From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ot0-f194.google.com ([74.125.82.194]:34217 "EHLO mail-ot0-f194.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S933683AbeEWQjF (ORCPT ); Wed, 23 May 2018 12:39:05 -0400 Received: by mail-ot0-f194.google.com with SMTP id i5-v6so25979151otf.1 for ; Wed, 23 May 2018 09:39:05 -0700 (PDT) MIME-Version: 1.0 In-Reply-To: <20180523163442.GB29519@linux.intel.com> References: <152539236455.31796.7516599166555186700.stgit@dwillia2-desk3.amr.corp.intel.com> <152539240242.31796.4162676712193177396.stgit@dwillia2-desk3.amr.corp.intel.com> <20180523163442.GB29519@linux.intel.com> From: Dan Williams Date: Wed, 23 May 2018 09:39:04 -0700 Message-ID: Subject: Re: [PATCH v3 7/9] dax: report bytes remaining in dax_iomap_actor() To: Ross Zwisler , Dan Williams , linux-nvdimm , "Luck, Tony" , Peter Zijlstra , Linux Kernel Mailing List , X86 ML , Christoph Hellwig , Andy Lutomirski , Ingo Molnar , Borislav Petkov , Al Viro , linux-fsdevel , Thomas Gleixner , Linus Torvalds , Andrew Morton Content-Type: text/plain; charset="UTF-8" Sender: linux-fsdevel-owner@vger.kernel.org List-ID: On Wed, May 23, 2018 at 9:34 AM, Ross Zwisler wrote: > On Thu, May 03, 2018 at 05:06:42PM -0700, Dan Williams wrote: >> In preparation for protecting the dax read(2) path from media errors >> with copy_to_iter_mcsafe() (via dax_copy_to_iter()), convert the >> implementation to report the bytes successfully transferred. >> >> Cc: >> Cc: Ingo Molnar >> Cc: Borislav Petkov >> Cc: Tony Luck >> Cc: Al Viro >> Cc: Thomas Gleixner >> Cc: Andy Lutomirski >> Cc: Peter Zijlstra >> Cc: Andrew Morton >> Cc: Linus Torvalds >> Signed-off-by: Dan Williams >> --- >> fs/dax.c | 20 +++++++++++--------- >> 1 file changed, 11 insertions(+), 9 deletions(-) >> >> diff --git a/fs/dax.c b/fs/dax.c >> index a64afdf7ec0d..34a2d435ae4b 100644 >> --- a/fs/dax.c >> +++ b/fs/dax.c >> @@ -991,6 +991,7 @@ dax_iomap_actor(struct inode *inode, loff_t pos, loff_t length, void *data, >> struct iov_iter *iter = data; >> loff_t end = pos + length, done = 0; >> ssize_t ret = 0; >> + size_t xfer; >> int id; >> >> if (iov_iter_rw(iter) == READ) { >> @@ -1054,19 +1055,20 @@ dax_iomap_actor(struct inode *inode, loff_t pos, loff_t length, void *data, >> * vfs_write(), depending on which operation we are doing. >> */ >> if (iov_iter_rw(iter) == WRITE) >> - map_len = dax_copy_from_iter(dax_dev, pgoff, kaddr, >> + xfer = dax_copy_from_iter(dax_dev, pgoff, kaddr, >> map_len, iter); >> else >> - map_len = dax_copy_to_iter(dax_dev, pgoff, kaddr, >> + xfer = dax_copy_to_iter(dax_dev, pgoff, kaddr, >> map_len, iter); >> - if (map_len <= 0) { >> - ret = map_len ? map_len : -EFAULT; >> - break; >> - } >> >> - pos += map_len; >> - length -= map_len; >> - done += map_len; >> + pos += xfer; >> + length -= xfer; >> + done += xfer; >> + >> + if (xfer == 0) >> + ret = -EFAULT; >> + if (xfer < map_len) >> + break; > > I'm confused by this error handling. So if we hit an error on a given iov and > we don't transfer the expected number of bytes, we have two cases: > > 1) We transferred *something* on this iov but not everything - return success. > 2) We didn't transfer anything on this iov - return -EFAULT. > > Both of these are true regardless of data transferred on previous iovs. > > Why the distinction? If a given iov is interrupted, regardless of whether it > transferred 0 bytes or 1, shouldn't the error path be the same? This is is the semantics of read(2) / write(2). Quoting the pwrite man page: Note that is not an error for a successful call to transfer fewer bytes than requested (see read(2) and write(2)).