From: Leon Yu <[email protected]>
This patch has been added to the 3.18 stable tree. If you have any
objections, please let us know.
===============
[ Upstream commit 3fe89b3e2a7bbf3e97657104b9b33a9d81b950b3 ]
I have constantly stumbled upon "kernel BUG at mm/rmap.c:399!" after
upgrading to 3.19 and had no luck with 4.0-rc1 neither.
So, after looking into new logic introduced by commit 7a3ef208e662 ("mm:
prevent endless growth of anon_vma hierarchy"), I found chances are that
unlink_anon_vmas() is called without incrementing dst->anon_vma->degree
in anon_vma_clone() due to allocation failure. If dst->anon_vma is not
NULL in error path, its degree will be incorrectly decremented in
unlink_anon_vmas() and eventually underflow when exiting as a result of
another call to unlink_anon_vmas(). That's how "kernel BUG at
mm/rmap.c:399!" is triggered for me.
This patch fixes the underflow by dropping dst->anon_vma when allocation
fails. It's safe to do so regardless of original value of dst->anon_vma
because dst->anon_vma doesn't have valid meaning if anon_vma_clone()
fails. Besides, callers don't care dst->anon_vma in such case neither.
Also suggested by Michal Hocko, we can clean up vma_adjust() a bit as
anon_vma_clone() now does the work.
[[email protected]: tweak comment]
Fixes: 7a3ef208e662 ("mm: prevent endless growth of anon_vma hierarchy")
Signed-off-by: Leon Yu <[email protected]>
Signed-off-by: Konstantin Khlebnikov <[email protected]>
Reviewed-by: Michal Hocko <[email protected]>
Acked-by: Rik van Riel <[email protected]>
Acked-by: David Rientjes <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>
Signed-off-by: Sasha Levin <[email protected]>
---
mm/mmap.c | 2 +-
mm/rmap.c | 7 +++++++
2 files changed, 8 insertions(+), 1 deletion(-)
diff --git a/mm/mmap.c b/mm/mmap.c
index 3c83bec..f88b4f9 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -778,10 +778,10 @@ again: remove_next = 1 + (end >
next->vm_end);
if (exporter && exporter->anon_vma && !importer->anon_vma) {
int error;
+ importer->anon_vma = exporter->anon_vma;
error = anon_vma_clone(importer, exporter);
if (error)
return error;
- importer->anon_vma = exporter->anon_vma;
}
}
diff --git a/mm/rmap.c b/mm/rmap.c
index 3e4c721..eeafed7 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -261,6 +261,13 @@ int anon_vma_clone(struct vm_area_struct *dst, struct
vm_area_struct *src)
return 0;
enomem_failure:
+ /*
+ * dst->anon_vma is dropped here otherwise its degree can be incorrectly
+ * decremented in unlink_anon_vmas().
+ * We can safely do this because callers of anon_vma_clone() don't care
+ * about dst->anon_vma if anon_vma_clone() failed.
+ */
+ dst->anon_vma = NULL;
unlink_anon_vmas(dst);
return -ENOMEM;
}
--
2.1.0
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