Re: [RFC 0/6] x86: prefetch_page() vDSO call

[Matthew Wilcox]

On Thu, Feb 25, 2021 at 04:56:50PM +0000, Nadav Amit wrote:
> 
> > On Feb 25, 2021, at 4:16 AM, Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
> > 
> > On Wed, Feb 24, 2021 at 11:29:04PM -0800, Nadav Amit wrote:
> >> Just as applications can use prefetch instructions to overlap
> >> computations and memory accesses, applications may want to overlap the
> >> page-faults and compute or overlap the I/O accesses that are required
> >> for page-faults of different pages.
> > 
> > Isn't this madvise(MADV_WILLNEED)?
> 
> Good point that I should have mentioned. In a way prefetch_page() a
> combination of mincore() and MADV_WILLNEED.
> 
> There are 4 main differences from MADV_WILLNEED:
> 
> 1. Much lower invocation cost if the readahead is not needed: this allows
> to prefetch pages more abundantly.

That seems like something that could be fixed in libc -- if we add a
page prefetch vdso call, an application calling posix_madvise() could
be implemented by calling this fast path.  Assuming the performance
increase justifies this extra complexity.

> 2. Return value: return value tells you whether the page is accessible.
> This makes it usable for coroutines, for instance. In this regard the
> call is more similar to mincore() than MADV_WILLNEED.

I don't quite understand the programming model you're describing here.

> 3. The PTEs are mapped if the pages are already present in the
> swap/page-cache, preventing an additional page-fault just to map them.

We could enhance madvise() to do this, no?

> 4. Avoiding heavy-weight reclamation on low memory (this may need to
> be selective, and can be integrated with MADV_WILLNEED).

Likewise.

I don't want to add a new Linux-specific call when there's already a
POSIX interface that communicates the exact same thing.  The return
value seems like the only problem.

https://pubs.opengroup.org/onlinepubs/9699919799/functions/posix_madvise.html