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

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Message-ID: <7e151e6b-3f81-4e37-b314-c6e9f19c1b82@redhat.com>
Date: Wed, 5 Mar 2025 20:49:13 +0100
From: David Hildenbrand <david@...hat.com>
To: Shakeel Butt <shakeel.butt@...ux.dev>
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 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.

Makes a lot of sense to me!

-- 
Cheers,

David / dhildenb