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

[Matthew Wilcox]

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.

> 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.

> @@ -2467,7 +2467,7 @@ int total_mapcount(struct page *page)
>  	if (likely(!PageCompound(page)))
>  		return atomic_read(&page->_mapcount) + 1;
>  
> -	compound = compound_mapcount(page);
> +	compound = head_mapcount(page);
>  	if (PageHuge(page))
>  		return compound;
>  	ret = compound;

Yes.  This function is a little confusing because it uses PageCompound()
all the way through when it really should be using PageHead ... because
the opening line of the function is: VM_BUG_ON_PAGE(PageTail(page), page);

> @@ -2531,7 +2531,7 @@ int page_trans_huge_mapcount(struct page *page, int *total_mapcount)
>  		ret -= 1;
>  		_total_mapcount -= HPAGE_PMD_NR;
>  	}
> -	mapcount = compound_mapcount(page);
> +	mapcount = head_mapcount(page);
>  	ret += mapcount;
>  	_total_mapcount += mapcount;
>  	if (total_mapcount)

Yes, we called compound_head() earlier in the function.  Safe.

> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 9ee4211835c6..c5e722de38b8 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -1673,7 +1673,7 @@ static int page_trans_huge_map_swapcount(struct page *page, int *total_mapcount,
>  		map_swapcount -= 1;
>  		_total_mapcount -= HPAGE_PMD_NR;
>  	}
> -	mapcount = compound_mapcount(page);
> +	mapcount = head_mapcount(page);
>  	map_swapcount += mapcount;
>  	_total_mapcount += mapcount;
>  	if (total_mapcount)

Yes.  page is a head page at this point.

> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index a086c104a9a6..72218cdfd902 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1248,7 +1248,7 @@ static unsigned int shrink_page_list(struct list_head *page_list,
>  					 * away. Chances are some or all of the
>  					 * tail pages can be freed without IO.
>  					 */
> -					if (!compound_mapcount(page) &&
> +					if (!head_mapcount(page) &&
>  					    split_huge_page_to_list(page,
>  								    page_list))
>  						goto activate_locked;

Yes.  We don't put (can't put!) tail pages on the lists, so this must be a
head page.