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bh=60vLc/49QFY8fDwLLEXKxkWu/t+AAawSZXYUWozx5iw=; b=BDY85WIXRDJS4Oy0rCgklQtRzxd1lavBzbDg8zUM78uUS5SMWoDaVZ+oVRN5jXM0DZV6Do EGMHNXBJWEu/6TCg== Received: from imap1.dmz-prg2.suse.org (localhost [127.0.0.1]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (4096 bits) server-digest SHA256) (No client certificate requested) by imap1.dmz-prg2.suse.org (Postfix) with ESMTPS id D5EC113957; Thu, 2 May 2024 20:34:24 +0000 (UTC) Received: from dovecot-director2.suse.de ([2a07:de40:b281:106:10:150:64:167]) by imap1.dmz-prg2.suse.org with ESMTPSA id sYDCJtD4M2ZUVAAAD6G6ig (envelope-from ); Thu, 02 May 2024 20:34:24 +0000 From: Fabiano Rosas To: Peter Xu Cc: qemu-devel@nongnu.org, berrange@redhat.com, armbru@redhat.com, Claudio Fontana , Jim Fehlig Subject: Re: [PATCH 0/9] migration/mapped-ram: Add direct-io support In-Reply-To: References: <20240426142042.14573-1-farosas@suse.de> Date: Thu, 02 May 2024 17:34:22 -0300 Message-ID: <87ikzwneb5.fsf@suse.de> MIME-Version: 1.0 Content-Type: text/plain X-Rspamd-Action: no action X-Rspamd-Queue-Id: 65E671FD13 X-Rspamd-Server: rspamd2.dmz-prg2.suse.org X-Spamd-Result: default: False [-4.51 / 50.00]; BAYES_HAM(-3.00)[100.00%]; NEURAL_HAM_LONG(-1.00)[-1.000]; R_DKIM_ALLOW(-0.20)[suse.de:s=susede2_rsa,suse.de:s=susede2_ed25519]; NEURAL_HAM_SHORT(-0.20)[-1.000]; MIME_GOOD(-0.10)[text/plain]; MX_GOOD(-0.01)[]; RCVD_VIA_SMTP_AUTH(0.00)[]; MISSING_XM_UA(0.00)[]; MIME_TRACE(0.00)[0:+]; ARC_NA(0.00)[]; TO_DN_SOME(0.00)[]; MID_RHS_MATCH_FROM(0.00)[]; RCVD_TLS_ALL(0.00)[]; RCPT_COUNT_FIVE(0.00)[6]; FROM_EQ_ENVFROM(0.00)[]; FROM_HAS_DN(0.00)[]; FUZZY_BLOCKED(0.00)[rspamd.com]; RCVD_COUNT_TWO(0.00)[2]; TO_MATCH_ENVRCPT_ALL(0.00)[]; DBL_BLOCKED_OPENRESOLVER(0.00)[suse.de:dkim]; DKIM_SIGNED(0.00)[suse.de:s=susede2_rsa,suse.de:s=susede2_ed25519]; DKIM_TRACE(0.00)[suse.de:+] Received-SPF: pass client-ip=195.135.223.131; envelope-from=farosas@suse.de; helo=smtp-out2.suse.de X-Spam_score_int: -43 X-Spam_score: -4.4 X-Spam_bar: ---- X-Spam_report: (-4.4 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_MED=-2.3, 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: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Peter Xu writes: > On Fri, Apr 26, 2024 at 11:20:33AM -0300, Fabiano Rosas wrote: >> If the user is not passing in a file name which QEMU can open at will, >> we must then require that the user pass the two file descriptors with >> the flags already properly set. We'll use the already existing fdset + >> QMP add-fd infrastructure for this. > > Yes I remember such requirement that one extra fd is needed for direct-io, > however today when I looked closer at the man page it looks like F_SETFL > works with O_DIRECT too? > > F_SETFL (int) > Set the file status flags to the value specified by arg. > File access mode (O_RDONLY, O_WRONLY, O_RDWR) and file > creation flags (i.e., O_CREAT, O_EXCL, O_NOCTTY, O_TRUNC) in > arg are ignored. On Linux, this command can change only the > O_APPEND, O_ASYNC, O_DIRECT, O_NOATIME, and O_NONBLOCK flags. > It is not possible to change the O_DSYNC and O_SYNC flags; > see BUGS, below. > > ====8<==== > $ cat fcntl.c > #define _GNU_SOURCE > #include > #include > #include > #include > > int main(void) > { > int fd, newfd, ret, flags; > > fd = open("test.txt", O_RDWR | O_CREAT, 0660); > assert(fd != -1); > > flags = fcntl(fd, F_GETFL); > printf("old fd flags: 0x%x\n", flags); > > newfd = dup(fd); > assert(newfd != -1); > > flags = fcntl(newfd, F_GETFL); > printf("new fd flags: 0x%x\n", flags); > > flags |= O_DIRECT; > ret = fcntl(newfd, F_SETFL, flags); > > flags = fcntl(fd, F_GETFL); > printf("updated new flags: 0x%x\n", flags); > > return 0; > } > $ make fcntl > cc fcntl.c -o fcntl > $ ./fcntl > old fd flags: 0x8002 > new fd flags: 0x8002 > updated new flags: 0xc002 > ====8<==== > > Perhaps I missed something important? The dup()'ed file descriptor shares file status flags with the original fd. Your code example proves just that. In the last two blocks you're doing F_SETFL on the 'newfd' and then seeing the change take effect on 'fd'. That's what we don't want to happen.