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

[Lorenzo Stoakes]

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@xxxxxxxxxx> 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@xxxxxxxxxx> 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.

Maybe I should fiddle with that and see...

Cheers, Lorenzo