linux-kernel - Re: [RFC 0/6] x86: prefetch

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Date:   Thu, 25 Feb 2021 16:56:50 +0000
From:   Nadav Amit <namit@...are.com>
To:     Matthew Wilcox <willy@...radead.org>
CC:     Linux-MM <linux-mm@...ck.org>, LKML <linux-kernel@...r.kernel.org>,
        Hugh Dickins <hughd@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Sean Christopherson <seanjc@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "x86@...nel.org" <x86@...nel.org>
Subject: Re: [RFC 0/6] x86: prefetch_page() vDSO call

> On Feb 25, 2021, at 4:16 AM, Matthew Wilcox <willy@...radead.org> 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).