linux-kernel - Re: [RFC PATCH 0/8] mm/madvise: support process_madvise(MADV

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Date:   Mon, 27 Sep 2021 11:24:09 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Nadav Amit <nadav.amit@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Peter Xu <peterx@...hat.com>, Nadav Amit <namit@...are.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Minchan Kim <minchan@...nel.org>,
        Colin Cross <ccross@...gle.com>,
        Suren Baghdasarya <surenb@...gle.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>
Subject: Re: [RFC PATCH 0/8] mm/madvise: support
 process_madvise(MADV_DONTNEED)

On 26.09.21 18:12, Nadav Amit wrote:
> From: Nadav Amit <namit@...are.com>
> 
> The goal of these patches is to add support for
> process_madvise(MADV_DONTNEED). Yet, in the process some (arguably)
> useful cleanups, a bug fix and performance enhancements are performed.
> 
> The patches try to consolidate the logic across different behaviors, and
> to a certain extent overlap/conflict with an outstanding patch that does
> something similar [1]. This consolidation however is mostly orthogonal
> to the aforementioned one and done in order to clarify what is done in
> respect to locks and TLB for each behavior and to batch these operations
> more efficiently on process_madvise().
> 
> process_madvise(MADV_DONTNEED) is useful for two reasons: (a) it allows
> userfaultfd monitors to unmap memory from monitored processes; and (b)
> it is more efficient than madvise() since it is vectored and batches TLB
> flushes more aggressively.

MADV_DONTNEED on MAP_PRIVATE memory is a target-visible operation; this 
is very different to all the other process_madvise() calls we allow, 
which are merely hints, but the target cannot be broken . I don't think 
this is acceptable.

-- 
Thanks,

David / dhildenb