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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