Re: [PATCH v2] mm, dump_page: do not crash with bad compound_mapcount()

[Vlastimil Babka <vbabka@suse.cz>]

On 8/6/20 3:48 PM, Matthew Wilcox wrote:
> On Thu, Aug 06, 2020 at 01:45:11PM +0200, Vlastimil Babka wrote:
>> How about this additional patch now that we have head_mapcoun()? (I wouldn't
>> go for squashing as the goal and scope is too different).
> 
> I like it.  It bothers me that the compiler doesn't know that
> compound_head(compound_head(x)) == compound_head(x).  I updated
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=32911 with a request to be
> able to tell the compiler that compound_head() is idempotent.

Yeah it would be nice to get the benefits everywhere automatically. But I guess
the compiler would have to discard the idempotence assumptions if there are
multiple consecutive (perhaps hidden behind page flag access)
compound_head(page) from a function, as soon as we modify the struct page somewhere.

>> The bloat-o-meter difference without DEBUG_VM is the following:
>> 
>> add/remove: 0/0 grow/shrink: 1/4 up/down: 32/-56 (-24)
>> Function                                     old     new   delta
>> __split_huge_pmd                            2867    2899     +32
>> shrink_page_list                            3860    3847     -13
>> reuse_swap_page                              762     748     -14
>> page_trans_huge_mapcount                     153     139     -14
>> total_mapcount                               187     172     -15
>> Total: Before=8687306, After=8687282, chg -0.00%
> 
> That's great.  I'm expecting improvements from my thp_head() macro when
> that lands (currently in Andrew's tree).  I have been reluctant to replace
> current callers of compound_head() with thp_head(), but I suspect PF_HEAD
> could use thp_head() and save a few bytes on a tinyconfig build.
> 
>> +++ b/mm/huge_memory.c
>> @@ -2125,7 +2125,7 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
>>  	 * Set PG_double_map before dropping compound_mapcount to avoid
>>  	 * false-negative page_mapped().
>>  	 */
>> -	if (compound_mapcount(page) > 1 && !TestSetPageDoubleMap(page)) {
>> +	if (head_mapcount(page) > 1 && !TestSetPageDoubleMap(page)) {
> 
> I'm a little nervous about this one.  The page does actually come from
> pmd_page(), and today that's guaranteed to be a head page.  But I'm
> not convinced that's going to still be true in twenty years.  With the
> current THP patchset, I won't allocate pages larger than PMD order, but
> I can see there being interest in tracking pages in chunks larger than
> 2MB in the future.  And then pmd_page() might well return a tail page.
> So it might be a good idea to not convert this one.

Hmm the function converts the compound mapcount of the whole page to a
HPAGE_PMD_NR of base pages. If suddenly the compound page was bigger than a pmd,
then I guess this wouldn't work properly anymore without changes anyway?
Maybe we could stick something like VM_BUG_ON(PageTransHuge(page)) there as
"enforced documentation" for now?