mm: add vma_start_write_killable()

mm: add vma_start_write_killable()

Patch series "vma_start_write_killable"", v2.

When we added the VMA lock, we made a major oversight in not adding a
killable variant.  That can run us into trouble where a thread takes the
VMA lock for read (eg handling a page fault) and then goes out to lunch
for an hour (eg doing reclaim).  Another thread tries to modify the VMA,
taking the mmap_lock for write, then attempts to lock the VMA for write.
That blocks on the first thread, and ensures that every other page fault
now tries to take the mmap_lock for read.  Because everything's in an
uninterruptible sleep, we can't kill the task, which makes me angry.

This patchset just adds vma_start_write_killable() and converts one caller
to use it.  Most users are somewhat tricky to convert, so expect follow-up
individual patches per call-site which need careful analysis to make sure
we've done proper cleanup.

This patch (of 2):

The vma can be held read-locked for a substantial period of time, eg if
memory allocation needs to go into reclaim.  It's useful to be able to
send fatal signals to threads which are waiting for the write lock.

Link: https://lkml.kernel.org/r/20251110203204.1454057-1-willy@infradead.org
Link: https://lkml.kernel.org/r/20251110203204.1454057-2-willy@infradead.org
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Suren Baghdasaryan <surenb@google.com>
Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Chris Li <chriscli@google.com>
Cc: Jann Horn <jannh@google.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>