On Thu, Mar 24, 2016 at 12:08:58PM +0100, Nicolai Stange wrote: > If > - generic_file_read_iter() gets called with a zero read length, > - the read offset is at a page boundary, > - IOCB_DIRECT is not set > - and the page in question hasn't made it into the page cache yet, > then do_generic_file_read() will trigger a readahead with a req_size hint > of zero. > > Since roundup_pow_of_two(0) is undefined, UBSAN reports > > UBSAN: Undefined behaviour in include/linux/log2.h:63:13 > shift exponent 64 is too large for 64-bit type 'long unsigned int' > CPU: 3 PID: 1017 Comm: sa1 Tainted: G L 4.5.0-next-20160318+ #14 > [...] > Call Trace: > [...] > [<ffffffff813ef61a>] ondemand_readahead+0x3aa/0x3d0 > [<ffffffff813ef61a>] ? ondemand_readahead+0x3aa/0x3d0 > [<ffffffff813c73bd>] ? find_get_entry+0x2d/0x210 > [<ffffffff813ef9c3>] page_cache_sync_readahead+0x63/0xa0 > [<ffffffff813cc04d>] do_generic_file_read+0x80d/0xf90 > [<ffffffff813cc955>] generic_file_read_iter+0x185/0x420 > [...] > [<ffffffff81510b06>] __vfs_read+0x256/0x3d0 > [...] > > when get_init_ra_size() gets called from ondemand_readahead(). > > The net effect is that the initial readahead size is arch dependent for > requested read lengths of zero: for example, since > > 1UL << (sizeof(unsigned long) * 8) > > evaluates to 1 on x86 while its result is 0 on ARMv7, the initial readahead > size becomes 4 on the former and 0 on the latter. > > What's more, whether or not the file access timestamp is updated for zero > length reads is decided differently for the two cases of IOCB_DIRECT > being set or cleared: in the first case, generic_file_read_iter() > explicitly skips updating that timestamp while in the latter case, it is > always updated through the call to do_generic_file_read(). > > According to POSIX, zero length reads "do not modify the last data access > timestamp" and thus, the IOCB_DIRECT behaviour is POSIXly correct. > > Let generic_file_read_iter() unconditionally check the requested read > length at its entry and return immediately with success if it is zero. > > Signed-off-by: Nicolai Stange <nicstange@xxxxxxxxx> Acked-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx> -- Mel Gorman SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>