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Mon, 09 Aug 2021 03:44:01 -0700 (PDT) X-Google-Smtp-Source: ABdhPJwmhbYo0CinZKFIgHoYQLZ2gPjSninIcBgtvNsiedBOiup+9chxg8uqRcDytESadHBvjqAgGg== X-Received: by 2002:adf:eac7:: with SMTP id o7mr24073019wrn.52.1628505841131; Mon, 09 Aug 2021 03:44:01 -0700 (PDT) Received: from dresden.str.redhat.com ([2a02:908:1e46:160:b272:8083:d5:bc7d]) by smtp.gmail.com with ESMTPSA id g35sm4461539wmp.9.2021.08.09.03.44.00 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 09 Aug 2021 03:44:00 -0700 (PDT) Subject: Re: [PATCH v3 02/10] virtiofsd: Add TempFd structure To: Vivek Goyal References: <20210730150134.216126-1-mreitz@redhat.com> <20210730150134.216126-3-mreitz@redhat.com> From: Max Reitz Message-ID: Date: Mon, 9 Aug 2021 12:44:00 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.11.0 MIME-Version: 1.0 In-Reply-To: Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=mreitz@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Content-Language: en-US Received-SPF: pass client-ip=216.205.24.124; envelope-from=mreitz@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -34 X-Spam_score: -3.5 X-Spam_bar: --- X-Spam_report: (-3.5 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.707, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, NICE_REPLY_A=-0.001, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H2=-0.001, 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: virtio-fs@redhat.com, qemu-devel@nongnu.org, Stefan Hajnoczi , "Dr . David Alan Gilbert" Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 06.08.21 16:41, Vivek Goyal wrote: > On Fri, Jul 30, 2021 at 05:01:26PM +0200, Max Reitz wrote: >> We are planning to add file handles to lo_inode objects as an >> alternative to lo_inode.fd. That means that everywhere where we >> currently reference lo_inode.fd, we will have to open a temporary file >> descriptor that needs to be closed after use. >> >> So instead of directly accessing lo_inode.fd, there will be a helper >> function (lo_inode_fd()) that either returns lo_inode.fd, or opens a new >> file descriptor with open_by_handle_at(). It encapsulates this result >> in a TempFd structure to let the caller know whether the FD needs to be >> closed after use (opened from the handle) or not (copied from >> lo_inode.fd). > I am wondering why this notion of "owned". Why not have this requirement > of always closing "fd". If we copied it from lo_inode.fd, then we will > need to dup() it. Otherwise we opened it from file handle and we will > need to close it anyway. > > I guess you are trying to avoid having to call dup() and that's why > this notion of "owned" fd. Yes, I don’t want to dup() it.  One reason is that I’d rather just not.  It’s something that we can avoid, and dup-ing every time wouldn’t make the code that much simpler (I think, without having tried). One other is because this affects the current behavior (with O_PATH FDs), which I don’t want to alter. Well, and finally, as a pragmatic reason, virtiofsd-rs uses the same structure and I don’t really want C virtiofsd and virtiofsd-rs to differ too much. >> By using g_auto(TempFd) to store this result, callers will not even have >> to care about closing a temporary FD after use. It will be done >> automatically once the object goes out of scope. >> >> Signed-off-by: Max Reitz >> Reviewed-by: Connor Kuehl >> --- >> tools/virtiofsd/passthrough_ll.c | 49 ++++++++++++++++++++++++++++++++ >> 1 file changed, 49 insertions(+) >> >> diff --git a/tools/virtiofsd/passthrough_ll.c b/tools/virtiofsd/passthrough_ll.c >> index 1f27eeabc5..fb5e073e6a 100644 >> --- a/tools/virtiofsd/passthrough_ll.c >> +++ b/tools/virtiofsd/passthrough_ll.c >> @@ -178,6 +178,28 @@ struct lo_data { >> int user_posix_acl, posix_acl; >> }; >> >> +/** >> + * Represents a file descriptor that may either be owned by this >> + * TempFd, or only referenced (i.e. the ownership belongs to some >> + * other object, and the value has just been copied into this TempFd). >> + * >> + * The purpose of this encapsulation is to be used as g_auto(TempFd) >> + * to automatically clean up owned file descriptors when this object >> + * goes out of scope. >> + * >> + * Use temp_fd_steal() to get an owned file descriptor that will not >> + * be closed when the TempFd goes out of scope. >> + */ >> +typedef struct { >> + int fd; >> + bool owned; /* fd owned by this object? */ >> +} TempFd; >> + >> +#define TEMP_FD_INIT ((TempFd) { .fd = -1, .owned = false }) >> + >> +static void temp_fd_clear(TempFd *temp_fd); >> +G_DEFINE_AUTO_CLEANUP_CLEAR_FUNC(TempFd, temp_fd_clear); >> + >> static const struct fuse_opt lo_opts[] = { >> { "sandbox=namespace", >> offsetof(struct lo_data, sandbox), >> @@ -255,6 +277,33 @@ static struct lo_data *lo_data(fuse_req_t req) >> return (struct lo_data *)fuse_req_userdata(req); >> } >> >> +/** >> + * Clean-up function for TempFds >> + */ >> +static void temp_fd_clear(TempFd *temp_fd) >> +{ >> + if (temp_fd->owned) { >> + close(temp_fd->fd); >> + *temp_fd = TEMP_FD_INIT; >> + } >> +} >> + >> +/** >> + * Return an owned fd from *temp_fd that will not be closed when >> + * *temp_fd goes out of scope. >> + * >> + * (TODO: Remove __attribute__ once this is used.) >> + */ >> +static __attribute__((unused)) int temp_fd_steal(TempFd *temp_fd) >> +{ >> + if (temp_fd->owned) { >> + temp_fd->owned = false; >> + return temp_fd->fd; >> + } else { >> + return dup(temp_fd->fd); >> + } >> +} > This also will be simpler if we always called dup() and every caller > will close() fd. > > I think only downside is having to call dup()/close(). Not sure if this > is an expensive operation or not. > > Vivek >