On Feb 25, 2021, at 9:32 AM, Matthew Wilcox [off-list ref] wrote:

On Thu, Feb 25, 2021 at 04:56:50PM +0000, Nadav Amit wrote:

quoted

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On Feb 25, 2021, at 4:16 AM, Matthew Wilcox [off-list ref] wrote:

On Wed, Feb 24, 2021 at 11:29:04PM -0800, Nadav Amit wrote:

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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.

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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.

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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?

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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.

I agree that this call does not have to be exposed to the application.

I am not sure there is a lot of extra complexity now, but obviously
some evaluations are needed.