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? 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
