Re: [PATCH v2 06/28] gup: Fix some contiguous memmap assumptions

[Matthew Wilcox]

On Mon, Jan 10, 2022 at 12:29:58AM -0800, Christoph Hellwig wrote:
> On Mon, Jan 10, 2022 at 04:23:44AM +0000, Matthew Wilcox (Oracle) wrote:
> > Several functions in gup.c assume that a compound page has virtually
> > contiguous page structs.  This isn't true for SPARSEMEM configs unless
> > SPARSEMEM_VMEMMAP is also set.  Fix them by using nth_page() instead of
> > plain pointer arithmetic.
> 
> So is this an actualy bug that need a Fixes tag, or do all architectures
> that support THP and sparsemem use SPARSEMEM_VMEMMAP?

As far as I can tell (and I am by no means an expert in this area),
this problem only affects pages of order MAX_ORDER or higher.  That is,
somebody using regular 2MB hugepages on x86 won't see a problem, whether
they're using VMEMMAP or not.  It only starts to become a problem for
1GB hugepages.

Since THPs are (currently) only allocated from the page allocator, it's
never a problem for THPs, only hugetlbfs.  Correcting the places which
can't see a 1GB page is just defense against copy-and-paste programming.

So I'll defer to Mike -- does this ever affect real systems and thus
warrant a backport?  I know this doesn't affect UEK because we enable
SPARSEMEM_VMEMMAP.

> > +	page = nth_page(head, (addr & (sz-1)) >> PAGE_SHIFT);
> 
> Would be nice to fix the indeation for sz - 1 while you're at it.

Done.