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

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Message-ID: <snby4wevysj2hr6rmqcwezcujhwmjgtby6ogkrc4wmqnzcqcsv@tu23rsyltc2m>
Date: Wed, 5 Mar 2025 11:46:31 -0800
From: Shakeel Butt <shakeel.butt@...ux.dev>
To: David Hildenbrand <david@...hat.com>
Cc: Matthew Wilcox <willy@...radead.org>, SeongJae Park <sj@...nel.org>, 
	"Liam R. Howlett" <howlett@...il.com>, Andrew Morton <akpm@...ux-foundation.org>, 
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, 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 08:19:41PM +0100, David Hildenbrand wrote:
> On 05.03.25 19:56, Matthew Wilcox wrote:
> > 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?
> 
> I had the same question. If this happens in an app, I would assume that a
> single MADV_DONTNEED call would usually not span multiples VMAs, and if it
> does, not that many (and that often) that we would really care about it.

IMHO madvise() is just an add-on and the real motivation behind this
series is your next point.

> 
> OTOH, optimizing tlb flushing when using a vectored MADV_DONTNEED version
> would make more sense to me. I don't recall if process_madvise() allows for
> that already, and if it does, is this series primarily tackling optimizing
> that?

Yes process_madvise() allows that and that is what SJ has benchmarked
and reported in the cover letter. In addition, we are adding
process_madvise() support in jemalloc which will land soon.