From f06f5d708c262f240bc23efe416340e3e1ef6280 Mon Sep 17 00:00:00 2001
From: Michael Lotz <mmlr@mlotz.ch>
Date: Sat, 30 May 2009 17:21:10 +0000
Subject: [PATCH] Make reading the write_overlay more efficient. Instead of
 always reading the original file and then overwriting it with what we have in
 memory, just linearly fill the buffer, closing gaps between our buffers with
 original file content as needed.

git-svn-id: file:///srv/svn/repos/haiku/haiku/trunk@30927 a95241bf-73f2-0310-859d-f6bbb57e9c96
---
 .../layers/write_overlay/write_overlay.cpp    | 91 +++++++++++--------
 1 file changed, 54 insertions(+), 37 deletions(-)

diff --git a/src/add-ons/kernel/file_systems/layers/write_overlay/write_overlay.cpp b/src/add-ons/kernel/file_systems/layers/write_overlay/write_overlay.cpp
index 3c58233300..5dd9d5e8f9 100644
--- a/src/add-ons/kernel/file_systems/layers/write_overlay/write_overlay.cpp
+++ b/src/add-ons/kernel/file_systems/layers/write_overlay/write_overlay.cpp
@@ -451,54 +451,71 @@ status_t
 OverlayInode::Read(void *_cookie, off_t position, void *buffer, size_t *length,
 	bool readPages)
 {
-	if (position >= fStat.st_size) {
-		*length = 0;
-		return B_OK;
-	}
+	uint8 *pointer = (uint8 *)buffer;
+	write_buffer *element = fWriteBuffers;
+	size_t bytesLeft = MIN(fStat.st_size - position, *length);
+	*length = bytesLeft;
 
-	if (position < fOriginalNodeLength) {
-		void *superCookie = NULL;
-		if (_cookie != NULL)
-			superCookie = ((open_cookie *)_cookie)->super_cookie;
+	void *superCookie = NULL;
+	if (!fIsVirtual && _cookie != NULL)
+		superCookie = ((open_cookie *)_cookie)->super_cookie;
 
-		size_t readLength = MIN(fOriginalNodeLength - position, *length);
-		status_t result = B_ERROR;
-
-		if (readPages) {
-			iovec vector;
-			vector.iov_base = buffer;
-			vector.iov_len = readLength;
-
-			result = fSuperVnode.ops->read_pages(SuperVolume(),
-				&fSuperVnode, superCookie, position, &vector, 1, &readLength);
-		} else {
-			result = fSuperVnode.ops->read(SuperVolume(), &fSuperVnode,
-				superCookie, position, buffer, &readLength);
+	while (bytesLeft > 0) {
+		size_t gapSize = bytesLeft;
+		if (element != NULL) {
+			gapSize = MIN(bytesLeft, element->position > position ?
+				element->position - position : 0);
 		}
 
-		if (result != B_OK)
-			return result;
+		if (gapSize > 0 && !fIsVirtual && position < fOriginalNodeLength) {
+			// there's a part missing between the read position and our
+			// next position, fill the gap with original file content
+			size_t readLength = MIN(fOriginalNodeLength - position, gapSize);
+			status_t result = B_ERROR;
+			if (readPages) {
+				iovec vector;
+				vector.iov_base = pointer;
+				vector.iov_len = readLength;
 
-		if (readLength < *length)
-			memset((uint8 *)buffer + readLength, 0, *length - readLength);
-	} else
-		memset(buffer, 0, *length);
+				result = fSuperVnode.ops->read_pages(SuperVolume(),
+					&fSuperVnode, superCookie, position, &vector, 1,
+					&readLength);
+			} else {
+				result = fSuperVnode.ops->read(SuperVolume(), &fSuperVnode,
+					superCookie, position, pointer, &readLength);
+			}
 
-	// overlay the read with whatever chunks we have written
-	write_buffer *element = fWriteBuffers;
-	*length = MIN(fStat.st_size - position, *length);
-	off_t end = position + *length;
-	while (element) {
-		if (element->position > end)
+			if (result != B_OK)
+				return result;
+
+			pointer += readLength;
+			position += readLength;
+			bytesLeft -= readLength;
+			gapSize -= readLength;
+		}
+
+		if (gapSize > 0) {
+			// there's a gap before our next position which we cannot
+			// fill with original file content, zero it out
+			memset(pointer, 0, gapSize);
+			bytesLeft -= gapSize;
+			position += gapSize;
+			pointer += gapSize;
+		}
+
+		// we've reached the end
+		if (bytesLeft == 0 || element == NULL)
 			break;
 
 		off_t elementEnd = element->position + element->length;
 		if (elementEnd > position) {
-			off_t copyPosition = MAX(position, element->position);
-			size_t copyLength = MIN(elementEnd - position, *length);
-			memcpy((uint8 *)buffer + (copyPosition - position),
-				element->buffer + (copyPosition - element->position),
+			size_t copyLength = MIN(elementEnd - position, bytesLeft);
+			memcpy(pointer, element->buffer + (position - element->position),
 				copyLength);
+
+			bytesLeft -= copyLength;
+			position += copyLength;
+			pointer += copyLength;
 		}
 
 		element = element->next;