linux-kernel - Re: [PATCH v1 04/17] mm: let _folio_nr_pages overlay memcg

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Message-ID: <ea19ca6b-b521-4316-a918-2a7ff5e518d3@redhat.com>
Date: Wed, 23 Oct 2024 13:40:28 +0200
From: David Hildenbrand <david@...hat.com>
To: "Kirill A. Shutemov" <kirill@...temov.name>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 cgroups@...r.kernel.org, x86@...nel.org, linux-fsdevel@...r.kernel.org,
 Andrew Morton <akpm@...ux-foundation.org>,
 "Matthew Wilcox (Oracle)" <willy@...radead.org>, Tejun Heo <tj@...nel.org>,
 Zefan Li <lizefan.x@...edance.com>, Johannes Weiner <hannes@...xchg.org>,
 Michal Koutný <mkoutny@...e.com>,
 Jonathan Corbet <corbet@....net>, Andy Lutomirski <luto@...nel.org>,
 Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
 Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: [PATCH v1 04/17] mm: let _folio_nr_pages overlay memcg_data in
 first tail page

On 23.10.24 13:38, Kirill A. Shutemov wrote:
> On Thu, Aug 29, 2024 at 06:56:07PM +0200, David Hildenbrand wrote:
>> Let's free up some more of the "unconditionally available on 64BIT"
>> space in order-1 folios by letting _folio_nr_pages overlay memcg_data in
>> the first tail page (second folio page). Consequently, we have the
>> optimization now whenever we have CONFIG_MEMCG, independent of 64BIT.
>>
>> We have to make sure that page->memcg on tail pages does not return
>> "surprises". page_memcg_check() already properly refuses PageTail().
>> Let's do that earlier in print_page_owner_memcg() to avoid printing
>> wrong "Slab cache page" information. No other code should touch that
>> field on tail pages of compound pages.
>>
>> Reset the "_nr_pages" to 0 when splitting folios, or when freeing them
>> back to the buddy (to avoid false page->memcg_data "bad page" reports).
>>
>> Note that in __split_huge_page(), folio_nr_pages() would stop working
>> already as soon as we start messing with the subpages.
>>
>> Most kernel configs should have at least CONFIG_MEMCG enabled, even if
>> disabled at runtime. 64byte "struct memmap" is what we usually have
>> on 64BIT.
>>
>> While at it, rename "_folio_nr_pages" to "_nr_pages".
>>
>> Signed-off-by: David Hildenbrand <david@...hat.com>
> 
> Reviewed-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> 
> BTW, have anybody evaluated how much (if anything) do we gain we a
> separate _nr_pages field in struct folio comparing to calculating it
> based on the order in _flags_1? Mask+shift should be pretty cheap.

I recall that Willy did, and it's mostly getting rid of a single 
instruction in loads of places.

$ git grep folio_nr_pages | wc -l
254

[my first intuition was also to just remove it, but this way seems easy 
to just maintain it for now]

-- 
Cheers,

David / dhildenb