Re: [PATCH]: buddy allocator: ext3 failed to alloc with __GFP_NOFAIL

[Kirill Korotaev]

Hello,

as Andrey Savochkin pointed to me, the previous patch is incorrect since 
makes allocations with PF_MEMALLOC to be always success for order <= 3 
which is not what we usually want.
I remade the patch to be more explicit about the issue - now it retries 
allocation _only_ if __GFP_NOFAIL is set.

------------- original comment below ------------------

we had the following ext3 problems once during stress testing (on 2.6.8):
-----------------------------------------------------------------
journal_get_undo_access: No memory for committed data
ext3_free_blocks: aborting transaction: Out of memory in 
__ext3_journal_get_undo_access<2>EXT3-fs error (device hda7) in 
ext3_free_blocks: Out of memory
Aborting journal on device hda7.
EXT3-fs error (device hda7) in ext3_ordered_commit_write: IO failure
ext3_abort called.
EXT3-fs abort (device hda7): ext3_journal_start: Detected aborted journal
Remounting filesystem read-only
ext3_free_blocks: aborting transaction: Journal has aborted in 
__ext3_journal_get_undo_access<2>EXT3-fs error (device hda7) in 
ext3_free_blocks: Journal has aborted
ext3_reserve_inode_write: aborting transaction: Journal has aborted in 
__ext3_journal_get_write_access<2>EXT3-fs error (device hda7) in 
ext3_reserve_inode_write: Journal has aborted
EXT3-fs error (device hda7) in ext3_truncate: Out of memory
ext3_reserve_inode_write: aborting transaction: Journal has aborted in 
__ext3_journal_get_write_access<2>EXT3-fs error (device hda7) in 
ext3_reserve_inode_write: Journal has aborted
EXT3-fs error (device hda7) in ext3_orphan_del: Journal has aborted
ext3_reserve_inode_write: aborting transaction: Journal has aborted in 
__ext3_journal_get_write_access<2>EXT3-fs error (device hda7) in 
ext3_reserve_inode_write: Journal has aborted
EXT3-fs error (device hda7) in ext3_delete_inode: Out of memory
__journal_remove_journal_head: freeing b_committed_data
..................
------------------------------------------------------------------

As it is seen from the messages journal_get_undo_access() failed to 
allocate some memory with jbd_kmalloc(), which in turn called kmalloc() 
with __GFP_NOFAIL flag.

How could it happen?
The only possible reason for this we suppose is a piece of code in 
__alloc_pages():

if ((p->flags & (PF_MEMALLOC | PF_MEMDIE)) && !in_interrupt()) {
        /* go through the zonelist yet again, ignoring mins */
        for (i = 0; zones[i] != NULL; i++) {
                struct zone *z = zones[i];

                page = buffered_rmqueue(z, order, gfp_mask);
                if (page) {
                        zone_statistics(zonelist, z);
                        goto got_pg;
                }
        }
        goto nopage;                <<<< HERE!!! FAIL...
}


So kswapd (which has PF_MEMALLOC flag) can fail to allocate memory even 
when it allocates it with __GFP_NOFAIL flag.

Signed-Off-By: Pavel Emelianov <xemul@xxxxx>
Signed-Off-By: Denis Lunev <den@xxxxx>
Signed-Off-By: Kirill Korotaev <dev@xxxxx>

The attached patch should fix the problem (against 2.6.14).

Kirill
--- ./mm/page_alloc.c.alpg	2005-11-09 21:42:50.000000000 +0300
+++ ./mm/page_alloc.c	2005-11-09 21:44:22.000000000 +0300
@@ -870,6 +870,7 @@ zone_reclaim_retry:
 	if (((p->flags & PF_MEMALLOC) || unlikely(test_thread_flag(TIF_MEMDIE)))
 			&& !in_interrupt()) {
 		if (!(gfp_mask & __GFP_NOMEMALLOC)) {
+nofail_alloc:
 			/* go through the zonelist yet again, ignoring mins */
 			for (i = 0; (z = zones[i]) != NULL; i++) {
 				if (!cpuset_zone_allowed(z, gfp_mask))
@@ -878,6 +879,10 @@ zone_reclaim_retry:
 				if (page)
 					goto got_pg;
 			}
+			if (gfp_mask & __GFP_NOFAIL) {
+				blk_congestion_wait(WRITE, HZ/50);
+				goto nofail_alloc;
+			}
 		}
 		goto nopage;
 	}