Re: [RFC PATCH] mm/filemap: avoid buffered read/write race to read inconsistent data

[Baokun Li]

On 2023/12/12 20:41, Jan Kara wrote:
> On Tue 12-12-23 17:36:34, Baokun Li wrote:
> > The following concurrency may cause the data read to be inconsistent with
> > the data on disk:
> >
> >               cpu1                           cpu2
> > ------------------------------|------------------------------
> >                                 // Buffered write 2048 from 0
> >                                 ext4_buffered_write_iter
> >                                  generic_perform_write
> >                                   copy_page_from_iter_atomic
> >                                   ext4_da_write_end
> >                                    ext4_da_do_write_end
> >                                     block_write_end
> >                                      __block_commit_write
> >                                       folio_mark_uptodate
> > // Buffered read 4096 from 0          smp_wmb()
> > ext4_file_read_iter                   set_bit(PG_uptodate, folio_flags)
> >   generic_file_read_iter            i_size_write // 2048
> >    filemap_read                     unlock_page(page)
> >     filemap_get_pages
> >      filemap_get_read_batch
> >      folio_test_uptodate(folio)
> >       ret = test_bit(PG_uptodate, folio_flags)
> >       if (ret)
> >        smp_rmb();
> >        // Ensure that the data in page 0-2048 is up-to-date.
> >
> >                                 // New buffered write 2048 from 2048
> >                                 ext4_buffered_write_iter
> >                                  generic_perform_write
> >                                   copy_page_from_iter_atomic
> >                                   ext4_da_write_end
> >                                    ext4_da_do_write_end
> >                                     block_write_end
> >                                      __block_commit_write
> >                                       folio_mark_uptodate
> >                                        smp_wmb()
> >                                        set_bit(PG_uptodate, folio_flags)
> >                                     i_size_write // 4096
> >                                     unlock_page(page)
> >
> >     isize = i_size_read(inode) // 4096
> >     // Read the latest isize 4096, but without smp_rmb(), there may be
> >     // Load-Load disorder resulting in the data in the 2048-4096 range
> >     // in the page is not up-to-date.
> >     copy_page_to_iter
> >     // copyout 4096
> >
> > In the concurrency above, we read the updated i_size, but there is no read
> > barrier to ensure that the data in the page is the same as the i_size at
> > this point, so we may copy the unsynchronized page out. Hence adding the
> > missing read memory barrier to fix this.
> >
> > This is a Load-Load reordering issue, which only occurs on some weak
> > mem-ordering architectures (e.g. ARM64, ALPHA), but not on strong
> > mem-ordering architectures (e.g. X86). And theoretically the problem
> AFAIK x86 can also reorder loads vs loads so the problem can in theory
> happen on x86 as well.
According to what I read in the perfbook at the link below,
 Loads Reordered After Loads does not happen on x86.
pdf sheet 562 corresponds to page 550,
Table 15.5: Summary of Memory Ordering
https://mirrors.edge.kernel.org/pub/linux/kernel/people/paulmck/perfbook/perfbook-1c.2023.06.11a.pdf 

> > doesn't only happen on ext4, filesystems that call filemap_read() but
> > don't hold inode lock (e.g. btrfs, f2fs, ubifs ...) will have this
> > problem, while filesystems with inode lock (e.g. xfs, nfs) won't have
> > this problem.
> >
> > Cc: stable@xxxxxxxxxx
> > Signed-off-by: Baokun Li <libaokun1@xxxxxxxxxx>
> > ---
> >   mm/filemap.c | 3 +++
> >   1 file changed, 3 insertions(+)
> >
> > diff --git a/mm/filemap.c b/mm/filemap.c
> > index 71f00539ac00..6324e2ac3e74 100644
> > --- a/mm/filemap.c
> > +++ b/mm/filemap.c
> > @@ -2607,6 +2607,9 @@ ssize_t filemap_read(struct kiocb *iocb, struct iov_iter *iter,
> >   			goto put_folios;
> >   		end_offset = min_t(loff_t, isize, iocb->ki_pos + iter->count);
> >   
> > +		/* Ensure that the page cache within isize is updated. */
> Barries have to be in pairs to work and it is a good practice to document
> this. So here I'd have comment like:
> 		/*
> 		 * Pairs with a barrier in
> 		 * block_write_end()->mark_buffer_dirty() or other page
> 		 * dirtying routines like iomap_write_end() to ensure
> 		 * changes to page contents are visible before we see
> 		 * increased inode size.
> 		 */
>
> 								Honza
That's a very accurate description! Thanks a lot!
I will add this comment in the next version.
> > +		smp_rmb();
> > +
> >   		/*
> >   		 * Once we start copying data, we don't want to be touching any
> >   		 * cachelines that might be contended:
> > --
> > 2.31.1
Thanks!
--
With Best Regards,
Baokun Li
.