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

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Date:   Mon, 27 Sep 2021 03:41:18 -0700
From:   Nadav Amit <nadav.amit@...il.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Peter Xu <peterx@...hat.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 Sep 27, 2021, at 2:24 AM, David Hildenbrand <david@...hat.com> wrote:
> 
> 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.

This is a fair point, which I expected, but did not address properly.

I guess an additional capability, such as CAP_SYS_PTRACE needs to be
required in this case. Would that ease your mind?