Re: nfs_page_async_flush returning 0 for fatal errors on writeback

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On 08/07/2021 12:50 am, Trond Myklebust wrote:
On Thu, 2021-07-08 at 00:13 +0100, Calum Mackay wrote:
On 07/07/2021 11:01 pm, Trond Myklebust wrote:
On Wed, 2021-07-07 at 19:51 +0100, Calum Mackay wrote:
hi Trond,

I had a question about these two old commits of yours, from v5.0
&
v5.2:

14bebe3c90b3 NFS: Don't interrupt file writeout due to fatal
errors
(2
years, 2 months ago)

8fc75bed96bb NFS: Fix up return value on fatal errors in
nfs_page_async_flush() (2 years, 5 months ago)


I am looking at a crash dump, with a kernel based on an older-
still
v4.14 stable which did not have either of the above commits.

          PANIC: "BUG: unable to handle kernel NULL pointer
dereference
at
0000000000000080"

       [exception RIP: _raw_spin_lock+20]

#10 [ffffb1493d78fcb8] nfs_updatepage at ffffffffc08f1791 [nfs]
#11 [ffffb1493d78fd10] nfs_write_end at ffffffffc08e094e [nfs]
#12 [ffffb1493d78fd58] generic_perform_write at ffffffffa71d458b
#13 [ffffb1493d78fde0] nfs_file_write at ffffffffc08dfdb4 [nfs]
#14 [ffffb1493d78fe18] __vfs_write at ffffffffa72848bc
#15 [ffffb1493d78fea0] vfs_write at ffffffffa7284ad2
#16 [ffffb1493d78fee0] sys_write at ffffffffa7284d35
#17 [ffffb1493d78ff28] do_syscall_64 at ffffffffa7003949

the real sequence, obscured by compiler inlining, is:

      nfs_updatepage
         nfs_writepage_setup
            nfs_setup_write_request
               nfs_inode_add_request
                  spin_lock(&mapping->private_lock);

and we crash since the as mapping pointer is NULL.


I thought I was able to construct a possible sequence that would
explain
the above, if we are in (from above):

      nfs_setup_write_request
       nfs_try_to_update_request
        nfs_wb_page
         nfs_writepage_locked
          nfs_do_writepage

and nfs_page_async_flush detects a fatal server error, and calls
nfs_write_error_remove_page, which results in the page->mapping
set
to NULL.

In that version of the code, without your commits above,
nfs_page_async_flush returns 0 in this case, which I thought
might
result in nfs_setup_write_request going ahead and calling
nfs_inode_add_request with that page, resulting in the crash
seen.


I then discovered your v5.0 commit:

8fc75bed96bb NFS: Fix up return value on fatal errors in
nfs_page_async_flush() (2 years, 5 months ago)

which appeared to correct that, having nfs_page_async_flush
return
the
error in this case, so we would not end up in
nfs_inode_add_request.


But I then spotted your later v5.2 commit:

14bebe3c90b3 NFS: Don't interrupt file writeout due to fatal
errors
(2
years, 2 months ago)

which changes things back, so that nfs_page_async_flush now again
returns 0, in the "launder" case, and that's how that code
remains
today.


If so, is there anything to stop the possible crash path that I
describe
above?


path I suggest above? Or perhaps I'm missing another commit that
stops
it happening, even after your second commit above?


In order for page->mapping to get set to NULL, we'd have to be
removing
the page from the page cache altogether. I'm not seeing where we'd
be
doing that here. It certainly isn't possible for some third party
to do
so, since our thread is holding the page lock and I'm not seeing
where
the call to nfs_write_error() might be doing so.

We do call nfs_inode_remove_request(), which removes the struct
nfs_page that is tracking the page dirtiness, however it shouldn't
ever
result in the removal of the pagecache page itself.

Am I misreading your email?

thanks very much Trond; much more likely I am misreading the code :)


My theory was that we have nfs_page_async_flush detecting
nfs_error_is_fatal_on_server, so calling nfs_write_error_remove_page
(this is an older v4.14.72-ish kernel).

That would then generic_error_remove_page -> truncate_inode_page ->
truncate_complete_page -> delete_from_page_cache

thus, as you say, removing the page from the page cache, with
__delete_from_page_cache clearing page->mapping.


Without your v5.0 commit, nfs_page_async_flush will then return 0,
via
nfs_do_writepage, nfs_writepage_locked, nfs_wb_page to
nfs_try_to_update_request, which then returns NULL to
nfs_setup_write_request.

nfs_inode_add_request, which itself then dereferences the mapping:

         spin_lock(&mapping->private_lock);

which is where we crash.


Obviously, there are a number of assumptions in the above, so I
thought
it must just be a possible path the code could take?

Does that sound plausible (given that v4.14.72-ish code)?



However, I note that in a subsequent v5.2 commit:

22876f540bdf NFS: Don't call generic_error_remove_page() while
holding locks

you remove the call to generic_error_remove_page from
nfs_write_error_remove_page(), and that is itself then renamed
nfs_write_error(), as part of a later v5.2 commit:

6fbda89b257f NFS: Replace custom error reporting mechanism with
generic one


Without those commits, and also without your adjustments to
nfs_page_async_flush I mentioned earlier, is it possible that the
code
path I present above, where the page /is/ removed from the page
cache,
could result in the crash we saw?



OK, yes that is plausible. The call to generic_error_remove_page()
would, as you said, remove the page from the page cache, and thus could
result in the crash you describe.


that's great, thanks very much indeed for the confirmation, Trond.

sorry to waste your time with older code…

cheers,
calum.

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