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 problemAFAIK 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
--
With Best Regards,
Baokun Li
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