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

[Lorenzo Stoakes]

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.

<insert rant about C macros here>

Cheers, Lorenzo