On Fri, Mar 01, 2024 at 09:51:18AM -0800, Linus Torvalds wrote: > On Fri, 1 Mar 2024 at 09:07, Vlastimil Babka <vbabka@xxxxxxx> wrote: > > > > This is just an example of using the kmem_cache_charge() API. I think > > it's placed in a place that's applicable for Linus's example [1] > > although he mentions do_dentry_open() - I have followed from strace() > > showing openat(2) to path_openat() doing the alloc_empty_file(). > > Thanks. This is not the right patch, but yes, patches 1-3 look very nice to me. > > > The idea is that filp_cachep stops being SLAB_ACCOUNT. Allocations that > > want to be accounted immediately can use GFP_KERNEL_ACCOUNT. I did that > > in alloc_empty_file_noaccount() (despite the contradictory name but the > > noaccount refers to something else, right?) as IIUC it's about > > kernel-internal opens. > > Yeah, the "noaccount" function is about not accounting it towards nr_files. > That said, I don't think it necessarily needs to do the memory > accounting either - it's literally for cases where we're never going > to install the file descriptor in any user space. Exactly. > Your change to use GFP_KERNEL_ACCOUNT isn't exactly wrong, but I don't > think it's really the right thing either, because > > > Why is this unfinished: > > > > - there are other callers of alloc_empty_file() which I didn't adjust so > > they simply became memcg-unaccounted. I haven't investigated for which > > ones it would make also sense to separate the allocation and accounting. > > Maybe alloc_empty_file() would need to get a parameter to control > > this. > > Right. I think the natural and logical way to deal with this is to > just say "we account when we add the file to the fdtable". > IOW, just have fd_install() do it. That's the really natural point, > and also makes it very logical why alloc_empty_file_noaccount() > wouldn't need to do the GFP_KERNEL_ACCOUNT. > > > - I don't know how to properly unwind the accounting failure case. It > > seems like a new case because when we succeed the open, there's no > > further error path at least in path_openat(). > > Yeah, let me think about this part. Becasue fd_install() is the right > point, but that too does not really allow for error handling. > > Yes, we could close things and fail it, but it really is much too late > at this point. It would also mean massaging 100+ callsites. And having a non-subsystems specific failure step between file allocation, fd reservation and fd_install() would be awkward and an invitation for bugs. > What I *think* I'd want for this case is > > (a) allow the accounting to go over by a bit > > (b) make sure there's a cheap way to ask (before) about "did we go > over the limit" > > IOW, the accounting never needed to be byte-accurate to begin with, > and making it fail (cheaply and early) on the next file allocation is > fine. I think that's a good idea.
