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

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Message-Id: <20250305205938.57904-1-sj@kernel.org>
Date: Wed,  5 Mar 2025 12:59:38 -0800
From: SeongJae Park <sj@...nel.org>
To: David Hildenbrand <david@...hat.com>
Cc: SeongJae Park <sj@...nel.org>,
	Shakeel Butt <shakeel.butt@...ux.dev>,
	Matthew Wilcox <willy@...radead.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, 5 Mar 2025 20:49:13 +0100 David Hildenbrand <david@...hat.com> wrote:

> On 05.03.25 20:46, Shakeel Butt wrote:
> > 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.

Shakeel is correct.  Thank you for making the early clarification Shakeel.

Also sorry for causing confuses.  I will make this point clearer on next spin.

> 
> Makes a lot of sense to me!

Seems Shakeel already addressed all question so far, but please feel free to
raise more question for anything not yet cleared!

> 
> -- 
> Cheers,
> 
> David / dhildenb


Thanks,
SJ