Re: [PATCH 1/3] mm: Print head flags in dump_page

[Matthew Wilcox <willy@infradead.org>]

On Thu, Jul 02, 2020 at 11:39:07PM +0300, Kirill A. Shutemov wrote:
> On Thu, Jul 02, 2020 at 06:19:08PM +0200, Vlastimil Babka wrote:
> > Really? We are calculatting index (pgoff) of tail page, which should be index of
> > head page plus n for n'th tail page; the unit is base pages.
> > But ff HugeTLB head pages use the unit of huge page in page->index, and
> > page_to_pgoff() translates it to unit of base pages, then we should do the same
> > when calculating the index of tail page, no? Otherwise we are adding up units of
> > huge pages (from head->index) with units of base page (n'th tail) and get
> > garbage as a result, AFAICS?
> 
> You are right. I guess we can get away with this because nobody calls
> page_to_pgoff() on tail pages of hugetlb page. Except when something goes
> wrong and dump_page() has to deal with it.
> 
> I'm not sure if it's worth fixing and whether the fix should be inside
> page_to_pgoff().
> 
> The best fix would be to remove hugetlb pages special-casing: it has to
> have ->index in base pagesize, not huge page.

https://lore.kernel.org/linux-mm/20200629152033.16175-1-willy@infradead.org/
would be a good place to start.  We'll use 8 entries for an order-9 page
instead of the 1 entry that we currently do, but that's a lot better
than using 512 entries.