Because we need to hold the i_mmap_sem until the page is inserted into the page tables to avoid racing with truncate. Therefore it is unlocked by the MM code.
Did you try this patch? It should have BUGged immediately that you hit this case. If not, you have insufficient CONFIG_DEBUG enabled.
________________________________________
From: Jared Hulbert [jaredeh@xxxxxxxxx]
Sent: January 29, 2016 1:38 PM
To: linux-fsdevel@xxxxxxxxxxxxxxx; Wilcox, Matthew R; ross.zwisler@xxxxxxxxxxxxxxx
Subject: DAX: bug in COW no page fault?
Why isn't this required?
diff --git a/fs/dax.c b/fs/dax.c
index a7f77e1..30f2abe 100644
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -416,6 +416,7 @@ int __dax_fault(struct vm_area_struct *vma, struct vm_fault
error = -EIO;
goto out;
}
+ i_mmap_unlock_read(mapping);
}
return VM_FAULT_LOCKED;
}
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