[Linaro-mm-sig] Re: [PATCH] epoll: try to be a _bit_ better about file lifetimes

On Sun, 5 May 2024 at 13:30, Al Viro viro@zeniv.linux.org.uk wrote:

0.  special-cased ->f_count rule for ->poll() is a wart and it's
   

better to get rid of it.

1.  fs/eventpoll.c is a steaming pile of shit and I'd be glad to see
   

git rm taken to it. Short of that, by all means, let's grab reference in there around the call of vfs_poll() (see (0)).

Agreed on 0/1.

2.  having ->poll() instances grab extra references to file passed
   

to them is not something that should be encouraged; there's a plenty of potential problems, and "caller has it pinned, so we are fine with grabbing extra refs" is nowhere near enough to eliminate those.

So it's not clear why you hate it so much, since those extra references are totally normal in all the other VFS paths.

I mean, they are perhaps not the *common* case, but we have a lot of random get_file() calls sprinkled around in various places when you end up passing a file descriptor off to some asynchronous operation thing.

Yeah, I think most of them tend to be special operations (eg the tty TIOCCONS ioctl to redirect the console), but it's not like vfs_ioctl() is *that* different from vfs_poll. Different operation, not somehow "one is more special than the other".

cachefiles and backing-file does it for regular IO, and drop it at IO completion - not that different from what dma-buf does. It's in ->read_iter() rather than ->poll(), but again: different operations, but not "one of them is somehow fundamentally different".

3.  dma-buf uses of get_file() are probably safe (epoll shite aside),
   

but they do look fishy. That has nothing to do with epoll.

Now, what dma-buf basically seems to do is to avoid ref-counting its own fundamental data structure, and replaces that by refcounting the 'struct file' that *points* to it instead.

And it is a bit odd, but it actually makes some amount of sense, because then what it passes around is that file pointer (and it allows passing it around from user space *as* that file).

And honestly, if you look at why it then needs to add its refcount to it all, it actually makes sense. dma-bufs have this notion of "fences" that are basically completion points for the asynchronous DMA. Doing a "poll()" operation will add a note to the fence to get that wakeup when it's done.

And yes, logically it takes a ref to the "struct dma_buf", but because of how the lifetime of the dma_buf is associated with the lifetime of the 'struct file', that then turns into taking a ref on the file.

Unusual? Yes. But not illogical. Not obviously broken. Tying the lifetime of the dma_buf to the lifetime of a file that is passed along makes _sense_ for that use.

I'm sure dma-bufs could add another level of refcounting on the 'struct dma_buf' itself, and not make it be 1:1 with the file, but it's not clear to me what the advantage would really be, or why it would be wrong to re-use a refcount that is already there.

Linus