[PATCH resend2] rd: fix data corruption on memory pressure

[Christian Borntraeger <borntraeger@de.ibm.com>]

Nick, Eric, Andrew,

we now have passed rc1. That means that Erics or Nicks rd rewrite is no longer
an option for 2.6.24. If I followed the last thread correctly all alternative
patches have one of the following issue
- too big for post rc1
- break reiserfs and maybe others
- call into vfs while being unrelated

So this is a resend of my patch, which is in my opinion the simplest fix for
the data corruption problem.
The patch was tested by our test department, thanks to Oliver Paukstadt and
Thorsten Diehl.

---

Subject: [PATCH] rd: fix data corruption on memory pressure
From: Christian Borntraeger <borntraeger@de.ibm.com>

We have seen ramdisk based install systems, where some pages of mapped 
libraries and programs were suddendly zeroed under memory pressure. This 
should not happen, as the ramdisk avoids freeing its pages by keeping them 
dirty all the time.

It turns out that there is a case, where the VM makes a ramdisk page clean, 
without telling the ramdisk driver.
On memory pressure shrink_zone runs and it starts to run shrink_active_list. 
There is a check for buffer_heads_over_limit, and if true, pagevec_strip is 
called. pagevec_strip calls try_to_release_page. If the mapping has no 
releasepage callback, try_to_free_buffers is called. try_to_free_buffers has 
now a special logic for some file systems to make a dirty page clean, if all 
buffers are clean. Thats what happened in our test case.

The simplest solution is to provide a noop-releasepage callback for the
ramdisk driver. This avoids try_to_free_buffers for ramdisk pages. 

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
---
 drivers/block/rd.c |   13 +++++++++++++
 1 file changed, 13 insertions(+)

Index: linux-2.6/drivers/block/rd.c
===================================================================
--- linux-2.6.orig/drivers/block/rd.c
+++ linux-2.6/drivers/block/rd.c
@@ -189,6 +189,18 @@ static int ramdisk_set_page_dirty(struct
 	return 0;
 }
 
+/*
+ * releasepage is called by pagevec_strip/try_to_release_page if
+ * buffers_heads_over_limit is true. Without a releasepage function
+ * try_to_free_buffers is called instead. That can unset the dirty
+ * bit of our ram disk pages, which will be eventually freed, even
+ * if the page is still in use.
+ */
+static int ramdisk_releasepage(struct page *page, gfp_t dummy)
+{
+	return 0;
+}
+
 static const struct address_space_operations ramdisk_aops = {
 	.readpage	= ramdisk_readpage,
 	.prepare_write	= ramdisk_prepare_write,
@@ -196,6 +208,7 @@ static const struct address_space_operat
 	.writepage	= ramdisk_writepage,
 	.set_page_dirty	= ramdisk_set_page_dirty,
 	.writepages	= ramdisk_writepages,
+	.releasepage	= ramdisk_releasepage,
 };
 
 static int rd_blkdev_pagecache_IO(int rw, struct bio_vec *vec, sector_t sector,

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