From: Usama Arif <usama.arif@linux•dev>
To: Jan Kara <jack@suse•cz>
Cc: Pedro Falcato <pfalcato@suse•de>,
willy@infradead•org, Andrew Morton <akpm@linux-foundation•org>,
david@kernel•org, ryan.roberts@arm•com, linux-mm@kvack•org,
r@hev•cc, Andrew Donnellan <andrew+kernel@donnellan•id.au>,
apopple@nvidia•com, baohua@kernel•org,
baolin.wang@linux•alibaba.com, brauner@kernel•org,
catalin.marinas@arm•com, dev.jain@arm•com, kees@kernel•org,
kevin.brodsky@arm•com, lance.yang@linux•dev,
"Liam R. Howlett" <liam@infradead•org>,
linux-arm-kernel@lists•infradead.org,
linux-fsdevel@vger•kernel.org, linux-kernel@vger•kernel.org,
ljs@kernel•org, mhocko@suse•com, npache@redhat•com,
pasha.tatashin@soleen•com, rmclure@linux•ibm.com,
rppt@kernel•org, surenb@google•com, vbabka@kernel•org,
Al Viro <viro@zeniv•linux.org.uk>,
wilts.infradead.org@pedro-suse•lan, ziy@nvidia•com,
hannes@cmpxchg•org, kas@kernel•org, shakeel.butt@linux•dev,
kernel-team@meta•com
Subject: Re: [PATCH v6 2/2] mm: use mapping_max_folio_order() for force_thp_readahead order
Date: Mon, 1 Jun 2026 10:43:03 +0100 [thread overview]
Message-ID: <d144b0a1-acd5-4d08-aab0-5b2dc8219219@linux.dev> (raw)
In-Reply-To: <u52xvwk2fjjk2izdb4wvuqq2zhc5neb4q6usimcxejluuyngxw@d64sc6rvojyp>
On 30/05/2026 16:16, Jan Kara wrote:
> On Fri 29-05-26 15:11:54, Usama Arif wrote:
>> On 29/05/2026 14:40, Pedro Falcato wrote:
>>> On Fri, May 29, 2026 at 01:19:03PM +0100, Usama Arif wrote:
>>>>
>>>> which means mapping_max_folio_order(mapping) <= MAX_PAGECACHE_ORDER <= HPAGE_PMD_ORDER is always
>>>> true, and you dont need the min3(..) in your diff.
>>>>
>>>> Now the question is if then why not just do:
>>>>
>>>> if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE) && (vm_flags & VM_HUGEPAGE)) {
>>>> if (mapping_large_folio_support(mapping)) {
>>>> force_thp_readahead = true;
>>>> thp_order = min_t(unsigned int,
>>>> mapping_max_folio_order(mapping),
>>>> get_order(SZ_2M));
>>>> }
>>>> }
>>>>
>>>>
>>>> This is because this will regress the 16K ARM case where we already got 32M
>>>> folios. Someone might upgrade the kernel and start getting 2M folios now.
>>>
>>> So maybe limit to 32MB? It's still arbitrary but at least you get simpler
>>> logic. If the architecture does not support 32MiB folios, it will clamp
>>> the maximum folio order to HPAGE_PMD_ORDER, and you get the same result.
>>>
>>> Does this sound correct?
>>>
>>
>> Yes, so if we replace it with SZ_32M, it sounds correct. I just think
>> the 32M size is too large. But as you pointed out, even 2M can be too large...
>
> So AFAIU the practical discussion is about two options:
>
> 1) limiting at 2MB with a slighly more complicated logic to keep mapping at
> PMD order for 16k pagesize on ARM but use 2MB pages for 64k pagesize on ARM
>
> or
>
> 2) limit at 32MB with simple logic which results in larger (32MB) folios
> with 16k and 64k pagesize on ARM and thus larger memory overhead.
>
> I'd like to maybe offer option 3): limit at 2MB with simple logic. This
> will reduce folio size on 16k pagesize ARM compared to 1) but do we really
> care? I.e., is there big enough practical performance impact with conpte
> and other tricks ARM is playing?
>
I think the logic isn't that complicated for 1, but I am happy with option 3.
>>> Bottom line is that changing things will always affect someone :) Particularly
>>> since the logic we have is not too careful at deciding what should or should
>>> not be a THP (both in anon and file cases). And if (once?) we make it smarter,
>>> it will surely also regress someone!
>>
>> Yes completely agree on this as well.
>>
>> So personally I do have a preference of keeping the cap at 2M atleast initially
>> while we currently try and solve the issues we see with 2M alone. As we are already
>> seeing reports of thrashing and compaction with just 2M, I dont think the logic
>> in this patch with just an if else is that complicated.
>>
>> Matthew, Jan, do you have any thoughts or strong preferences on cap size?
>
> Frankly, no strong opinion. I'd think 3) is worth trying for its simplicity
> and seeing whether somebody complains, otherwise I can live with both 1)
> and 2).
Thanks! Yes, let me send this.
Andrew I will send this as a new and hopefully last revision as I am not sure
if you would like another fixup on top of this series! Thanks!
>
> Honza
next prev parent reply other threads:[~2026-06-01 9:43 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <20260528165635.2068012-1-usama.arif@linux.dev>
2026-05-28 20:27 ` [PATCH v6 0/2] mm: improve large folio readahead for exec memory Andrew Morton
[not found] ` <20260528165635.2068012-2-usama.arif@linux.dev>
2026-05-29 9:47 ` [PATCH v6 1/2] mm: bypass mmap_miss heuristic for VM_EXEC readahead Pedro Falcato
[not found] ` <20260528165635.2068012-3-usama.arif@linux.dev>
2026-05-29 10:01 ` [PATCH v6 2/2] mm: use mapping_max_folio_order() for force_thp_readahead order Pedro Falcato
2026-05-29 12:19 ` Usama Arif
2026-05-29 13:40 ` Pedro Falcato
2026-05-29 14:11 ` Usama Arif
2026-05-30 15:16 ` Jan Kara
2026-06-01 9:43 ` Usama Arif [this message]
2026-06-02 17:35 ` Pedro Falcato
2026-06-03 10:10 ` Usama Arif
2026-06-03 11:51 ` Pedro Falcato
2026-06-02 17:46 ` Pedro Falcato
2026-05-29 12:36 ` Usama Arif
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