On Wed, 2019-10-16 at 01:00 +0000, Su, Yanjun wrote: > Hi trond, > Because My mail system cant receive nfs mail list’s mails, I reply > your patch here. > I have some question for the patch. > > > No. Basic O_DIRECT does not guarantee atomicity of requests, which > > is > > why we do not have generic locking at the VFS level when reading > > and > > writing. The only guarantee being offered is that O_DIRECT and > > buffered > > writes do not collide. > Do you mean other fs also cant guarantee atomicity of O_DIRECT > request or just nfs? > > > IOW: I think the basic premise for this test is just broken (as I > > commented in the patch series I sent) because it is assuming a > > behaviour that is simply not guaranteed. > So the generic/465 of xfstests can’t apply to nfs for now, am I > right? As far as I can see, it is does not belong in the 'generic' category at all. > > > BTW: note that buffered writes have the same property. They are > > ordered > > when being written into the page cache, meaning that reads on the > > same > > client will see no holes, however if you try to read from another > > client, then you will see the same behaviour, with temporary holes > > magically appearing in the file. > As you say, nfs buffered write also has the hole problem with > multiple r/w on different clients. > I want to know if the problem exists in other local fs such as > xfs,ext4? > There is no VFS locking that enforces any serialisation for O_DIRECT. So unless the filesystem implements its own serialisation (which xfs does) then there is no guarantee. -- Trond Myklebust Linux NFS client maintainer, Hammerspace trond.myklebust@xxxxxxxxxxxxxxx
