mm: abstract the vma_merge()/split_vma() pattern for mprotect() et al.

mprotect() and other functions which change VMA parameters over a range
each employ a pattern of:-

1. Attempt to merge the range with adjacent VMAs.
2. If this fails, and the range spans a subset of the VMA, split it
   accordingly.

This is open-coded and duplicated in each case. Also in each case most of
the parameters passed to vma_merge() remain the same.

Create a new function, vma_modify(), which abstracts this operation,
accepting only those parameters which can be changed.

To avoid the mess of invoking each function call with unnecessary
parameters, create inline wrapper functions for each of the modify
operations, parameterised only by what is required to perform the action.

We can also significantly simplify the logic - by returning the VMA if we
split (or merged VMA if we do not) we no longer need specific handling for
merge/split cases in any of the call sites.

Note that the userfaultfd_release() case works even though it does not
split VMAs - since start is set to vma->vm_start and end is set to
vma->vm_end, the split logic does not trigger.

In addition, since we calculate pgoff to be equal to vma->vm_pgoff + (start
- vma->vm_start) >> PAGE_SHIFT, and start - vma->vm_start will be 0 in this
instance, this invocation will remain unchanged.

We eliminate a VM_WARN_ON() in mprotect_fixup() as this simply asserts that
vma_merge() correctly ensures that flags remain the same, something that is
already checked in is_mergeable_vma() and elsewhere, and in any case is not
specific to mprotect().

Link: https://lkml.kernel.org/r/0dfa9368f37199a423674bf0ee312e8ea0619044.1697043508.git.lstoakes@gmail.com
Signed-off-by: Lorenzo Stoakes <lstoakes@gmail.com>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Liam R. Howlett <Liam.Howlett@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 4 insertions, 25 deletions

diff --git a/mm/mprotect.c b/mm/mprotect.c
index 03e2cec3e669..f1dc8f8c84ef 100644
--- a/mm/mprotect.c
+++ b/mm/mprotect.c
@@ -581,7 +581,6 @@ mprotect_fixup(struct vma_iterator *vmi, struct mmu_gather *tlb,
 	long nrpages = (end - start) >> PAGE_SHIFT;
 	unsigned int mm_cp_flags = 0;
 	unsigned long charged = 0;
-	pgoff_t pgoff;
 	int error;
 
 	if (newflags == oldflags) {
@@ -631,34 +630,14 @@ mprotect_fixup(struct vma_iterator *vmi, struct mmu_gather *tlb,
 		newflags &= ~VM_ACCOUNT;
 	}
 
-	/*
-	 * First try to merge with previous and/or next vma.
-	 */
-	pgoff = vma->vm_pgoff + ((start - vma->vm_start) >> PAGE_SHIFT);
-	*pprev = vma_merge(vmi, mm, *pprev, start, end, newflags,
-			   vma->anon_vma, vma->vm_file, pgoff, vma_policy(vma),
-			   vma->vm_userfaultfd_ctx, anon_vma_name(vma));
-	if (*pprev) {
-		vma = *pprev;
-		VM_WARN_ON((vma->vm_flags ^ newflags) & ~VM_SOFTDIRTY);
-		goto success;
+	vma = vma_modify_flags(vmi, *pprev, vma, start, end, newflags);
+	if (IS_ERR(vma)) {
+		error = PTR_ERR(vma);
+		goto fail;
 	}
 
 	*pprev = vma;
 
-	if (start != vma->vm_start) {
-		error = split_vma(vmi, vma, start, 1);
-		if (error)
-			goto fail;
-	}
-
-	if (end != vma->vm_end) {
-		error = split_vma(vmi, vma, end, 0);
-		if (error)
-			goto fail;
-	}
-
-success:
 	/*
 	 * vm_flags and vm_page_prot are protected by the mmap_lock
 	 * held in write mode.