Re: [PATCH v4 37/39] netfs: Optimise away reads above the point at which there can be no data

[Jeff Layton]

On Wed, 2023-12-13 at 15:23 +0000, David Howells wrote:
> Track the file position above which the server is not expected to have any
> data (the "zero point") and preemptively assume that we can satisfy
> requests by filling them with zeroes locally rather than attempting to
> download them if they're over that line - even if we've written data back
> to the server.  Assume that any data that was written back above that
> position is held in the local cache.  Note that we have to split requests
> that straddle the line.
> 
> Make use of this to optimise away some reads from the server.  We need to
> set the zero point in the following circumstances:
> 
>  (1) When we see an extant remote inode and have no cache for it, we set
>      the zero_point to i_size.
> 
>  (2) On local inode creation, we set zero_point to 0.
> 
>  (3) On local truncation down, we reduce zero_point to the new i_size if
>      the new i_size is lower.
> 
>  (4) On local truncation up, we don't change zero_point.
> 

The above seems odd, but I guess the assumption is that if there are any
writes by a 3rd party above the old zero point, that that would cause an
invalidation?

>  (5) On local modification, we don't change zero_point.
> 
>  (6) On remote invalidation, we set zero_point to the new i_size.
> 
>  (7) If stored data is discarded from the pagecache or culled from fscache,
>      we must set zero_point above that if the data also got written to the
>      server.
> 
>  (8) If dirty data is written back to the server, but not fscache, we must
>      set zero_point above that.
> 
>  (9) If a direct I/O write is made, set zero_point above that.
> 
> Assuming the above, any read from the server at or above the zero_point
> position will return all zeroes.
> 
> The zero_point value can be stored in the cache, provided the above rules
> are applied to it by any code that culls part of the local cache.
> 
> Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
> cc: Jeff Layton <jlayton@xxxxxxxxxx>
> cc: linux-cachefs@xxxxxxxxxx
> cc: linux-fsdevel@xxxxxxxxxxxxxxx
> cc: linux-mm@xxxxxxxxx
> ---
>  fs/afs/inode.c            | 22 +++++++++++++---------
>  fs/netfs/buffered_write.c |  2 +-
>  fs/netfs/direct_write.c   |  4 ++++
>  fs/netfs/io.c             | 10 ++++++++++
>  fs/netfs/misc.c           |  5 +++++
>  fs/smb/client/cifsfs.c    |  4 ++--
>  include/linux/netfs.h     | 14 ++++++++++++--
>  7 files changed, 47 insertions(+), 14 deletions(-)
> 
> diff --git a/fs/afs/inode.c b/fs/afs/inode.c
> index c43112dcbbbb..dfd940a64e0f 100644
> --- a/fs/afs/inode.c
> +++ b/fs/afs/inode.c
> @@ -166,6 +166,7 @@ static void afs_apply_status(struct afs_operation *op,
>  	struct inode *inode = &vnode->netfs.inode;
>  	struct timespec64 t;
>  	umode_t mode;
> +	bool unexpected_jump = false;
>  	bool data_changed = false;
>  	bool change_size = vp->set_size;
>  
> @@ -230,6 +231,7 @@ static void afs_apply_status(struct afs_operation *op,
>  		}
>  		change_size = true;
>  		data_changed = true;
> +		unexpected_jump = true;
>  	} else if (vnode->status.type == AFS_FTYPE_DIR) {
>  		/* Expected directory change is handled elsewhere so
>  		 * that we can locally edit the directory and save on a
> @@ -251,6 +253,8 @@ static void afs_apply_status(struct afs_operation *op,
>  		vnode->netfs.remote_i_size = status->size;
>  		if (change_size || status->size > i_size_read(inode)) {
>  			afs_set_i_size(vnode, status->size);
> +			if (unexpected_jump)
> +				vnode->netfs.zero_point = status->size;
>  			inode_set_ctime_to_ts(inode, t);
>  			inode_set_atime_to_ts(inode, t);
>  		}
