Messages in this thread |  | | Date | Sat, 6 Dec 2025 16:43:57 +0000 | | From | Lorenzo Stoakes <> | | Subject | Re: [PATCH] mm: avoid use of BIT() macro for initialising VMA flags |
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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@oracle.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@oracle.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.
> > At the moment I'm trying to fix bitfield.h so you don't get compile errors > on lines that are 18KB long.
:)
> > Found a new version in linux-next - has its own set of new bugs as well > as more of the old ones. > > David > > > > > <insert rant about C macros here> > > > > Cheers, Lorenzo >
Cheers, Lorenzo
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