[RFC][PATCH -mm] swsusp: use less memory during resume

[Rafael J. Wysocki]

Hi,

Currently during resume swsusp puts the image data in the page frames that
don't conflict with the original locations of the data (ie. the locations the
data will be put in when the saved system state is restored from the image).
These page frames are considered as "safe" and the other page frames are
treadet as "unsafe".

Of course we cannot force the memory allocator to allocate "safe" pages only,
so if an "unsafe" page is allocated, swsusp treats it as an "eaten page" and
attempts to allocate another page in the hope that it'll be "safe" etc.
swsusp tries to allocate as many "safe" pages as necessary to store the
image data, so it "eats" a considerable number of "unsafe" pages in the
process.  Next, it reads the image and puts the data into the allocated "safe"
pages.  Finally, the data are copied to their "original" locations.

This approach, although it works nicely, is quite inefficient from the memory
utilization point of view and it also turns out to be unnecessary.  Namely,
for each "unsafe" page frame returned by the memory allocator there's exactly
one page in the image that finally should be placed in this page frame.
Therefore we can put the right data into this page frame as soon as they're
read from the image and we won't have to copy these data later on.  This way
we'll only need to allocate as many pages as necessary to store the image
data and we won't have to "eat" the "unsafe" pages.

The appended patch implements this idea.  It makes swsusp allocate as
many pages as it'll need to store the data read from the image in one
shot, creating a list of allocated "safe" pages, and uses the observation that
all pages allocated by swsusp are marked with the PG_nosave and PG_nosave_free
flags set.  Namely, when it's about to read a page, it checks whether the page
frame corresponding to the "original" location of this page has been allocated
(ie. if the page frame has the PG_nosave and PG_nosave_free flags set) and if
so, it reads the page directly into this page frame.  Otherwise it uses an
allocated "safe" page from the list to store the data that will be copied to
their "original" location later on.

On my box this patch allows us to save as many as approx. 90000 page copyings
and 90000 (at least - probably twice as many, because it's an x86_64 box) page
allocations for an image of approx. 100000 of pages.  Also it will allow us to
read images greater than 50% of the normal zone (when we learn how to create
them ;-)).

[This patch is on top of the
swsusp-prevent-possible-image-corruption-on-resume.patch
(recently added to the -mm tree).]

Comments welcome.

Greetings,
Rafael

---
 kernel/power/power.h    |    2 
 kernel/power/snapshot.c |  141 ++++++++++++++++++++++++++++--------------------
 2 files changed, 85 insertions(+), 58 deletions(-)

Index: linux-2.6.17-rc1-mm2/kernel/power/power.h
===================================================================
--- linux-2.6.17-rc1-mm2.orig/kernel/power/power.h	2006-04-17 13:47:58.000000000 +0200
+++ linux-2.6.17-rc1-mm2/kernel/power/power.h	2006-04-17 13:59:16.000000000 +0200
@@ -55,7 +55,7 @@ struct snapshot_handle {
 	unsigned int	page;
 	unsigned int	page_offset;
 	unsigned int	prev;
-	struct pbe	*pbe;
+	struct pbe	*pbe, *last_pbe;
 	void		*buffer;
 	unsigned int	buf_offset;
 };
Index: linux-2.6.17-rc1-mm2/kernel/power/snapshot.c
===================================================================
--- linux-2.6.17-rc1-mm2.orig/kernel/power/snapshot.c	2006-04-17 13:58:07.000000000 +0200
+++ linux-2.6.17-rc1-mm2/kernel/power/snapshot.c	2006-04-17 14:05:01.000000000 +0200
@@ -293,62 +293,29 @@ static inline void create_pbe_list(struc
 	}
 }
 
-/**
- *	On resume it is necessary to trace and eventually free the unsafe
- *	pages that have been allocated, because they are needed for I/O
- *	(on x86-64 we likely will "eat" these pages once again while
- *	creating the temporary page translation tables)
- */
-
-struct eaten_page {
-	struct eaten_page *next;
-	char padding[PAGE_SIZE - sizeof(void *)];
-};
-
-static struct eaten_page *eaten_pages = NULL;
-
-static void release_eaten_pages(void)
-{
-	struct eaten_page *p, *q;
-
-	p = eaten_pages;
-	while (p) {
-		q = p->next;
-		/* We don't want swsusp_free() to free this page again */
-		ClearPageNosave(virt_to_page(p));
-		free_page((unsigned long)p);
-		p = q;
-	}
-	eaten_pages = NULL;
-}
+static unsigned int unsafe_pages;
 
 /**
  *	@safe_needed - on resume, for storing the PBE list and the image,
  *	we can only use memory pages that do not conflict with the pages
- *	which had been used before suspend.
+ *	used before suspend.
  *
  *	The unsafe pages are marked with the PG_nosave_free flag
- *
- *	Allocated but unusable (ie eaten) memory pages should be marked
- *	so that swsusp_free() can release them
+ *	and we count them using unsafe_pages
  */
 
 static inline void *alloc_image_page(gfp_t gfp_mask, int safe_needed)
 {
 	void *res;
 
+	res = (void *)get_zeroed_page(gfp_mask);
 	if (safe_needed)
-		do {
+		while (res && PageNosaveFree(virt_to_page(res))) {
+			/* The page is unsafe, mark it for swsusp_free() */
+			SetPageNosave(virt_to_page(res));
+			unsafe_pages++;
 			res = (void *)get_zeroed_page(gfp_mask);
-			if (res && PageNosaveFree(virt_to_page(res))) {
-				/* This is for swsusp_free() */
-				SetPageNosave(virt_to_page(res));
-				((struct eaten_page *)res)->next = eaten_pages;
-				eaten_pages = res;
-			}
-		} while (res && PageNosaveFree(virt_to_page(res)));
-	else
-		res = (void *)get_zeroed_page(gfp_mask);
+		}
 	if (res) {
 		SetPageNosave(virt_to_page(res));
 		SetPageNosaveFree(virt_to_page(res));
@@ -642,6 +609,8 @@ static int mark_unsafe_pages(struct pbe 
 			return -EFAULT;
 	}
 
+	unsafe_pages = 0;
+
 	return 0;
 }
 
@@ -719,42 +688,99 @@ static inline struct pbe *unpack_orig_ad
 }
 
 /**
- *	create_image - use metadata contained in the PBE list
+ *	prepare_image - use metadata contained in the PBE list
  *	pointed to by pagedir_nosave to mark the pages that will
  *	be overwritten in the process of restoring the system
- *	memory state from the image and allocate memory for
- *	the image avoiding these pages
+ *	memory state from the image ("unsafe" pages) and allocate
+ *	memory for the image
+ *
+ *	The idea is to allocate the PBE list first and then
+ *	allocate as many pages as it's needed for the image data,
+ *	but not to assign these pages to the PBEs initially.
+ *	Instead, we just mark them as allocated and create a list
+ *	of "safe" which will be used later
  */
 
-static int create_image(struct snapshot_handle *handle)
+struct safe_page {
+	struct safe_page *next;
+	char padding[PAGE_SIZE - sizeof(void *)];
+};
+
+static struct safe_page *safe_pages;
+
+static int prepare_image(struct snapshot_handle *handle)
 {
 	int error = 0;
-	struct pbe *p, *pblist;
+	unsigned int nr_pages = nr_copy_pages;
+	struct pbe *p, *pblist = NULL;
 
 	p = pagedir_nosave;
 	error = mark_unsafe_pages(p);
 	if (!error) {
-		pblist = alloc_pagedir(nr_copy_pages, GFP_ATOMIC, 1);
+		pblist = alloc_pagedir(nr_pages, GFP_ATOMIC, 1);
 		if (pblist)
 			copy_page_backup_list(pblist, p);
 		free_pagedir(p, 0);
 		if (!pblist)
 			error = -ENOMEM;
 	}
-	if (!error)
-		error = alloc_data_pages(pblist, GFP_ATOMIC, 1);
+	safe_pages = NULL;
+	if (!error && nr_pages > unsafe_pages) {
+		nr_pages -= unsafe_pages;
+		while (nr_pages--) {
+			struct safe_page *ptr;
+
+			ptr = (struct safe_page *)get_zeroed_page(GFP_ATOMIC);
+			if (!ptr) {
+				error = -ENOMEM;
+				break;
+			}
+			if (!PageNosaveFree(virt_to_page(ptr))) {
+				/* The page is "safe", add it to the list */
+				ptr->next = safe_pages;
+				safe_pages = ptr;
+			}
+			/* Mark the page as allocated */
+			SetPageNosave(virt_to_page(ptr));
+			SetPageNosaveFree(virt_to_page(ptr));
+		}
+	}
 	if (!error) {
-		release_eaten_pages();
 		pagedir_nosave = pblist;
 	} else {
-		pagedir_nosave = NULL;
 		handle->pbe = NULL;
-		nr_copy_pages = 0;
-		nr_meta_pages = 0;
+		swsusp_free();
 	}
 	return error;
 }
 
+static void *get_buffer(struct snapshot_handle *handle)
+{
+	struct pbe *pbe = handle->pbe, *last = handle->last_pbe;
+	struct page *page = virt_to_page(pbe->orig_address);
+
+	if (PageNosave(page) && PageNosaveFree(page)) {
+		/*
+		 * We have allocated the "original" page frame and we can
+		 * use it directly to store the read page
+		 */
+		pbe->address = 0;
+		if (last && last->next)
+			last->next = NULL;
+		return (void *)pbe->orig_address;
+	}
+	/*
+	 * The "original" page frame has not been allocated and we have to
+	 * use a "safe" page frame to store the read page
+	 */
+	pbe->address = (unsigned long)safe_pages;
+	safe_pages = safe_pages->next;
+	if (last)
+		last->next = pbe;
+	handle->last_pbe = pbe;
+	return (void *)pbe->address;
+}
+
 /**
  *	snapshot_write_next - used for writing the system memory snapshot.
  *
@@ -799,15 +825,16 @@ int snapshot_write_next(struct snapshot_
 		} else if (handle->prev <= nr_meta_pages) {
 			handle->pbe = unpack_orig_addresses(buffer, handle->pbe);
 			if (!handle->pbe) {
-				error = create_image(handle);
+				error = prepare_image(handle);
 				if (error)
 					return error;
 				handle->pbe = pagedir_nosave;
-				handle->buffer = (void *)handle->pbe->address;
+				handle->last_pbe = NULL;
+				handle->buffer = get_buffer(handle);
 			}
 		} else {
 			handle->pbe = handle->pbe->next;
-			handle->buffer = (void *)handle->pbe->address;
+			handle->buffer = get_buffer(handle);
 		}
 		handle->prev = handle->page;
 	}
-
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