Subject: Bad use of highmem with buffer_migrate_page?

From: [email-masked] (Laura Abbott)
Date: Thu, 10 May 2012 13:07:41 -0700

Hi,

I did a backport of the Contiguous Memory Allocator to a 3.0.8 tree. I 
wrote fairly simple test case that, in 1MB chunks, allocs up to 40MB 
from a reserved area, maps, writes, unmaps and then frees in an infinite 
loop. When running this with another program in parallel to put some 
stress on the filesystem, I hit data aborts in the filesystem/journal 
layer, although not always the same backtrace. As an example:

[<c02907a4>] (__ext4_check_dir_entry+0x20/0x184) from [<c029e1a8>] 
(add_dirent_to_buf+0x70/0x2ac)
[<c029e1a8>] (add_dirent_to_buf+0x70/0x2ac) from [<c029f3f0>] 
(ext4_add_entry+0xd8/0x4bc)
[<c029f3f0>] (ext4_add_entry+0xd8/0x4bc) from [<c029fe90>] 
(ext4_add_nondir+0x14/0x64)
[<c029fe90>] (ext4_add_nondir+0x14/0x64) from [<c02a04c4>] 
(ext4_create+0xd8/0x120)
[<c02a04c4>] (ext4_create+0xd8/0x120) from [<c022e134>] 
(vfs_create+0x74/0xa4)
[<c022e134>] (vfs_create+0x74/0xa4) from [<c022ed3c>] (do_last+0x588/0x8d4)
[<c022ed3c>] (do_last+0x588/0x8d4) from [<c022fe64>] 
(path_openat+0xc4/0x394)
[<c022fe64>] (path_openat+0xc4/0x394) from [<c0230214>] 
(do_filp_open+0x30/0x7c)
[<c0230214>] (do_filp_open+0x30/0x7c) from [<c0220cb4>] 
(do_sys_open+0xd8/0x174)
[<c0220cb4>] (do_sys_open+0xd8/0x174) from [<c0105ea0>] 
(ret_fast_syscall+0x0/0x30)

Every panic had the same issue where a struct buffer_head [1] had a 
b_data that was unexpectedly NULL.

During the course of CMA, buffer_migrate_page could be called to migrate 
from a CMA page to a new page. buffer_migrate_page calls set_bh_page[2] 
to set the new page for the buffer_head. If the new page is a highmem 
page though, the bh->b_data ends up as NULL, which could produce the 
panics seen above.

This seems to indicate that highmem pages are not not appropriate for 
use as pages to migrate to. The following made the problem go away for me:

--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -5753,7 +5753,7 @@ static struct page *
  __alloc_contig_migrate_alloc(struct page *page, unsigned long private,
                              int **resultp)
  {
-       return alloc_page(GFP_HIGHUSER_MOVABLE);
+       return alloc_page(GFP_USER | __GFP_MOVABLE);
  }

Does this seem like an actual issue or is this an artifact of my 
backport to 3.0? I'm not familiar enough with the filesystem layer to be 
able to tell where highmem can actually be used.

Thanks,
Laura

[1] http://lxr.free-electrons.com/source/include/linux/buffer_head.h#L59
[2] http://lxr.free-electrons.com/source/fs/buffer.c?v=3.0#L1441
-- 
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