On Tue, Feb 14, 2023 at 08:52:23AM +0800, Ming Lei wrote: > On Mon, Feb 13, 2023 at 12:04:27PM -0800, Linus Torvalds wrote: > > On Sat, Feb 11, 2023 at 5:39 PM Ming Lei <ming.lei@xxxxxxxxxx> wrote: > > > > > > > > > > > (a) what's the point of MAY_READ? A non-readable page sounds insane > > > > and wrong. All sinks expect to be able to read. > > > > > > For example, it is one page which needs sink end to fill data, so > > > we needn't to zero it in source end every time, just for avoiding > > > leak kernel data if (unexpected)sink end simply tried to read from > > > the spliced page instead of writing data to page. > > > > I still don't understand. > > > > A sink *reads* the data. It doesn't write the data. > > > > There's no point trying to deal with "if unexpectedly doing crazy > > things". If a sink writes the data, the sinkm is so unbelievably buggy > > that it's not even funny. > > > > And having two flags that you then say "have to be used together" is pointless. > > Actually I think it is fine to use the pipe buffer flags separately, > if MAY_READ/MAY_WRITE is set in source end, the sink side need to respect > it. All current in-tree source end actually implies both MAY_READ & MAY_WRITE. > > > It's not two different flags if you can't use them separately! > > > > So I think your explanations are anything *but* explaining what you > > want. They are just strange and not sensible. > > > > Please explain to me in small words and simple sentences what it is > > you want. And no, if the explanation is "the sink wants to write to > > the buffer", then that's not an explanation, it's just insanity. > > > > We *used* to have the concept of "gifting" the buffer explicitly to > > the sink, so that the sink could - instead of reading from it - decide > > to just use the whole buffer as-is long term. The idea was that tthe > > buffer woudl literally be moved from the source to the destination, > > ownership and all. > > > > But if that's what you want, then it's not about "sink writes". It's > > literally about the splice() wanting to move not just the data, but > > the whole ownership of the buffer. > > Yeah, it is actually transferring the buffer ownership, and looks > SPLICE_F_GIFT is exactly the case, but the driver side needs to set > QUEUE_FLAG_STABLE_WRITES for avoiding writeback to touch these pages. > > Follows the idea: > > file(devices(such as, fuse, ublk), produce pipe buffer) -> direct pipe -> file(consume the pipe buffer) > > The 'consume' could be READ or WRITE. > > So once SPLICE_F_GIFT is set from source side, the two buffer flags > aren't needed any more, right? Sorry, I meant PIPE_BUF_FLAG_GIFT actually. > > Please see the detailed explanation & use case in following link: > > https://lore.kernel.org/linux-block/409656a0-7db5-d87c-3bb2-c05ff7af89af@xxxxxxxxx/T/#m237e5973571b3d85df9fa519cf2c9762440009ba > Thanks, Ming
