Commit 33bfad54 authored Jan 30, 2009 by Linus Torvalds

Allow opportunistic merging of VM_CAN_NONLINEAR areas



Commit de33c8db ("Fix OOPS in
mmap_region() when merging adjacent VM_LOCKED file segments") unified
the vma merging of anonymous and file maps to just one place, which
simplified the code and fixed a use-after-free bug that could cause an
oops.

But by doing the merge opportunistically before even having called
->mmap() on the file method, it now compares two different 'vm_flags'
values: the pre-mmap() value of the new not-yet-formed vma, and previous
mappings of the same file around it.

And in doing so, it refused to merge the common file case, which adds a
marker to say "I can be made non-linear".

This fixes it by just adding a set of flags that don't have to match,
because we know they are ok to merge.  Currently it's only that single
VM_CAN_NONLINEAR flag, but at least conceptually there could be others
in the future.

Reported-and-acked-by: Hugh Dickins <hugh@veritas.com>
Cc: Lee Schermerhorn <Lee.Schermerhorn@hp.com>
Cc: Nick Piggin <npiggin@suse.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Greg KH <gregkh@suse.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent c01a25e7

mm/mmap.c

+4 −1

Original line number	Diff line number	Diff line
		@@ -658,6 +658,9 @@ again: remove_next = 1 + (end > next->vm_end);
		validate_mm(mm);
		}

		/* Flags that can be inherited from an existing mapping when merging */
		#define VM_MERGEABLE_FLAGS (VM_CAN_NONLINEAR)

		/*
		* If the vma has a ->close operation then the driver probably needs to release
		* per-vma resources, so we don't attempt to merge those.
		@@ -665,7 +668,7 @@ again: remove_next = 1 + (end > next->vm_end);
		static inline int is_mergeable_vma(struct vm_area_struct *vma,
		struct file *file, unsigned long vm_flags)
		{
		if (vma->vm_flags != vm_flags)
		if ((vma->vm_flags ^ vm_flags) & ~VM_MERGEABLE_FLAGS)
		return 0;
		if (vma->vm_file != file)
		return 0;