On Thu, May 19, 2022 at 11:39:01AM +0200, Miklos Szeredi wrote: > On Tue, 17 May 2022 at 12:08, Dharmendra Singh <dharamhans87@xxxxxxxxx> wrote: > > > > In FUSE, as of now, uncached lookups are expensive over the wire. > > E.g additional latencies and stressing (meta data) servers from > > thousands of clients. These lookup calls possibly can be avoided > > in some cases. Incoming three patches address this issue. > > > > > > Fist patch handles the case where we are creating a file with O_CREAT. > > Before we go for file creation, we do a lookup on the file which is most > > likely non-existent. After this lookup is done, we again go into libfuse > > to create file. Such lookups where file is most likely non-existent, can > > be avoided. > > I'd really like to see a bit wider picture... > > We have several cases, first of all let's look at plain O_CREAT > without O_EXCL (assume that there were no changes since the last > lookup for simplicity): Hi Miklos, Thanks for providing this breakup. There are too many cases here and this data helps a lot with that. I feel this should really be captured in commit logs to show the current paths and how these have been optimized with ATOMIC_OPEN/CREATE_EXT. > > [not cached, negative] > ->atomic_open() > LOOKUP > CREATE > > [not cached, positive] > ->atomic_open() > LOOKUP > ->open() > OPEN > > [cached, negative, validity timeout not expired] > ->d_revalidate() > return 1 > ->atomic_open() > CREATE > > [cached, negative, validity timeout expired] > ->d_revalidate() > return 0 > ->atomic_open() > LOOKUP > CREATE > > [cached, positive, validity timeout not expired] > ->d_revalidate() > return 1 > ->open() > OPEN > > [cached, positive, validity timeout expired] > ->d_revalidate() > LOOKUP > return 1 > ->open() > OPEN > > (Caveat emptor: I'm just looking at the code and haven't actually > tested what happens.) > > Apparently in all of these cases we are doing at least one request, so > it would make sense to make them uniform: > > [not cached] > ->atomic_open() > CREATE_EXT > > [cached] > ->d_revalidate() > return 0 So fuse_dentry_revalidate() will return 0 even if timeout has not expired (if server supports so called atomic_open()). And that will lead to calling d_invalidate() on existing positive dentry always. IOW, if I am calling open() on a dentry, dentry will always be dropped and a new dentry will always be created from ->atomic_open() path, is that right. I am not sure what does it mean from VFS perspective to always call d_invalidate() on a cached positive dentry when open() is called. /** * d_invalidate - detach submounts, prune dcache, and drop * @dentry: dentry to invalidate (aka detach, prune and drop) */ Thanks Vivek > ->atomic_open() > CREATE_EXT > > Similarly we can look at the current O_CREAT | O_EXCL cases: > > [not cached, negative] > ->atomic_open() > LOOKUP > CREATE > > [not cached, positive] > ->atomic_open() > LOOKUP > return -EEXIST > > [cached, negative] > ->d_revalidate() > return 0 (see LOOKUP_EXCL check) > ->atomic_open() > LOOKUP > CREATE > > [cached, positive] > ->d_revalidate() > LOOKUP > return 1 > return -EEXIST > > Again we are doing at least one request, so we can unconditionally > replace them with CREATE_EXT like the non-O_EXCL case. > > > > > > Second patch handles the case where we open first time a file/dir > > but do a lookup first on it. After lookup is performed we make another > > call into libfuse to open the file. Now these two separate calls into > > libfuse can be combined and performed as a single call into libfuse. > > And here's my analysis: > > [not cached, negative] > ->lookup() > LOOKUP > return -ENOENT > > [not cached, positive] > ->lookup() > LOOKUP > ->open() > OPEN > > [cached, negative, validity timeout not expired] > ->d_revalidate() > return 1 > return -ENOENT > > [cached, negative, validity timeout expired] > ->d_revalidate() > return 0 > ->atomic_open() > LOOKUP > return -ENOENT > > [cached, positive, validity timeout not expired] > ->d_revalidate() > return 1 > ->open() > OPEN > > [cached, positive, validity timeout expired] > ->d_revalidate() > LOOKUP > return 1 > ->open() > OPEN > > There's one case were no request is sent: a valid cached negative > dentry. Possibly we can also make this uniform, e.g.: > > [not cached] > ->atomic_open() > OPEN_ATOMIC > > [cached, negative, validity timeout not expired] > ->d_revalidate() > return 1 > return -ENOENT > > [cached, negative, validity timeout expired] > ->d_revalidate() > return 0 > ->atomic_open() > OPEN_ATOMIC > > [cached, positive] > ->d_revalidate() > return 0 > ->atomic_open() > OPEN_ATOMIC > > It may even make the code simpler to clearly separate the cases where > the atomic variants are supported and when not. I'd also consider > merging CREATE_EXT into OPEN_ATOMIC, since a filesystem implementing > one will highly likely want to implement the other as well. > > Thanks, > Miklos >
