc: Miklos Szeredi <miklos@szeredi.hu>
cc: Trond Myklebust <trond.myklebust@hammerspace.com>
cc: Christoph Hellwig <hch@lst.de>
cc: Andrew Morton <akpm@linux-foundation.org>
cc: Alexander Viro <viro@zeniv.linux.org.uk>
cc: Christian Brauner <brauner@kernel.org>
cc: Jeff Layton <jlayton@kernel.org>
cc: linux-mm@kvack.org
cc: linux-fsdevel@vger.kernel.org
cc: netfs@lists.linux.dev
cc: v9fs@lists.linux.dev
cc: linux-afs@lists.infradead.org
cc: ceph-devel@vger.kernel.org
cc: linux-cifs@vger.kernel.org
cc: linux-nfs@vger.kernel.org
cc: devel@lists.orangefs.org
---
 include/linux/pagemap.h |    1 +
 mm/filemap.c            |   48 ++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 49 insertions(+)

diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index 2df35e65557d..4eb3d4177a53 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -40,6 +40,7 @@ int filemap_fdatawait_keep_errors(struct address_space *mapping);
 int filemap_fdatawait_range(struct address_space *, loff_t lstart, loff_t lend);
 int filemap_fdatawait_range_keep_errors(struct address_space *mapping,
 		loff_t start_byte, loff_t end_byte);
+int filemap_invalidate_inode(struct inode *inode, bool flush);
 
 static inline int filemap_fdatawait(struct address_space *mapping)
 {
diff --git a/mm/filemap.c b/mm/filemap.c
index 25983f0f96e3..98f439bedb44 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -4134,6 +4134,54 @@ bool filemap_release_folio(struct folio *folio, gfp_t gfp)
 }
 EXPORT_SYMBOL(filemap_release_folio);
 
+/**
+ * filemap_invalidate_inode - Invalidate/forcibly write back an inode's pagecache
+ * @inode: The inode to flush
+ * @flush: Set to write back rather than simply invalidate.
+ *
+ * Invalidate all the folios on an inode, possibly writing them back first.
+ * Whilst the operation is undertaken, the invalidate lock is held to prevent
+ * new folios from being installed.
+ */
+int filemap_invalidate_inode(struct inode *inode, bool flush)
+{
+	struct address_space *mapping = inode->i_mapping;
+
+	if (!mapping || !mapping->nrpages)
+		goto out;
+
+	/* Prevent new folios from being added to the inode. */
+	filemap_invalidate_lock(mapping);
+
+	if (!mapping->nrpages)
+		goto unlock;
+
+	/* Assume there are probably PTEs only if there are mmaps. */
+	if (unlikely(!RB_EMPTY_ROOT(&mapping->i_mmap.rb_root)))
+		unmap_mapping_pages(mapping, 0, ULONG_MAX, false);
+
+	/* Write back the data if we're asked to. */
+	if (flush) {
+		struct writeback_control wbc = {
+			.sync_mode	= WB_SYNC_ALL,
+			.nr_to_write	= LONG_MAX,
+			.range_start	= 0,
+			.range_end	= LLONG_MAX,
+		};
+
+		filemap_fdatawrite_wbc(mapping, &wbc);
+	}
+
+	/* Wait for writeback to complete on all folios and discard. */
+	truncate_inode_pages_range(mapping, 0, LLONG_MAX);
+
+unlock:
+	filemap_invalidate_unlock(mapping);
+out:
+	return filemap_check_errors(mapping);
+}
+EXPORT_SYMBOL(filemap_invalidate_inode);
+
 #ifdef CONFIG_CACHESTAT_SYSCALL
 /**
  * filemap_cachestat() - compute the page cache statistics of a mapping

Re: [RFC PATCH v2] mm, netfs: Provide a means of invalidation without using launder_folio

[Matthew Wilcox]

On Wed, Mar 27, 2024 at 05:55:45PM +0000, David Howells wrote:
> +int filemap_invalidate_inode(struct inode *inode, bool flush)
> +{
> +	struct address_space *mapping = inode->i_mapping;
> +
> +	if (!mapping || !mapping->nrpages)
> +		goto out;
> +
> +	/* Prevent new folios from being added to the inode. */
> +	filemap_invalidate_lock(mapping);

I'm kind of surprised that the callers wouldn't want to hold that lock
over a call to this function.  I guess you're working on the callers,
so you'd know better than I would, but I would have used lockdep to
assert that invalidate_lock was held.

> +	if (!mapping->nrpages)
> +		goto unlock;
> +
> +	/* Assume there are probably PTEs only if there are mmaps. */
> +	if (unlikely(!RB_EMPTY_ROOT(&mapping->i_mmap.rb_root)))
> +		unmap_mapping_pages(mapping, 0, ULONG_MAX, false);

Is this optimisation worth it?  We're already doing some expensive
operations here, does saving cycling the i_mmap_lock really help
anything?  You'll note that unmap_mapping_pages() already does this
check inside the lock.

Re: [RFC PATCH v2] mm, netfs: Provide a means of invalidation without using launder_folio

[David Howells]

Matthew Wilcox <willy@infradead.org> wrote:

> > +	/* Prevent new folios from being added to the inode. */
> > +	filemap_invalidate_lock(mapping);
> 
> I'm kind of surprised that the callers wouldn't want to hold that lock
> over a call to this function.  I guess you're working on the callers,
> so you'd know better than I would, but I would have used lockdep to
> assert that invalidate_lock was held.

I'm not sure.  None of the places that look like they'd be calling this
currently take that lock (though possibly they should).

Also, should I provide it with explicit range, I wonder?

> > +	if (unlikely(!RB_EMPTY_ROOT(&mapping->i_mmap.rb_root)))
> > +		unmap_mapping_pages(mapping, 0, ULONG_MAX, false);
> 
> Is this optimisation worth it?

Perhaps not.

David