Re: Out of order read() completion and buffer filling beyond returned amount

[Matthew Wilcox]

On Mon, Jan 17, 2022 at 12:19:29PM +0200, Linus Torvalds wrote:
> On Mon, Jan 17, 2022 at 11:57 AM David Howells <dhowells@xxxxxxxxxx> wrote:
> >
> > Do you have an opinion on whether it's permissible for a filesystem to write
> > into the read() buffer beyond the amount it claims to return, though still
> > within the specified size of the buffer?
> 
> I'm pretty sure that would seriously violate POSIX in the general
> case, and maybe even break some programs that do fancy buffer
> management (ie I could imagine some circular buffer thing that expects
> any "unwritten" ('unread'?) parts to stay with the old contents)
> 
> That said, that's for generic 'read()' cases for things like tty's or
> pipes etc that can return partial reads in the first place.
> 
> If it's a regular file, then any partial read *already* violates
> POSIX, and nobody sane would do any such buffer management because
> it's supposed to be a 'can't happen' thing.
> 
> And since you mention DIO, that's doubly true, and is already outside
> basic POSIX, and has already violated things like "all or nothing"
> rules for visibility of writes-vs-reads (which admittedly most Linux
> filesystems have violated even outside of DIO, since the strictest
> reading of the rules are incredibly nasty anyway). But filesystems
> like XFS which took some of the strict rules more seriously already
> ignored them for DIO, afaik.

I think for DIO, you're sacrificing the entire buffer with any filesystem.
If the underlying file is split across multiple drives, or is even
just fragmented on a single drive, we'll submit multiple BIOs which
will complete independently (even for SCSI which writes sequentially;
never mind NVMe which can DMA blocks asynchronously).  It might be
more apparent in a networking situation where errors are more common,
but it's always been a possibility since Linux introduced DIO.