[RFC] vmalloc: add warning in __vmalloc

[Minchan Kim]

Now there are several places to use __vmalloc with GFP_ATOMIC,
GFP_NOIO, GFP_NOFS but unfortunately __vmalloc calls map_vm_area
which calls alloc_pages with GFP_KERNEL to allocate page tables.
It means it's possible to happen deadlock.
I don't know why it doesn't have reported until now.

Firstly, I tried passing gfp_t to lower functions to support __vmalloc
with such flags but other mm guys don't want and decided that
all of caller should be fixed.

http://marc.info/?l=linux-kernel&m=133517143616544&w=2

To begin with, let's listen other's opinion whether they can fix it
by other approach without calling __vmalloc with such flags.

So this patch adds warning to detect and to be fixed hopely.
I Cced related maintainers.
If I miss someone, please Cced them.

side-note:
  I added WARN_ON instead of WARN_ONCE to detect all of callers
  and each WARN_ON for each flag to detect to use any flag easily.
  After we fix all of caller or reduce such caller, we can merge
  a warning with WARN_ONCE.

Cc: Neil Brown <neilb@xxxxxxx>
Cc: Artem Bityutskiy <dedekind1@xxxxxxxxx>
Cc: David Woodhouse <dwmw2@xxxxxxxxxxxxx>
Cc: "Theodore Ts'o" <tytso@xxxxxxx>
Cc: Adrian Hunter <adrian.hunter@xxxxxxxxx>
Cc: Steven Whitehouse <swhiteho@xxxxxxxxxx>
Cc: "David S. Miller" <davem@xxxxxxxxxxxxx>
Cc: James Morris <jmorris@xxxxxxxxx>
Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: Sage Weil <sage@xxxxxxxxxxxx>
Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx>
---
 mm/vmalloc.c |    9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index 94dff88..36beccb 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -1700,6 +1700,15 @@ static void *__vmalloc_node(unsigned long size, unsigned long align,
 			    gfp_t gfp_mask, pgprot_t prot,
 			    int node, void *caller)
 {
+	/*
+	 * This function calls map_vm_area so that it allocates
+	 * page table with GFP_KERNEL so caller should avoid using
+	 * GFP_NOIO, GFP_NOFS and !__GFP_WAIT.
+	 */
+	WARN_ON(!(gfp_mask & __GFP_WAIT));
+	WARN_ON(!(gfp_mask & __GFP_IO));
+	WARN_ON(!(gfp_mask & __GFP_FS));
+
 	return __vmalloc_node_range(size, align, VMALLOC_START, VMALLOC_END,
 				gfp_mask, prot, node, caller);
 }
-- 
1.7.9.5

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