On Sat, 14 May 2016 00:25:28 +0800 Eryu Guan <guaneryu@xxxxxxxxx> wrote: > Currently direct writes inside i_size on a DIO_SKIP_HOLES filesystem are > not allowed to allocate blocks(get_more_blocks() sets 'create' to 0 > before calling get_block() callback), if it's a sparse file, direct > writes fall back to buffered writes to avoid stale data exposure from > concurrent buffered read. But there're two cases that can result in > stale data exposure are not correctly detected. > > 1. The detection for "writing inside i_size" is not sufficient, writes > can be treated as "extending writes" wrongly. For example, direct write > 1FSB to a 1FSB sparse file on ext2/3/4, starting from offset 0, in this > case it's writing inside i_size, but 'create' is non-zero, because > 'block_in_file' and '(i_size_read(inode) >> blkbits' are both zero. um, what is an "FSB"? > 2. Direct writes starting from or beyong i_size (not inside i_size) also > could trigger block allocation and expose stale data. For example, > consider a sparse file with i_size of 2k, and a write to offset 2k or 3k > into the file, with a filesystem block size of 4k. (Thanks to Jeff Moyer > for pointing this case out in his review.) > > The first problem can be demostrated by running ltp-aiodio test ADSP045 > many times. When testing on extN filesystems, I see test failures > occasionally, buffered read could read non-zero (stale) data. > > ADSP045: dio_sparse -a 4k -w 4k -s 2k -n 1 > > dio_sparse 0 TINFO : Dirtying free blocks > dio_sparse 0 TINFO : Starting I/O tests > non zero buffer at buf[0] => 0xffffffaa,ffffffaa,ffffffaa,ffffffaa > non-zero read at offset 0 > dio_sparse 0 TINFO : Killing childrens(s) > dio_sparse 1 TFAIL : dio_sparse.c:191: 1 children(s) exited abnormally > > The second problem can also be reproduced easily by a hacked dio_sparse > program, which accepts an option to specify the write offset. > > What we should really do is to disable block allocation for writes that > could result in filling holes inside i_size. > -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
