On 11/24/21 8:42 PM, Clay Harris wrote: > > I seem to recall making a few comments the last time a getdents64 > for io_uring was proposed; in particular I wanted to bring up one > here. This applies only to altering the internal interface, which > io_uring would use, although wiring up a new syscall would be a nice > addition. > > The current interface has 2 issues: > > 1) > getdents64 requires at least two calls to read a directory. > One or more to get the dents and a final call to see the EOF. > With small directories, this means literally 50% of the calls > are wasted. > > 2) > The fpos cannot be changed atomically with a read, so it is not > possible to to safely perform concurrent reads on the same fd. > > But, the kernel knows (most, if not all of the time) that it is at > EOF at the time it returns the last buffer. So, it would be very > useful to get an EOF indicator back with the final buffer. This > could just a flag, or for instance make an fpos parameter which is > both input and output, returning the (post read) fpos or zero at > EOF. > > Futhermore, for input, one could supply: > 0: Start from directory beginning > -1: Read from current position > other: (output from previous call) Read from here > While I can understand the wish to optimize the getdents call, this has its own set of challenges: - The getdents API is following the logic of other read API's. None of these API's has the logic you described above. This would be inconsistent. - The eof needs to be stored in another field. The dirent structure does not have space in the field, so a new data structure needs to be defined. - However the goal is to provide a familiar interface to the user. - If the user wants to reduce the number of calls he can still provide a bigger user buffer. > On Wed, Nov 24 2021 at 15:16:57 -0800, Stefan Roesch quoth thus: > >> This series adds support for getdents64 in liburing. The intent is to >> provide a more complete I/O interface for io_uring. >> >> Patch 1: fs: add parameter use_fpos to iterate_dir() >> This adds a new parameter to the function iterate_dir() so the >> caller can specify if the position is the file position or the >> position stored in the buffer context. >> >> Patch 2: fs: split off vfs_getdents function from getdents64 system call >> This splits of the iterate_dir part of the syscall in its own >> dedicated function. This allows to call the function directly from >> liburing. >> >> Patch 3: io_uring: add support for getdents64 >> Adds the functions to io_uring to support getdents64. >> >> There is also a patch series for the changes to liburing. This includes >> a new test. The patch series is called "liburing: add getdents support." >> >> The following tests have been performed: >> - new liburing getdents test program has been run >> - xfstests have been run >> - both tests have been repeated with the kernel memory leak checker >> and no leaks have been reported. >> >> Signed-off-by: Stefan Roesch <shr@xxxxxx> >> --- >> V2: Updated the iterate_dir calls in fs/ksmbd, fs/ecryptfs and arch/alpha with >> the additional parameter. >> >> Stefan Roesch (3): >> fs: add parameter use_fpos to iterate_dir function >> fs: split off vfs_getdents function of getdents64 syscall >> io_uring: add support for getdents64 >> >> arch/alpha/kernel/osf_sys.c | 2 +- >> fs/ecryptfs/file.c | 2 +- >> fs/exportfs/expfs.c | 2 +- >> fs/internal.h | 8 +++++ >> fs/io_uring.c | 52 ++++++++++++++++++++++++++++ >> fs/ksmbd/smb2pdu.c | 2 +- >> fs/ksmbd/vfs.c | 4 +-- >> fs/nfsd/nfs4recover.c | 2 +- >> fs/nfsd/vfs.c | 2 +- >> fs/overlayfs/readdir.c | 6 ++-- >> fs/readdir.c | 64 ++++++++++++++++++++++++++--------- >> include/linux/fs.h | 2 +- >> include/uapi/linux/io_uring.h | 1 + >> 13 files changed, 121 insertions(+), 28 deletions(-) >> >> >> base-commit: f0afafc21027c39544a2c1d889b0cff75b346932 >> -- >> 2.30.2
