linux-kernel - Re: [PATCH v3 2/3] liveupdate: kho: Increase metadata bitmap size to PAGE

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Message-ID: <CALzav=fD47OCZRMNqDmHs798qpZzeU6eHHuNA+wyhmZ19aJ9vA@mail.gmail.com>
Date: Mon, 27 Oct 2025 16:01:37 -0700
From: David Matlack <dmatlack@...gle.com>
To: Pasha Tatashin <pasha.tatashin@...een.com>
Cc: akpm@...ux-foundation.org, brauner@...nel.org, corbet@....net, 
	graf@...zon.com, jgg@...pe.ca, linux-kernel@...r.kernel.org, 
	linux-kselftest@...r.kernel.org, linux-mm@...ck.org, masahiroy@...nel.org, 
	ojeda@...nel.org, pratyush@...nel.org, rdunlap@...radead.org, rppt@...nel.org, 
	tj@...nel.org, jasonmiu@...gle.com, skhawaja@...gle.com
Subject: Re: [PATCH v3 2/3] liveupdate: kho: Increase metadata bitmap size to PAGE_SIZE

On Mon, Oct 27, 2025 at 3:56 PM David Matlack <dmatlack@...gle.com> wrote:
>
> On Mon, Oct 20, 2025 at 5:09 PM Pasha Tatashin
> <pasha.tatashin@...een.com> wrote:
>
> > -static void *xa_load_or_alloc(struct xarray *xa, unsigned long index, size_t sz)
> > +static void *xa_load_or_alloc(struct xarray *xa, unsigned long index)
> >  {
> >         void *res = xa_load(xa, index);
> >
> >         if (res)
> >                 return res;
> >
> > -       void *elm __free(kfree) = kzalloc(sz, GFP_KERNEL);
> > +       void *elm __free(kfree) = kzalloc(PAGE_SIZE, GFP_KERNEL);
> >
> >         if (!elm)
> >                 return ERR_PTR(-ENOMEM);
> >
> > -       if (WARN_ON(kho_scratch_overlap(virt_to_phys(elm), sz)))
> > +       if (WARN_ON(kho_scratch_overlap(virt_to_phys(elm), PAGE_SIZE)))
> >                 return ERR_PTR(-EINVAL);
>
> Reading xa_load_or_alloc() is a bit confusing now.
>
> It seems very generic (returns a void *) but now hard-codes a size
> (PAGE_SIZE). You have to look at the caller to see it is allocating
> for a struct kho_mem_phys_bits, and then at the definition of struct
> kho_mem_phys_bits to see the static_assert() that this struct is
> always PAGE_SIZE.
>
> I would either keep letting the caller passing in size (if you think
> this code is going to be re-used) or just commit to making
> xa_load_or_alloc() specific to kho_mem_phys_bits. e.g. Change the
> return type to struct kho_mem_phys_bits * and use sizeof() instead of
> PAGE_SIZE.

I see that you replace kzalloc() with get_zeroed_page() in the next
patch. So the latter option is probably better, and maybe move static
assert down here and use BUILD_BUG_ON()? That way readers can easily
see that we are allocating for struct kho_mem_phys_bits *and* that
that struct is guaranteed to be PAGE_SIZE'd.