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

[Vlastimil Babka <vbabka@suse.cz>]

On 7/2/20 5:53 PM, Kirill A. Shutemov wrote:
> On Thu, Jul 02, 2020 at 04:59:14PM +0200, Vlastimil Babka wrote:
>> On 6/30/20 1:59 PM, Matthew Wilcox wrote:
>> > On Tue, Jun 30, 2020 at 10:02:50AM +0200, Vlastimil Babka wrote:
>> >> I would also prefer this approach.It would be also nice to know the tail index.
>> >> As long as page pointer wasn't hashed, it was possible to figure this out, but
>> >> now it's not. Maybe print pfn of both page and head?
>> > 
>> >> On 6/30/20 1:35 AM, John Hubbard wrote:
>> >> > ...so with that fix, along with your line break approach in the other thread,
>> >> > a tail page dump of a FOLL_PIN page looks like this:
>> >> > 
>> >> > [   38.027987] page:00000000abaef9ae refcount:513 mapcount:1 mapping:0000000000000000 index:0x11
>> > 
>> > index is the last thing printed on this line.  If it's something like
>> > index:0x12345678, you can reduce it mod 1<<order, as printed on the next
>> > line.
>> 
>> Hmm, I guess that works as long as head pages really have index aligned to 1 <<
>> order ... they should, right?
>> 
>> But does it fail for HugeTLB? CC Kirill (git blamed) and Mike.
>> page_to_pgoff() has for PageHeadHuge this:
>> 	return page->index << compound_order(page)
>> 
>> but page_to_index() does simply
>>         pgoff = compound_head(page)->index;
>>         pgoff += page - compound_head(page);
>> 
>> Shouldn't it also do a <<compound_order(head) for HugeTLB?
> 
> PageHeadHuge() is only true for head pages, so we are fine here.

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?