On Fri, May 24, 2019 at 11:55:44AM -0700, Linus Torvalds wrote: > On Fri, May 24, 2019 at 11:39 AM Alexey Dobriyan <adobriyan@xxxxxxxxx> wrote: > > > > > Would there ever be any other reason to traverse unknown open files > > > than to close them? > > > > This is what lsof(1) does: > > I repeat: Would there ever be any other reason to traverse unknown > open files than to close them? > > lsof is not AT ALL a relevant argument. > > lsof fundamentally wants /proc, because lsof looks at *other* > processes. That has absolutely zero to do with fdmap. lsof does *not* > want fdmap at all. It wants "list other processes files". Which is > very much what /proc is all about. > > So your argument that "fdmap is more generic" is bogus. > > fdmap is entirely pointless unless you can show a real and relevant > (to performance) use of it. > > When you would *possibly* have a "let me get a list of all the file > descriptors I have open, because I didn't track them myself" > situation? That makes no sense. Particularly from a performance > standpoint. > > In contrast, "close_range()" makes sense as an operation. What about orthogonality of interfaces? fdmap() bulk_close() Now fdmap() can be reused for lsof/criu and it is only 2 system calls for close-everything usecase which is OK because readdir is 4(!) minimum: open getdents getdents() = 0 close Writing all of this I understood how fdmap can be made more faster which neither getdents() nor even read() have the luxury of: it can return a flag if more data is available so that application would do next fdmap() only if truly necessary. > I can > explain exactly when it would be used, and I can easily see a > situation where "I've opened a ton of files, now I want to release > them" is a valid model of operation. And it's a valid optimization to > do a bulk operation like that. > > IOW, close_range() makes sense as an operation even if you could just > say "ok, I know exactly what files I have open". But it also makes > sense as an operation for the case of "I don't even care what files I > have open, I just want to close them". > > In contrast, the "I have opened a ton of files, and I don't even know > what the hell I did, so can you list them for me" makes no sense. > > Because outside of "close them", there's no bulk operation that makes > sense on random file handles that you don't know what they are. Unless > you iterate over them and do the stat thing or whatever to figure it > out - which is lsof, but as mentioned, it's about *other* peoples > files. What you're doing is making exactly one usecase take exactly one system call and leaving everything else deal with /proc. Stracing lsof shows very clearly how stupid and how wasteful it is. Especially now that we're post-meltdown era caring about system call costs (yeah suure). I'm suggesting make close-universe usecase take only 2 system calls. which is still better than anything /proc can offer.
