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SubjectGFP_NOFS broken
While I was travelling disconnected from the internet the last week (I
will buy a GPRS phone soon ;) I also spotted and fixed the __GFP_BUFFER
deadlock that I was triggering on my 128m laptop while browsing large
email folders on top of crypto loop:

--- 2.4.6pre6aa1/include/linux/mm.h.~1~ Sat Jun 30 00:49:54 2001
+++ 2.4.6pre6aa1/include/linux/mm.h Sat Jun 30 02:20:57 2001
@@ -538,9 +538,8 @@
#define __GFP_WAIT 0x10
#define __GFP_HIGH 0x20
#define __GFP_IO 0x40
-#define __GFP_BUFFER 0x80

-#define GFP_BUFFER (__GFP_HIGH | __GFP_WAIT | __GFP_BUFFER)
+#define GFP_BUFFER (__GFP_HIGH | __GFP_WAIT)
#define GFP_ATOMIC (__GFP_HIGH)
#define GFP_USER ( __GFP_WAIT | __GFP_IO)
#define GFP_HIGHUSER ( __GFP_WAIT | __GFP_IO | __GFP_HIGHMEM)
--- 2.4.6pre6aa1/include/linux/slab.h.~1~ Sat Jun 30 00:49:56 2001
+++ 2.4.6pre6aa1/include/linux/slab.h Sat Jun 30 02:21:09 2001
@@ -22,7 +22,7 @@
#define SLAB_NFS GFP_NFS
#define SLAB_DMA GFP_DMA

-#define SLAB_LEVEL_MASK (__GFP_WAIT|__GFP_HIGH|__GFP_IO|__GFP_BUFFER)
+#define SLAB_LEVEL_MASK (__GFP_WAIT|__GFP_HIGH|__GFP_IO)
#define SLAB_NO_GROW 0x00001000UL /* don't grow a cache */

/* flags to pass to kmem_cache_create().
--- 2.4.6pre6aa1/mm/vmscan.c.~1~ Sat Jun 30 00:44:36 2001
+++ 2.4.6pre6aa1/mm/vmscan.c Sat Jun 30 02:21:25 2001
@@ -425,7 +425,6 @@
*/
#define MAX_LAUNDER (4 * (1 << page_cluster))
#define CAN_DO_IO (gfp_mask & __GFP_IO)
-#define CAN_DO_BUFFERS (gfp_mask & __GFP_BUFFER)
int page_launder(int gfp_mask, int sync)
{
int launder_loop, maxscan, cleaned_pages, maxlaunder;
@@ -613,7 +612,7 @@
* loads, flush out the dirty pages before we have to wait on
* IO.
*/
- if ((CAN_DO_IO || CAN_DO_BUFFERS) && !launder_loop && free_shortage()) {
+ if (CAN_DO_IO && !launder_loop && free_shortage()) {
launder_loop = 1;
/* If we cleaned pages, never do synchronous IO. */
if (cleaned_pages)

However the more finegrined fix that I found in 2.4.7pre2 and 2.4.6
seems broken unlike my above one.

getblk called from the fs calls grow_buffers with GFP_NOFS which doesn't
inhibit shrink_dcache_memory to re-enter the fs in prune_dcache because
__GFP_IO is set. And it will deadlock as usual when shrink_dcache reenter
the fs that way.

This should cure the deadlock (against 2.4.7pre2) but beware it's
untested.

--- 2.4.7pre2aa1/fs/dcache.c.~1~ Thu Jul 5 17:13:50 2001
+++ 2.4.7pre2aa1/fs/dcache.c Thu Jul 5 18:52:03 2001
@@ -566,7 +566,7 @@
* We should make sure we don't hold the superblock lock over
* block allocations, but for now:
*/
- if (!(gfp_mask & __GFP_IO))
+ if (!(gfp_mask & __GFP_FS))
return;

if (priority)
--- 2.4.7pre2aa1/fs/inode.c.~1~ Thu Jul 5 17:13:48 2001
+++ 2.4.7pre2aa1/fs/inode.c Thu Jul 5 18:52:08 2001
@@ -696,7 +696,7 @@
* want to recurse into the FS that called us
* in clear_inode() and friends..
*/
- if (!(gfp_mask & __GFP_IO))
+ if (!(gfp_mask & __GFP_FS))
return;

if (priority)

Secondly this cleanups a bit in page_launder. If we are allowed to enter
the FS we can certainly also do I/O.

--- 2.4.7pre2aa1/mm/vmscan.c.~1~ Thu Jul 5 17:13:48 2001
+++ 2.4.7pre2aa1/mm/vmscan.c Thu Jul 5 18:59:33 2001
@@ -612,7 +612,7 @@
* loads, flush out the dirty pages before we have to wait on
* IO.
*/
- if ((CAN_DO_IO || CAN_DO_FS) && !launder_loop && free_shortage()) {
+ if (CAN_DO_IO && !launder_loop && free_shortage()) {
launder_loop = 1;
/* If we cleaned pages, never do synchronous IO. */
if (cleaned_pages)
I will include them in my next tree as it is at least certainly better
than the previous code.

Andrea
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