On Thu, Oct 18, 2018 at 6:04 PM, David Howells <dhowells@xxxxxxxxxx> wrote: > Miklos Szeredi <miklos@xxxxxxxxxx> wrote: > >> I'm trying to implement statx for fuse and ran into the following issues: >> >> - Need a STATX_ATTRIBUTES bit, so that userspace can explicitly ask >> for stx_attribute; otherwise if querying has non-zero cost, then >> filesystem cannot do it without regressing performance. > > Okay, though the way your patch implements it makes it superfluous. I presume > you have further patches that will actually make use of it from the fuse side? Being worked on, yes. > >> - STATX_ALL definition is unclear, can this change, or is it fixed? >> If it's the former, than that's a backward compatibility nightmare. >> If it's the latter, then what's the point? > > It's the set of supported attributes known by the headers, and such can > only be added to over time. But yes, it's probably unnecessary. Asking > fsinfo() will be a better way of doing things. > >> - STATX_ATIME is cleared from stx_mask on SB_RDONLY, > > Ummm... Where? It's cleared on IS_NOATIME() in generic_fillattr(). I made > the assumption that IS_NOATIME() == false indicates that there isn't an atime > to be had. Look at IS_NOATIME definition in <linux/fs.h> You probably wanted inode->i_sb->s_flags & SB_NOATIME instead. >> and on NFS it is also cleared on MNT_NOATIME, but not on MNT_RDONLY. We >> need some sort of guideline in the documentation about what constitutes >> "unsupported": does atime become unsupported because filesystem is remounted >> r/o? If so, why isn't this case handled consistently in the VFS and >> filesystems? > > STATX_ATIME should mean there is an actual atime from the "medium" in > stx_atime, rather than something made up by the filesystem driver; it doesn't > necessarily promise that this will be updated. In this case generic_fillattr() and nfs_getattr() are simply buggy. > > There can still be an atime if the medium is read-only. > > atime is even more complicated with MNT_NOATIME or MNT_RDONLY because that > doesn't stop the atime from actually being updated through another mountpoint > on the same system. > > Note that stx_atime should always contain something that can be used directly > to fill in st_atime if emulating stat() - even if STATX_ATIME is cleared. > >> - What about fields that are not cached when statx() is called with >> AT_STATX_DONT_SYNC? E.g. stx_btime is supported by the filesystem, >> but getting it requires a roundtrip to the server. > > Not necessarily. It's not cached in *struct inode*, but that doesn't mean > that the filesystem can't cache it elsewhere. > >> Requesting STATX_BTIME in the mask and adding AT_STATX_DONT_SYNC to the >> flags means the filesystem has to decide which it will honor. My feeling is >> that it should honor AT_STATX_DONT_SYNC and clear STATX_BTIME in stx_mask. >> Documentation has no word about this case. > > From the manpage: > > AT_STATX_DONT_SYNC > Don't synchronize anything, but rather just take whatever the > system has cached if possible. ... > > Note the "if possible". If it's not possible, you still need to go get it if > they explicitly asked for it. Okay. Thanks, Miklos