> @@ -689,17 +693,17 @@ static void afs_setattr_success(struct afs_operation *op)
>  static void afs_setattr_edit_file(struct afs_operation *op)
>  {
>  	struct afs_vnode_param *vp = &op->file[0];
> -	struct inode *inode = &vp->vnode->netfs.inode;
> +	struct afs_vnode *vnode = vp->vnode;
>  
>  	if (op->setattr.attr->ia_valid & ATTR_SIZE) {
>  		loff_t size = op->setattr.attr->ia_size;
>  		loff_t i_size = op->setattr.old_i_size;
>  
> -		if (size < i_size)
> -			truncate_pagecache(inode, size);
> -		if (size != i_size)
> -			fscache_resize_cookie(afs_vnode_cache(vp->vnode),
> -					      vp->scb.status.size);
> +		if (size != i_size) {
> +			truncate_setsize(&vnode->netfs.inode, size);
> +			netfs_resize_file(&vnode->netfs, size, true);
> +			fscache_resize_cookie(afs_vnode_cache(vnode), size);
> +		}
>  	}
>  }
>  
> @@ -767,11 +771,11 @@ int afs_setattr(struct mnt_idmap *idmap, struct dentry *dentry,
>  		 */
>  		if (!(attr->ia_valid & (supported & ~ATTR_SIZE & ~ATTR_MTIME)) &&
>  		    attr->ia_size < i_size &&
> -		    attr->ia_size > vnode->status.size) {
> -			truncate_pagecache(inode, attr->ia_size);
> +		    attr->ia_size > vnode->netfs.remote_i_size) {
> +			truncate_setsize(inode, attr->ia_size);
> +			netfs_resize_file(&vnode->netfs, size, false);
>  			fscache_resize_cookie(afs_vnode_cache(vnode),
>  					      attr->ia_size);
> -			i_size_write(inode, attr->ia_size);
>  			ret = 0;
>  			goto out_unlock;
>  		}
> diff --git a/fs/netfs/buffered_write.c b/fs/netfs/buffered_write.c
> index dce6995fb644..d7ce424b9188 100644
> --- a/fs/netfs/buffered_write.c
> +++ b/fs/netfs/buffered_write.c
> @@ -73,7 +73,7 @@ static enum netfs_how_to_modify netfs_how_to_modify(struct netfs_inode *ctx,
>  	if (folio_test_uptodate(folio))
>  		return NETFS_FOLIO_IS_UPTODATE;
>  
> -	if (pos >= ctx->remote_i_size)
> +	if (pos >= ctx->zero_point)
>  		return NETFS_MODIFY_AND_CLEAR;
>  
>  	if (!maybe_trouble && offset == 0 && len >= flen)
> diff --git a/fs/netfs/direct_write.c b/fs/netfs/direct_write.c
> index bb0c2718f57b..aad05f2349a4 100644
> --- a/fs/netfs/direct_write.c
> +++ b/fs/netfs/direct_write.c
> @@ -134,6 +134,7 @@ ssize_t netfs_unbuffered_write_iter(struct kiocb *iocb, struct iov_iter *from)
>  	struct file *file = iocb->ki_filp;
>  	struct inode *inode = file->f_mapping->host;
>  	struct netfs_inode *ictx = netfs_inode(inode);
> +	unsigned long long end;
>  	ssize_t ret;
>  
>  	_enter("%llx,%zx,%llx", iocb->ki_pos, iov_iter_count(from), i_size_read(inode));
> @@ -155,6 +156,9 @@ ssize_t netfs_unbuffered_write_iter(struct kiocb *iocb, struct iov_iter *from)
>  	ret = kiocb_invalidate_pages(iocb, iov_iter_count(from));
>  	if (ret < 0)
>  		goto out;
> +	end = iocb->ki_pos + iov_iter_count(from);
> +	if (end > ictx->zero_point)
> +		ictx->zero_point = end;
>  
>  	fscache_invalidate(netfs_i_cookie(ictx), NULL, i_size_read(inode),
>  			   FSCACHE_INVAL_DIO_WRITE);
> diff --git a/fs/netfs/io.c b/fs/netfs/io.c
> index 5d9098db815a..41a6113aa7fa 100644
> --- a/fs/netfs/io.c
> +++ b/fs/netfs/io.c
> @@ -569,6 +569,7 @@ netfs_rreq_prepare_read(struct netfs_io_request *rreq,
>  			struct iov_iter *io_iter)
>  {
>  	enum netfs_io_source source = NETFS_DOWNLOAD_FROM_SERVER;
> +	struct netfs_inode *ictx = netfs_inode(rreq->inode);
>  	size_t lsize;
>  
>  	_enter("%llx-%llx,%llx", subreq->start, subreq->start + subreq->len, rreq->i_size);
> @@ -586,6 +587,14 @@ netfs_rreq_prepare_read(struct netfs_io_request *rreq,
>  		 * to make serial calls, it can indicate a short read and then
>  		 * we will call it again.
>  		 */
> +		if (rreq->origin != NETFS_DIO_READ) {
> +			if (subreq->start >= ictx->zero_point) {
> +				source = NETFS_FILL_WITH_ZEROES;
> +				goto set;
> +			}
