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

[Trond Myklebust <trondmy@hammerspace.com>]

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