linux-kernel - Re: [PATCH] mm: avoid use of BIT() macro for initialising VMA flags

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Message-ID: <7006fa60-f4d3-4e7d-8c2b-974e9e4a1224@suse.cz>
Date: Tue, 9 Dec 2025 09:28:10 +0100
From: Vlastimil Babka <vbabka@...e.cz>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 David Laight <david.laight.linux@...il.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
 David Hildenbrand <david@...nel.org>,
 "Liam R . Howlett" <Liam.Howlett@...cle.com>, Mike Rapoport
 <rppt@...nel.org>, Suren Baghdasaryan <surenb@...gle.com>,
 Michal Hocko <mhocko@...e.com>, oliver.sang@...el.com, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, Mateusz Guzik <mjguzik@...il.com>
Subject: Re: [PATCH] mm: avoid use of BIT() macro for initialising VMA flags

On 12/8/25 17:42, Lorenzo Stoakes wrote:
> On Sat, Dec 06, 2025 at 04:43:57PM +0000, Lorenzo Stoakes wrote:
>> On Fri, Dec 05, 2025 at 09:34:49PM +0000, David Laight wrote:
>> > On Fri, 5 Dec 2025 19:18:56 +0000
>> > Lorenzo Stoakes <lorenzo.stoakes@...cle.com> wrote:
>> >
>> > > On Fri, Dec 05, 2025 at 06:43:42PM +0000, David Laight wrote:
>> > > > On Fri,  5 Dec 2025 17:50:37 +0000
>> > > > Lorenzo Stoakes <lorenzo.stoakes@...cle.com> wrote:
>> > > >
>> > > > > Commit 2b6a3f061f11 ("mm: declare VMA flags by bit") significantly changed
>> > > > > how VMA flags are declared, utilising an enum of VMA bit values and
>> > > > > ifdef-fery VM_xxx flag declarations via macro.
>> > > > >
>> > > > > As part of this change, it uses INIT_VM_FLAG() to define VM_xxx flags from
>> > > > > the newly introduced VMA bit numbers.
>> > > > >
>> > > > > However, use of this macro results in apparently unfortunate macro
>> > > > > expansion and resulted in a performance degradation.This appears to be due
>> > > > > to the (__force int), which is required for the sparse typechecking to
>> > > > > work.
>> > > >
>> > > > Does sparse complain if you just add 0? As in:
>> > > > #define INIT_VM_FLAG(name) BIT(VMA_ ## name ## _BIT + 0u)
>> > > >
>> > > > That should change the type without affecting what BIT() expands to.
>> > >
>> > > Thanks, checked that and unfortunately that doesn't satisfy sparse :)
>> > >
>> > > I don't think it's too crazy to use 1UL << here, just very frustrating (TM)
>> > > that this is an issue.
>> >
>> > I might use some of my copious spare time (ha) to see why BIT() fails.
>> > I bet it is just too complex for its own good.
>> > Personally I'm fine with both explicit (1ul << n) and hex constants.
>> > The latter are definitely most useful if you ever look at hexdumps.
>>
>> Thanks :) yeah I just didn't want to go down that rabbit hole myself as I seemed
>> to have the answer and wanted to get it fixed, but obviously am quite curious as
>> to what on earth is causing that.
> 
> I did wonder about _calc_vm_trans(), given the 'interesting' stuff it does.

It's unlikely that this affects anything in what the benchmark stresses.
As Mateusz pointed out off-list, the profiles look like mutexes are doing
less optimistic spinning and more sleeping. Which IMHO isn't something that
this change can directly affect.

My own bloat-o-meter test before/after the fix suggests no changed code
generation (as I would indeed expect):

 At least in my case it doesn't seem to be altering the generated code (and
I would expect it wouldn't) except some weird symbols that don't look
related at all:

> ./scripts/bloat-o-meter vmlinux.o vmlinux.o.after
add/remove: 0/0 grow/shrink: 0/7 up/down: 0/-277 (-277)
Function                                     old     new   delta
print_fmt_dax_pte_fault_class               1167    1160      -7
print_fmt_dax_pmd_load_hole_class            308     301      -7
print_fmt_dax_pmd_fault_class               1252    1245      -7
saved_rsp                                3117296 3117232     -64
saved_rdi                                3117312 3117248     -64
saved_rbx                                3117304 3117240     -64
saved_rbp                                3117328 3117264     -64
Total: Before=3350750464, After=3350750187, chg -0.00%

I don't know what happened to those functions above. Could be just
insufficient build reproducibility?
Maybe the effect is just that something slightly shifts in the cpu caches?