> +			if (subreq->len > ictx->zero_point - subreq->start)
> +				subreq->len = ictx->zero_point - subreq->start;
> +		}
>  		if (subreq->len > rreq->i_size - subreq->start)
>  			subreq->len = rreq->i_size - subreq->start;
>  		if (rreq->rsize && subreq->len > rreq->rsize)
> @@ -607,6 +616,7 @@ netfs_rreq_prepare_read(struct netfs_io_request *rreq,
>  		}
>  	}
>  
> +set:
>  	if (subreq->len > rreq->len)
>  		pr_warn("R=%08x[%u] SREQ>RREQ %zx > %zx\n",
>  			rreq->debug_id, subreq->debug_index,
> diff --git a/fs/netfs/misc.c b/fs/netfs/misc.c
> index 40421ced4cd3..31e45dfad5b0 100644
> --- a/fs/netfs/misc.c
> +++ b/fs/netfs/misc.c
> @@ -240,6 +240,11 @@ EXPORT_SYMBOL(netfs_invalidate_folio);
>  bool netfs_release_folio(struct folio *folio, gfp_t gfp)
>  {
>  	struct netfs_inode *ctx = netfs_inode(folio_inode(folio));
> +	unsigned long long end;
> +
> +	end = folio_pos(folio) + folio_size(folio);
> +	if (end > ctx->zero_point)
> +		ctx->zero_point = end;
>  
>  	if (folio_test_private(folio))
>  		return false;
> diff --git a/fs/smb/client/cifsfs.c b/fs/smb/client/cifsfs.c
> index 96a65cf9b5ec..07cd88897c33 100644
> --- a/fs/smb/client/cifsfs.c
> +++ b/fs/smb/client/cifsfs.c
> @@ -1220,7 +1220,7 @@ static int cifs_precopy_set_eof(struct inode *src_inode, struct cifsInodeInfo *s
>  	if (rc < 0)
>  		goto set_failed;
>  
> -	netfs_resize_file(&src_cifsi->netfs, src_end);
> +	netfs_resize_file(&src_cifsi->netfs, src_end, true);
>  	fscache_resize_cookie(cifs_inode_cookie(src_inode), src_end);
>  	return 0;
>  
> @@ -1351,7 +1351,7 @@ static loff_t cifs_remap_file_range(struct file *src_file, loff_t off,
>  			smb_file_src, smb_file_target, off, len, destoff);
>  		if (rc == 0 && new_size > i_size_read(target_inode)) {
>  			truncate_setsize(target_inode, new_size);
> -			netfs_resize_file(&target_cifsi->netfs, new_size);
> +			netfs_resize_file(&target_cifsi->netfs, new_size, true);
>  			fscache_resize_cookie(cifs_inode_cookie(target_inode),
>  					      new_size);
>  		}
> diff --git a/include/linux/netfs.h b/include/linux/netfs.h
> index fc77f7be220a..2005ad3b0e25 100644
> --- a/include/linux/netfs.h
> +++ b/include/linux/netfs.h
> @@ -136,6 +136,8 @@ struct netfs_inode {
>  	struct fscache_cookie	*cache;
>  #endif
>  	loff_t			remote_i_size;	/* Size of the remote file */
> +	loff_t			zero_point;	/* Size after which we assume there's no data
> +						 * on the server */
>  	unsigned long		flags;
>  #define NETFS_ICTX_ODIRECT	0		/* The file has DIO in progress */
>  #define NETFS_ICTX_UNBUFFERED	1		/* I/O should not use the pagecache */
> @@ -465,22 +467,30 @@ static inline void netfs_inode_init(struct netfs_inode *ctx,
>  {
>  	ctx->ops = ops;
>  	ctx->remote_i_size = i_size_read(&ctx->inode);
> +	ctx->zero_point = ctx->remote_i_size;
>  	ctx->flags = 0;
>  #if IS_ENABLED(CONFIG_FSCACHE)
>  	ctx->cache = NULL;
>  #endif
> +	/* ->releasepage() drives zero_point */
> +	mapping_set_release_always(ctx->inode.i_mapping);
>  }
>  
>  /**
>   * netfs_resize_file - Note that a file got resized
>   * @ctx: The netfs inode being resized
>   * @new_i_size: The new file size
> + * @changed_on_server: The change was applied to the server
>   *
>   * Inform the netfs lib that a file got resized so that it can adjust its state.
>   */
> -static inline void netfs_resize_file(struct netfs_inode *ctx, loff_t new_i_size)
> +static inline void netfs_resize_file(struct netfs_inode *ctx, loff_t new_i_size,
> +				     bool changed_on_server)
>  {
> -	ctx->remote_i_size = new_i_size;
> +	if (changed_on_server)
> +		ctx->remote_i_size = new_i_size;
> +	if (new_i_size < ctx->zero_point)
> +		ctx->zero_point = new_i_size;
>  }
>  
>  /**
> 

-- 
Jeff Layton <jlayton@xxxxxxxxxx>