Re: [PATCHv2 02/14] mm/sparse: Check memmap alignment

[Kiryl Shutsemau <kas@kernel.org>]

On Tue, Dec 23, 2025 at 10:38:26AM +0100, David Hildenbrand (Red Hat) wrote:
> On 12/22/25 15:55, Muchun Song wrote:
> > 
> > 
> > > On Dec 22, 2025, at 22:18, David Hildenbrand (Red Hat) <david@kernel.org> wrote:
> > > 
> > > On 12/22/25 15:02, Kiryl Shutsemau wrote:
> > > > > On Mon, Dec 22, 2025 at 04:34:40PM +0800, Muchun Song wrote:
> > > > > 
> > > > > 
> > > > > On 2025/12/18 23:09, Kiryl Shutsemau wrote:
> > > > > > The upcoming changes in compound_head() require memmap to be naturally
> > > > > > aligned to the maximum folio size.
> > > > > > 
> > > > > > Add a warning if it is not.
> > > > > > 
> > > > > > A warning is sufficient as MAX_FOLIO_ORDER is very rarely used, so the
> > > > > > kernel is still likely to be functional if this strict check fails.
> > > > > 
> > > > > Different architectures default to 2 MB alignment (mainly to
> > > > > enable huge mappings), which only accommodates folios up to
> > > > > 128 MB. Yet 1 GB huge pages are still fairly common, so
> > > > > validating 16 GB (MAX_FOLIO_SIZE) alignment seems likely to
> > > > > miss the most frequent case.
> > > > I don't follow. 16 GB check is more strict that anything smaller.
> > > > How can it miss the most frequent case?
> > > > > I’m concerned that this might plant a hidden time bomb: it
> > > > > could detonate at any moment in later code, silently triggering
> > > > > memory corruption or similar failures. Therefore, I don’t
> > > > > think a WARNING is a good choice.
> > > > We can upgrade it BUG_ON(), but I want to understand your logic here
> > > > first.
> > > 
> > > Definitely no BUG_ON(). I would assume this is something we would find early during testing, so even a VM_WARN_ON_ONCE() should be good enough?
> > > 
> > > This smells like a possible problem, though, as soon as some architecture wants to increase the folio size. What would be the expected step to ensure the alignment is done properly?
> > > 
> > > But OTOH, as I raised Willy's work will make all of that here obsolete either way, so maybe not worth worrying about that case too much,
> > 
> > Hi David,
> > 
> 
> Hi! :)
> 
> > I hope you're doing well. I must admit I have limited knowledge of Willy's work, and I was wondering if you might be kind enough to share any publicly available links where I could learn more about the future direction of this project. I would be truly grateful for your guidance.
> > Thank you very much in advance.
> 
> There is some information to be had at [1], but more at [2]. Take a look at
> [2] in "After those projects are complete - Then we can shrink struct page
> to 32 bytes:"
> 
> In essence, all pages (belonging to a memdesc) will have a "memdesc" pointer
> (that replaces the compound_head pointer).
> 
> "Then we make page->compound_head point to the dynamically allocated memdesc
> rather than the first page. Then we can transition to the above layout. "

I am not sure I understand how it is going to work.

32-byte layout indicates that flags will stay in the statically
allocated part, but most (all?) flags are in the head page and we would
need a way to redirect from tail to head in the statically allocated
pages.

> The "memdesc" could be a pointer to a "struct folio" that is allocated from
> the slab.
> 
> So in the new memdesc world, all pages part of a folio will point at the
> allocated "struct folio", not the head page where "struct folio" currently
> overlays "struct page".
> 
> That would mean that the proposal in this patch set will have to be reverted
> again.
> 
> 
> At LPC, Willy said that he wants to have something out there in the first
> half of 2026.

Okay, seems ambitious to me.

Last time I asked, we had no idea how much performance would additional
indirection cost us. Do we have a clue?

I like memdesc idea, but indirection cost always bothered me.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov