• David Howells's avatar
    [PATCH] add page_mkwrite() vm_operations method · 9637a5ef
    David Howells authored
    Add a new VMA operation to notify a filesystem or other driver about the
    MMU generating a fault because userspace attempted to write to a page
    mapped through a read-only PTE.
    This facility permits the filesystem or driver to:
     (*) Implement storage allocation/reservation on attempted write, and so to
         deal with problems such as ENOSPC more gracefully (perhaps by generating
     (*) Delay making the page writable until the contents have been written to a
         backing cache. This is useful for NFS/AFS when using FS-Cache/CacheFS.
         It permits the filesystem to have some guarantee about the state of the
     (*) Account and limit number of dirty pages. This is one piece of the puzzle
         needed to make shared writable mapping work safely in FUSE.
    Needed by cachefs (Or is it cachefiles?  Or fscache? <head spins>).
    At least four other groups have stated an interest in it or a desire to use
    the functionality it provides: FUSE, OCFS2, NTFS and JFFS2.  Also, things like
    EXT3 really ought to use it to deal with the case of shared-writable mmap
    encountering ENOSPC before we permit the page to be dirtied.
    From: Peter Zijlstra <a.p.zijlstra@chello.nl>
      get_user_pages(.write=1, .force=1) can generate COW hits on read-only
      shared mappings, this patch traps those as mkpage_write candidates and fails
      to handle them the old way.
    Signed-off-by: default avatarDavid Howells <dhowells@redhat.com>
    Cc: Miklos Szeredi <miklos@szeredi.hu>
    Cc: Joel Becker <Joel.Becker@oracle.com>
    Cc: Mark Fasheh <mark.fasheh@oracle.com>
    Cc: Anton Altaparmakov <aia21@cantab.net>
    Cc: David Woodhouse <dwmw2@infradead.org>
    Cc: Hugh Dickins <hugh@veritas.com>
    Signed-off-by: default avatarAndrew Morton <akpm@osdl.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@osdl.org>
mprotect.c 7.2 KB