mm/mremap: perform some simple cleanups

Patch series "mm/mremap: permit mremap() move of multiple VMAs", v4.

Historically we've made it a uAPI requirement that mremap() may only
operate on a single VMA at a time.

For instances where VMAs need to be resized, this makes sense, as it
becomes very difficult to determine what a user actually wants should they
indicate a desire to expand or shrink the size of multiple VMAs (truncate?
Adjust sizes individually? Some other strategy?).

However, in instances where a user is moving VMAs, it is restrictive to
disallow this.

This is especially the case when anonymous mapping remap may or may not be
mergeable depending on whether VMAs have or have not been faulted due to
anon_vma assignment and folio index alignment with vma->vm_pgoff.

Often this can result in surprising impact where a moved region is faulted,
then moved back and a user fails to observe a merge from otherwise
compatible, adjacent VMAs.

This change allows such cases to work without the user having to be
cognizant of whether a prior mremap() move or other VMA operations has
resulted in VMA fragmentation.

In order to do this, this series performs a large amount of refactoring,
most pertinently - grouping sanity checks together, separately those that
check input parameters and those relating to VMAs.

we also simplify the post-mmap lock drop processing for uffd and mlock()'d
VMAs.

With this done, we can then fairly straightforwardly implement this
functionality.

This works exclusively for mremap() invocations which specify
MREMAP_FIXED. It is not compatible with VMAs which use userfaultfd, as the
notification of the userland fault handler would require us to drop the
mmap lock.

It is also not compatible with file-backed mappings with customised
get_unmapped_area() handlers as these may not honour MREMAP_FIXED.

The input and output addresses ranges must not overlap. We carefully
account for moves which would result in VMA iterator invalidation.

While there can be gaps between VMAs in the input range, there can be no
gap before the first VMA in the range.


This patch (of 10):

We const-ify the vrm flags parameter to indicate this will never change.

We rename resize_is_valid() to remap_is_valid(), as this function does not
only apply to cases where we resize, so it's simply confusing to refer to
that here.

We remove the BUG() from mremap_at(), as we should not BUG() unless we are
certain it'll result in system instability.

We rename vrm_charge() to vrm_calc_charge() to make it clear this simply
calculates the charged number of pages rather than actually adjusting any
state.

We update the comment for vrm_implies_new_addr() to explain that
MREMAP_DONTUNMAP does not require a set address, but will always be moved.

Additionally consistently use 'res' rather than 'ret' for result values.

No functional change intended.

Link: https://lkml.kernel.org/r/cover.1752770784.git.lorenzo.stoakes@oracle.com
Link: https://lkml.kernel.org/r/d35ad8ce6b2c33b2f2f4ef7ec415f04a35cba34f.1752770784.git.lorenzo.stoakes@oracle.com
Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Jan Kara <jack@suse.cz>
Cc: Jann Horn <jannh@google.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Rik van Riel <riel@surriel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>