[PATCH v3 44/59] netfs: Optimise away reads above the point at which there can be no data

[David Howells <dhowells@redhat.com>]

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.

 (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@redhat.com>
cc: Jeff Layton <jlayton@kernel.org>
cc: linux-cachefs@redhat.com
cc: linux-fsdevel@vger.kernel.org
cc: linux-mm@kvack.org
---
 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 37485ae31471..2b44a342b4a1 100644
--- a/fs/afs/inode.c
+++ b/fs/afs/inode.c
@@ -168,6 +168,7 @@ 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;
 
@@ -231,6 +232,7 @@ 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
@@ -252,6 +254,8 @@ 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);
 		}
@@ -865,17 +869,17 @@ 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);
+		}
 	}
 }
 
@@ -943,11 +947,11 @@ 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 461be124b35d..64d84a0544ae 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 45165a3d5f99..37923dfeaae4 100644
--- a/fs/netfs/direct_write.c
+++ b/fs/netfs/direct_write.c
@@ -334,6 +334,7 @@ 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));
@@ -355,6 +356,9 @@ 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 ae014efe8728..5f112afcf578 100644
--- a/fs/netfs/io.c
+++ b/fs/netfs/io.c
@@ -573,6 +573,7 @@ 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);
@@ -590,6 +591,14 @@ 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)
@@ -611,6 +620,7 @@ 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 08040da65336..29072b6418c3 100644
--- a/include/linux/netfs.h
+++ b/include/linux/netfs.h
@@ -137,6 +137,8 @@ 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 */
@@ -482,6 +484,7 @@ 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;
 	ctx->min_bshift = 0;
 	ctx->crypto_bshift = 0;
@@ -489,18 +492,25 @@ static inline void netfs_inode_init(struct netfs_inode *ctx,
 #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;
 }
 
 /**