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.
