Re: io_uring force_nonblock vs POSIX_FADV_WILLNEED

[Andres Freund <andres@anarazel.de>]

Hi Matthew,

On 2020-02-02 22:40:47 -0800, Matthew Wilcox wrote:
> On Sat, Feb 01, 2020 at 01:43:09AM -0800, Andres Freund wrote:
> > As far as I can tell POSIX_FADV_WILLNEED synchronously starts readahead,
> > including page allocation etc, which of course might trigger quite
> > blocking. The fs also quite possibly needs to read metadata.
> > 
> > 
> > Seems like either WILLNEED would have to always be deferred, or
> > force_page_cache_readahead, __do_page_cache_readahead would etc need to
> > be wired up to know not to block. Including returning EAGAIN, despite
> > force_page_cache_readahead and generic_readahead() intentially ignoring
> > return values / errors.
> 
> The first step is going to be letting the readahead code know that it
> should have this behaviour, which is tricky because the code flow looks
> like this:
> 
> io_fadvise
>   vfs_fadvise
>     file->f_op->fadvise()

Yea.


> ... and we'd be breaking brand new ground trying to add a gfp_t to a
> file_operations method.  Which is not to say it couldn't be done, but
> would mean changing filesystems, just so we could pass the gfp
> flags through from the top level to the low level.  It wouldn't be
> too bad; only two filesystems implement an ->fadvise op today.

I was wondering if the right approach could be to pass through a kiocb
instead of gfp_t. There's obviously precedent for that in
file_operations, and then IOCB_NOWAIT could be used to represent the the
intent to not block. It'd be a bit weird in the sense that currently
there'd probably be no callback - but that seems fairly minor. And who
knows, 


> Next possibility, we could add a POSIX_FADV_WILLNEED_ASYNC advice
> flag.

POSIX_FADV_DONTNEED has similar problems to POSIX_FADV_WILLNEED, so it'd
be nice to come up with an API change to vfs_fadvise that'd support
both. Obviously there also could be a POSIX_FADV_DONTNEED_ASYNC, but ...


> Something I already want to see in an entirely different context is
> a flag in the task_struct which says, essentially, "don't block in
> memory allocations" -- ie behave as if __GFP_NOWAIT | __GFP_NOWARN
> is set.  See my proposal here:

I'm a bit out of my depth here: Would __GFP_NOWAIT actually be suitable
to indicate that no blocking IO is to be executed by the FS? E.g. for
metadata? As far as I can tell that's also a problem, not just reclaim
to make space for the to-be-read data.


> I've got my head stuck in the middle of the readahead code right now,
> so this seems like a good time to add this functionality.  Once I'm done
> with finding out who broke my test VM, I'll take a shot at adding
> this.

Cool!

Greetings,

Andres Freund