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Mon, 13 Jul 2020 09:29:43 -0700 (PDT) X-Google-Smtp-Source: ABdhPJz03bQnJ9pwmm344o5bvrJG1VzwosbGojMEjj4vx/kqE1yAjZj3xXtxTnXsrwXx56siQ6gWi6nOLJ8gyDxQZ9U= X-Received: by 2002:a05:6808:486:: with SMTP id z6mr219334oid.56.1594657783040; Mon, 13 Jul 2020 09:29:43 -0700 (PDT) MIME-Version: 1.0 References: <20200710142149.40962-1-kwolf@redhat.com> <20200710142149.40962-3-kwolf@redhat.com> In-Reply-To: <20200710142149.40962-3-kwolf@redhat.com> From: Nir Soffer Date: Mon, 13 Jul 2020 19:29:26 +0300 Message-ID: Subject: Re: [PATCH for-5.1 2/2] file-posix: Allow byte-aligned O_DIRECT with NFS To: Kevin Wolf X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset="UTF-8" Received-SPF: pass client-ip=207.211.31.81; envelope-from=nsoffer@redhat.com; helo=us-smtp-delivery-1.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/07/13 02:19:41 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] [fuzzy] X-Spam_score_int: -30 X-Spam_score: -3.1 X-Spam_bar: --- X-Spam_report: (-3.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-1, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H3=-0.01, RCVD_IN_MSPIKE_WL=-0.01, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: QEMU Developers , qemu-block , Max Reitz Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Fri, Jul 10, 2020 at 5:22 PM Kevin Wolf wrote: > > Since commit a6b257a08e3 ('file-posix: Handle undetectable alignment'), > we assume that if we open a file with O_DIRECT and alignment probing > returns 1, we just couldn't find out the real alignment requirement > because some filesystems make the requirement only for allocated blocks. > In this case, a safe default of 4k is used. > > This is too strict NFS, which does actually allow byte-aligned requests > even with O_DIRECT. Because we can't distinguish both cases with generic > code, let's just look at the file system magic and disable > s->needs_alignment for NFS. This way, O_DIRECT can still be used on NFS > for images that are not aligned to 4k. > > Signed-off-by: Kevin Wolf > --- > block/file-posix.c | 26 +++++++++++++++++++++++++- > 1 file changed, 25 insertions(+), 1 deletion(-) > > diff --git a/block/file-posix.c b/block/file-posix.c > index 0c4e07c415..4e9dac461b 100644 > --- a/block/file-posix.c > +++ b/block/file-posix.c > @@ -62,10 +62,12 @@ > #include > #include > #include > +#include > #include > #include > #include > #include > +#include > #include > #ifdef __s390__ > #include > @@ -300,6 +302,28 @@ static int probe_physical_blocksize(int fd, unsigned int *blk_size) > #endif > } > > +/* > + * Returns true if no alignment restrictions are necessary even for files > + * opened with O_DIRECT. > + * > + * raw_probe_alignment() probes the required alignment and assume that 1 means > + * the probing failed, so it falls back to a safe default of 4k. This can be > + * avoided if we know that byte alignment is okay for the file. > + */ > +static bool dio_byte_aligned(int fd) > +{ > +#ifdef __linux__ > + struct statfs buf; > + int ret; > + > + ret = fstatfs(fd, &buf); > + if (ret == 0 && buf.f_type == NFS_SUPER_MAGIC) { > + return true; > + } > +#endif > + return false; > +} > + > /* Check if read is allowed with given memory buffer and length. > * > * This function is used to check O_DIRECT memory buffer and request alignment. > @@ -631,7 +655,7 @@ static int raw_open_common(BlockDriverState *bs, QDict *options, > > s->has_discard = true; > s->has_write_zeroes = true; > - if ((bs->open_flags & BDRV_O_NOCACHE) != 0) { > + if ((bs->open_flags & BDRV_O_NOCACHE) != 0 && !dio_byte_aligned(s->fd)) { > s->needs_alignment = true; I did not know we have needs_alignment. Isn't this the same as using request_alignment = 1? For example we can check if we are on NFS and avoid the fallback to max_align: if (!bs->bl.request_alignment) { int i; size_t align; buf = qemu_memalign(max_align, max_align); for (i = 0; i < ARRAY_SIZE(alignments); i++) { align = alignments[i]; if (raw_is_io_aligned(fd, buf, align)) { /* Fallback to safe value. */ bs->bl.request_alignment = (align != 1) ? align : max_align; break; } } qemu_vfree(buf); } After this we will have correct bl.request_alignment and buf_align. Hopefully this will not break code expecting request_alignment >= 512. Assuming that needs_alignment is well tested, this patch may be safer. Nir > } > > -- > 2.25.4 >