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

[Nadav Amit]

> 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.

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.

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.

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