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. > 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 > + smp_rmb(); > + > /* > * Once we start copying data, we don't want to be touching any > * cachelines that might be contended: > -- > 2.31.1 > -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR
