linux-kernel - Re: [RFC PATCH 00/16] mm/madvise: batch tlb flushes for MADV_DONTNEED and MADV

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Message-ID: <Z8ieZVFEa6vAouvu@casper.infradead.org>
Date: Wed, 5 Mar 2025 18:56:37 +0000
From: Matthew Wilcox <willy@...radead.org>
To: SeongJae Park <sj@...nel.org>
Cc: "Liam R. Howlett" <howlett@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Hildenbrand <david@...hat.com>,
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
	Shakeel Butt <shakeel.butt@...ux.dev>,
	Vlastimil Babka <vbabka@...e.cz>, kernel-team@...a.com,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [RFC PATCH 00/16] mm/madvise: batch tlb flushes for
 MADV_DONTNEED and MADV_FREE

On Wed, Mar 05, 2025 at 10:15:55AM -0800, SeongJae Park wrote:
> For MADV_DONTNEED[_LOCKED] or MADV_FREE madvise requests, tlb flushes
> can happen for each vma of the given address ranges.  Because such tlb
> flushes are for address ranges of same process, doing those in a batch
> is more efficient while still being safe.  Modify madvise() and
> process_madvise() entry level code path to do such batched tlb flushes,
> while the internal unmap logics do only gathering of the tlb entries to
> flush.

Do real applications actually do madvise requests that span multiple
VMAs?  It just seems weird to me.  Like, each vma comes from a separate
call to mmap [1], so why would it make sense for an application to
call madvise() across a VMA boundary?

[1] Yes, I know we sometimes merge and/or split VMAs