Re: [PATCH v5 7/9] drivers/base/memory: count inherited poisoned frames into the block

[Breno Leitao <leitao@debian.org>]

On Wed, Sep 16, 2026 at 04:51:40PM +0200, David Hildenbrand (Arm) wrote:
> On 9/16/26 11:35, Breno Leitao wrote:
> >> That just slows down boot unnecessarily on 99.9999999999999999999% of all
> >> systems out there.
> >
> > hmmm, I am not sure I see it that way.
> >
> > The loop only runs for a block the bitmap marks. On a machine with nothing
> > recorded the bitmap is all zeros, the range_contains_poisoned_memory() check
> > right above it returns false, and the loop never executes.
> >
> > What every boot does pay is that check, once per block. The stub installs
> > the table whether or not anything was ever recorded in it, so this is not a
> > NULL test: it is two 64-bit divisions by the unit size plus a
> > find_next_bit() over the single word a 128M block covers at one bit per 2M.
> >
> > The real cost (that "for loop above"), comes when you kexec (not on cold
> > boot -- given the bitmap is empty), and you are trying to init
> > a memory block that has poisoned pages into it. Which seems the right
> > trade-off, no?
> >
> > That said, can we do better? Yes. The silly win is to let the table say
> > whether anything was ever recorded in it, something like a
> > linux_efi_poisoned_memory->empty that the first recorded frame clears,
> > and return on that before the bitmap is reached at all.
> >
> > Is this what you are looking for, or something more drastic?
>
> Ah, that magical "range_contains_poisoned_memory" does a bitmap scan?
>
> I'm sorry, but that is absolutely confusing.
>
> There is no way someone will figure out that range_contains_poisoned_memory()
> queries some efi specific bitmap that won't even be able to represent any memory
> outside of it's range.
>
> I don't really have time to give a better solution, but starting with the
> naming, range_contains_poisoned_memory() is just absolutely misleading.

Fair point, I'll clean up the naming in the next revision.

I'll also add that ->empty field, which should help locate this bit
faster and may let us skip the bitmap query entirely on the happy path.

Anything else you'd like addressed?

Good to know this moved the needle from "David hates this feature" to
"David only hates the naming" -- I'll take that as progress.  :-P