Question about LTS 4.19 patch "89047634f5ce NFS: Don't interrupt file writeout due to fatal errors"

[ChenXiaoSong]

Hi Trond and Greg:

LTS 4.19 reported null-ptr-deref BUG as follows:

BUG: unable to handle kernel NULL pointer dereference at 0000000000000080
Call Trace:
 nfs_inode_add_request+0x1cc/0x5b8
 nfs_setup_write_request+0x1fa/0x1fc
 nfs_writepage_setup+0x2d/0x7d
 nfs_updatepage+0x8b8/0x936
 nfs_write_end+0x61d/0xd45
 generic_perform_write+0x19a/0x3f0
 nfs_file_write+0x2cc/0x6e5
 new_sync_write+0x442/0x560
 __vfs_write+0xda/0xef
 vfs_write+0x176/0x48b
 ksys_write+0x10a/0x1e9
 __se_sys_write+0x24/0x29
 __x64_sys_write+0x79/0x93
 do_syscall_64+0x16d/0x4bb
 entry_SYSCALL_64_after_hwframe+0x5c/0xc1

The reason is: generic_error_remove_page set page->mapping to NULL when 
nfs server have a fatal error:

nfs_updatepage
  nfs_writepage_setup
    nfs_setup_write_request
      nfs_try_to_update_request // return NULL
        nfs_wb_page // return 0
          nfs_writepage_locked // return 0
            nfs_do_writepage // return 0
              nfs_page_async_flush // return 0
                nfs_error_is_fatal_on_server
                generic_error_remove_page
                  truncate_inode_page
                    delete_from_page_cache
                      __delete_from_page_cache
                        page_cache_tree_delete
                          page->mapping = NULL // this is point
      nfs_create_request
        req->wb_page    = page // the page is freed
      nfs_inode_add_request
        mapping = page_file_mapping(req->wb_page)
          return page->mapping
        spin_lock(&mapping->private_lock) // mapping is NULL

It is reasonable by reverting the patch "89047634f5ce NFS: Don't 
interrupt file writeout due to fatal errors" to fix this bug?


This patch is one patch of patchset [Fix up soft mounts for 
NFSv4.x](https://lore.kernel.org/all/20190407175912.23528-1-trond.myklebust@xxxxxxxxxxxxxxx/), 
the patchset replace custom error reporting mechanism. it seams that we 
should merge all the patchset to LTS 4.19, or all patchs should not be 
merged. And the "Fixes:" label is not correct, this patch is a 
refactoring patch, not for fixing bugs.